Tag Archives: milky way

The Milky Way collided with a galaxy in the past — and this shaped our galaxy

The Milky Way and Andromeda are on a collision course, a galactic smash-up of gargantuan proportions. But there’s not much reason why you should worry about this. For starters, it’s likely to happen some 4.5 billion years in the future — and also, as a new study found, our galaxy has merged with others before.

In fact, around 10 billion years ago with a dwarf galaxy called Gaia-Sausage-Enceladus (GSE) –  one of the most recent collisions.

A major event in the formation of the Milky Way with Gaia-Sausage-Enceladus, around 10 billion years ago.
Credit: ESA (artist’s impression and composition); Koppelman, Villalobos and Helmi (simulation); NASA/ESA/Hubble (galaxy image), CC BY-SA 3.0 IGO

The GSE was discovered with the European Space Agency (ESA)’s Gaia observatory. In the 22 first months of observations, Gaia found a distribution of 30,000 stars in a trajectory opposite to most stars. Their path in the galaxy resembles a sausage (hence the ‘Sausage’ in the name) and their brightness indicates they belong to a particular stellar population.

Recently, a team simulated the collision between the galaxies and tried to understand if it was head-on or did it involve a decaying orbit. The scientists based their simulation on both Gaia and Multiple Mirror Telescope (MMT) Observatory of the Smithsonian Institution and the University of Arizona. The idea is to create a simulation as close to reality as possible.

In the results, the team successfully obtained the structure present today and how the collision formed. Their analysis matches with the trajectories and composition of the stars that exist today. GSE was approaching the Milky Way in opposite direction from our galaxy’s rotation until the dwarf galaxy merged with its bigger neighbor. In other words, judging by these trajectories, the Milky Way and GSE have collided in the past.

Anatomy of the Milky Way. Credit: Left: NASA/JPL-Caltech; right: ESA; layout: ESA/ATG medialab

This study contributes to more information about the history of the Milky Way. We now have an idea of how our galaxy became the way it is today. Merging with Gaia-Sausage-Enceladus had about 500 million stars and the event provided 20% of our galaxies’ dark matter and 50% of its stellar halo – part of a galaxy that contains stars beyond the primary distribution. 

The study was published in The Astrophysical Journal.

Astronomers zoom in on stars around Milky Way’s supermassive black hole

These annotated images, obtained with the GRAVITY instrument on ESO’s Very Large Telescope Interferometer (VLTI) between March and July 2021, show stars orbiting very close to Sgr A*, the supermassive black hole at the heart of the Milky Way. Credit: ESO.

By studying the motions of distant stars around the galactic centers, scientists showed that there’s a supermassive black hole at the heart of the Milky Way whose mass exceeds 4 million Suns. Continuing this line of research, which was awarded the 2020 Nobel Prize in Physics, astronomers affiliated with the European Southern Observatory’s Very Large Telescope Interferometer (VLTI) have computed new images of the closest stars observed circling the supermassive black hole so far. The new images zoom in 20 times closer than what was previously possible.

Scientists have long-suspected the Milky Way harbors a massive black hole, like other similarly-sized galaxies in the universe. But proving it is another thing. After all, the gravitational anomaly could also be explained by tight clusters of neutron stars, for instance.

But all shroud of doubt lifted in the spring of 2002 when astrophysicists led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Germany used optical imaging showing a tiny speck of light — a star now known as SO-2 — that passed within 17 light-hours of the galactic center at an astonishing speed. That’s a minuscule distance at the cosmic scale, only three times the distance between Pluto and the sun. There’s only one object in the known universe that is compact enough and has enough mass to accelerate stars to such a high speed, and that’s a supermassive black hole.

Elsewhere, astronomer Andrea Ghez’s Galactic Center Group at UCLA used the 10-meter Keck Telescope in Hawaii to track the motion of S2, reporting in 2000 that the star’s path is curved, a telltale sign it was orbiting something super massive at the galactic center. The UCLA team later also found S2 orbits the galactic center, known as Sagittarius A*, very closely. Genzel, Ghez, and Roger Penrose shared the 2020 Nobel Prize in Physics “for the discovery of a supermassive compact object at the center of our galaxy.”

Research in studying the galactic center is an endless work in progress, though. Genzel’s latest work with the GRAVITY collaboration continues to expand an almost three-decade-long study of stars orbiting Sagittarius A*. GRAVITY is the second-generation VLTI instrument for precision narrow-angle astrometry and interferometric imaging. It brings the most advanced vision to the VLT: with its fiber-fed integrated optics, wavefront sensors, fringe tracker, beam stabilization, and a novel metrology concept, GRAVITY pushes the sensitivity and accuracy far beyond what is offered today. Using novel analysis techniques on GRAVITY data, the scientists obtained the deepest and sharpest images of the galactic center thus far.

In the process, the researchers made more precise measurements of previously identified stars as they approached the black hole. One such star is S29, which in May 2021 passed the galactic center at a distance of just 13 billion kilometers, equivalent to 90 times the distance from Earth to the Sun, at a speed of 8740 kilometers per second. No other star has been found to travel to this close or this fast around the supermassive black hole.

They also found a new, previously hidden star called S300, which demonstrated the power of the new method that combines the light of all four 8.2-meter telescopes of ESO’s Very Large Telescope located in Chile, using a technique known as interferometry. A machine-learning technique called Information Field Theory simulated how GRAVITY should see the new images of the stars around Sagittarius A*, which was then compared to actual GRAVITY observations.

“We have been building GRAVITY for a decade. How many things can go wrong when building such a complex machine? And, indeed, one of the main challenges really was that one needs to control & monitor the four telescopes of the VLT and the common interferometric laboratory to sufficient precision. Many, many control loops are running to keep things stable. And what perhaps is technically amazing: We use the 8m diameter telescopes, with domes as big as a tennis court, bundle the light, feed it into fibers that guide the light in channels with a diameter smaller than 1mm, and combine the light finally in a glass chip, that is only a few centimeters in size. The recorded wave patterns encode in a complex way the image of the sky – but with clever techniques, this can be recovered,” Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics and co-author of the new study told ZME Science.

This image show stars orbiting very close to Sgr A* (centre), the supermassive black hole at the heart of the Milky Way. They were obtained with the GRAVITY instrument on ESO’s Very Large Telescope Interferometer (VLTI) at the end of June 2021. Credit: ESO.

The new observations also confirm that the paths of the stars in close proximity to Sagittarius A* are exactly those predicted by Einstein’s Theory of General Relativity. Using the new data, the researchers refined the mass of Sagittarius A*, now computed at 4.3 million times that of the Sun, as well as its distance, finding it is about 27,000 light-years away.

But despite the most precise measurements to date of Sagittarius A*, the galactic center remains largely mysterious. Previous work by scientists from the Event Horizon Telescope led to the first and now iconic image of the shadow of a black hole at the heart of galaxy M87. However, The Milky Way’s black hole is trickier to image because it is much more active. “I would not be surprised if also the new stars behave as chaotically as the ones we know at larger radii already. But we better find that out via observations,” Gillessen said.

Later this decade, GRAVITY will be upgraded in order to improve sensitivity and reveal fainter stars that perhaps lurk even closer to the black hole. ESO’s upcoming Extremely Large Telescope (ELT), currently under construction in the Chilean Atacama Desert, will further supercharge these efforts, allowing scientists to measure the velocity of these stars with the highest precision. In the meantime, even the tools currently at our disposal can deliver stunning results when in the right hands.

“Perhaps what is most stunning is how big a step forward it is. With interferometry we can see a factor 15 sharper than what was possible with single telescopes. That is really a huge improvement. Imagine, someone would increase your salary by that factor – it is simply huge. And we are very thrilled by the new opportunities. The Galactic Center continues to be very rich and exciting!” Gillessen said. 

Strange radio waves picked up from center of Milky Way baffle astronomers

The Milky Way still contains plenty of unknowns. (Photo: Pixabay)

The Milky Way offers up a lot of unknows, one reason that scientists are always on the lookout for new stuff. An international team of researchers might have just found some when they discovered unusual radio waves coming from the direction of the galactic core. The waves fit no current pattern of radio sources and could mean a brand spanking new class of stellar objects.

“The strangest property of this new signal is that it is has a very high polarization. This means its light oscillates in only one direction, but that direction rotates with time,” said Ziteng Wang, lead author of the new study and a Ph.D. student in the School of Physics at the University of Sydney. “The brightness of the object also varies dramatically, by a factor of 100, and the signal switches on and off apparently at random. We’ve never seen anything like it.”

Many types of stars emit some sort of variable light across the electromagnetic spectrum. Radio waves have become a larger part of discoveries with ongoing advances. Some of the more significant finds from radio telescopes are pulsars, supernovae, flaring stars and fast radio bursts (and even Apollo landing sites).

“At first we thought it could be a pulsar – a very dense type of spinning dead star – or else a type of star that emits huge solar flares,” said Wang. “But the signals from this new source don’t match what we expect from these types of celestial objects.”

An artist’s impression of the ASKAP J173608.2-321635 source. Image credit: Sebastian Zentilomo.

Wang and his group found the unknown waves using the CSIRO’s ASKAP radio telescope in Western Australia. Follow-up observations were performed with the South African Radio Astronomy Observatory’s MeerKAT telescope. They had been surveying the universe since 2020 through a project known as Variables and Slow Transients (VAST).

Coined “ASKAP J173608.2-321635” after its coordinates (which doesn’t exactly roll off the tongue), the object started out invisible, became bright, faded away and then reappeared, a behavior that Tara Murphy, Wang’s Ph.D. supervisor called “extraordinary.”

After detecting six radio signals from the source over nine months in 2020, the astronomers tried to find the object in visual light. They found nothing. So they turned to the Parkes radio telescope in Australia. Again nothing.

After a switch to the more sensitive MeerKAT radio telescope in South Africa, they were able to pick up the lead.

“Luckily, the signal returned, but we found that the behavior of the source was dramatically different – the source disappeared in a single day, even though it had lasted for weeks in our previous ASKAP observations,” said Murphy.

The astronomers plan to keep their eyes peeled to look for more clues as to what it might be. They hope to learn more with the unveiling of the transcontinental Square Kilometre Array radio telescope. The total collecting area of the new telescope will be a square kilometer, making it 50 times more sensitive than any current radio instrument.

The findings appeared in the Astrophysical Journal.

This NASA/ESA Hubble Space Telescope image shows the massive galaxy cluster MACSJ 1206. Embedded within the cluster are the distorted images of distant background galaxies, seen as arcs and smeared features. These distortions are caused by the dark matter in the cluster, whose gravity bends and magnifies the light from faraway galaxies, an effect called gravitational lensing. This phenomenon allows astronomers to study remote galaxies that would otherwise be too faint to see. (NASA, ESA, G. Caminha (University of Groningen), M. Meneghetti (Observatory of Astrophysics and Space Science of Bologna), P. Natarajan (Yale University), the CLASH team, and M. Kornmesser (ESA/Hubble))

Could this be the oldest galaxy in the universe?

Back in the day of GN-z11, galaxies were made of stronger stuff and it just doesn’t understand young whipper-snappers like the Milky Way. Such is the life of older galaxies. In this case, the oldest that we know of. So distant, in fact, it defines the very boundary of the observable universe itself.

The oldest known galaxy is a whopping 13.4 billion light years from Earth (Photo: Pixabay)

Professor Nobunari Kashikawa from the Department of Astronomy at the University of Tokyo sought the most distant galaxy one can observe in order to find out how and when it came to be. In his quest, he and his team were able to more accurately find the distance to the aging galaxy.

While it has been known for a while that GN-z11 was the oldest known galaxy, measuring the distance to it turned out to be quite the challenge.

“From previous studies, the galaxy GN-z11 seems to be the farthest detectable galaxy from us, at 13.4 billion light years, or 134 nonillion kilometers (that’s 134 followed by 30 zeros),” said Kashikawa. “But measuring and verifying such a distance is not an easy task.”

In order to measure the distance, Kashikawa measured the redshift, or how much light has shifted toward the red end of the spectrum as galaxies move away from each other with the universal expansion. The farther away the galaxy is, the more redshift. Using the Keck I telescope, the astronomers were able to get a decent fix on GN-z11.

“We looked at ultraviolet light specifically, as that is the area of the electromagnetic spectrum we expected to find the redshifted chemical signatures,” said Kashikawa. “The Hubble Space Telescope detected the signature multiple times in the spectrum of GN-z11. However, even the Hubble cannot resolve ultraviolet emission lines to the degree we needed. So we turned to a more up-to-date ground-based spectrograph, an instrument to measure emission lines, called MOSFIRE.”

When working with distances at these enormous scales, it just isn’t sensible to use our familiar units of kilometers and miles, or even multiples of them. Instead, astronomers use a value known as the redshift number denoted by z.

Using MOSFIRE (Multi-Object Spectrometer For Infra-Red Exploration), the team captured the emission lines from GN-z11 in detail, which allowed them to make a much better estimation on its distance than was possible from previous data, confirming the galaxy’s ‘farthest’ status.

The study was published in the journal Nature Astronomy on December 14.

Astronomers find ‘fossil galaxy’ buried deep inside the Milky Way

We know so much about long-extinct creatures such as dinosaurs thanks to their fossilized bones which act as time capsules, revealing not only their size and shape but also clues about their diet, behavior or even mating patterns. Similarly, astronomers can learn about the Milky Way’s galactic history billions of years into the past by studying stellar relics.

In a new study, researchers describe a newly discovered ‘fossil galaxy’, which until now had been hidden deep inside our Milky Way.

 The colored rings show the rough extent of the fossil galaxy known as Heracles. Credit:  NASA/JPL-Caltech, and the SDSS.

Large galaxies such as the Milky Way, which contains more than 100 billion stars, take time to accrete matter and grow to their gargantuan sizes. Besides developing their own stars in galactic nurseries, large galaxies also merge with small galaxies over time, which increases their mass.

Over its 12 billion-year-old history, the Milky Way has gone through a dozen such mergers, devouring a neighbor’s stars and mixing them into an ever-growing stew of commandeered suns. With each galactic merger, the shape, size, and motion of our galaxy changed, until it formed its now-iconic spiral.

But it is possible to unwind this spiral and in the process reverse engineer the Milky Way’s previous mergers.

Just a week after Dr. Diederik Kruijssen at the Center for Astronomy at the University of Heidelberg (ZAH) reported newly identified globular clusters that match the properties of a previously unknown collision with what the team has dubbed the ‘Kraken’ galaxy, a new study is now reporting yet another hidden galaxy.

Using data from the Sloan Digital Sky Surveys’ Apache Point Observatory Galactic Evolution Experiment (APOGEE), astronomers have found the remnants of an ancient collision between the Milky Way and an early galaxy. The event, which took place 10 billion years ago when our galaxy was still in its infancy, is responsible for roughly one-third of the Milky Way’s spherical halo.

“To find a fossil galaxy like this one, we had to look at the detailed chemical makeup and motions of tens of thousands of stars,” Ricardo Schiavon from Liverpool John Moores University (LJMU) in the UK said in a press release. “That is especially hard to do for stars in the center of the Milky Way, because they are hidden from view by clouds of interstellar dust. APOGEE lets us pierce through that dust and see deeper into the heart of the Milky Way than ever before.”

The astronomers dubbed the galactic fossil Heracles, after the ancient Greek hero also called Hercules, who was granted the gift of immortality when the Milky Way was created — or so the myth goes.

 The colored rings show the approximate extent of the stars that came from the fossil galaxy known as Heracles. Credit: ESA/Gaia, and the SDSS.

In order to distinguish Heracles’ stars out of all the Milky Ways’ billions of objects, the astronomers had to measure the stars’ chemical composition and velocities using APOGEE. A few hundred of these stars had chemical makeup and velocities that were radically different from all the rest. The only reasonable explanation is that these objects belonged to a galaxy other than the Milky Way.

“These stars are so different that they could only have come from another galaxy. By studying them in detail, we could trace out the precise location and history of this fossil galaxy,” Danny Horta from LJMU, lead author of the new study published in the Monthly Notices of the Royal Astronomical Society, said in a statement.

This is just the tip of the iceberg. In the future, the fifth phase of the Sloan Digital Sky Survey and its “Milky Way Mapper” will measure spectra for ten times as many stars across the entire galaxy. Who knows what cosmic fossils astronomers will be able to discover then.

“As our cosmic home, the Milky Way is already special to us, but this ancient galaxy buried within makes it even more special,” Schiavon says.

And since you must be wondering, the next merger is scheduled to occur 2.5 billion years from now with the neighboring Andromeda Galaxy.

An artist's impression of a gravitational microlensing event by a free-floating planet. (Jan Skowron / Astronomical Observatory, University of Warsaw)

‘Lonely’ Rogue Planet Discovered Wandering the Milky Way

Researchers believe that our galaxy is teeming with cosmic orphans, planets wandering free of a parent star. Though common, these rogue planets are difficult to spot, especially when they are in the size range of the earth. 

Despite this difficulty; an international team of astronomers including Przemek Mróz, a postdoctoral scholar at the California Institute of Technology (Caltech) and Radosław Poleski from the Astronomical Observatory of the University of Warsaw, have spotted what they believe to be a free-floating planet with a size and mass somewhere in the range of Mars and Earth, wandering the Milky Way. 

The discovery represents a major step forward in the field of exoplanet investigation as it is the first earth-sized ‘rogue planet’ ever observed.

An artist's impression of a gravitational microlensing event by a free-floating planet. (Jan Skowron / Astronomical Observatory, University of Warsaw)
An artist’s impression of a gravitational microlensing event by a free-floating planet. (Jan Skowron / Astronomical Observatory, University of Warsaw)

“We found a planet that seems extremely lonely and small, far away in the Universe,” Poleski tells ZME Science. “If you can imagine, Earth is in a sandbox surrounded by lots of other planets, and light from the Sun. This planet isn’t. It’s truly alone.”

The rogue planet the team found — OGLE-2016-BLG-1928 — is believed to be the smallest free-floating planet ever discovered. It was found in data collected by Optical Gravitational Lensing Experiment (OGLE), a Polish astronomical project based at the University of Warsaw. Previously discovered rogues — such as the first-ever recorded free-floating planet also found by OGLE in 2016 — are closer in size to Jupiter.

The gravity of a free-floating planet may deflect and focus light from a distant star when passing close in front of it. Due to the distorted image, the star temporarily seems much brighter. (v)

“We discovered the smallest free-floating planet candidate to date. The planet is likely smaller than Earth, which is consistent with the predictions of planet-formation theories,” Mróz — lead author of the team’s study published in Astrophysical Journal Letters — explains to ZME Science. “Free-floating planets are too faint to be observed directly — we can detect them using gravitational microlensing via their light-bending gravity.”

The Gravity of the Situation

The team spotted this wandering planet using the technique of gravitational microlensing, often utilised to spot exoplanets — planets outside our solar system. Exoplanets can’t often be observed directly, and when they can it’s a result of interaction with radiation from their parent star — for example, the dimming effect exoplanets have when they cross in front of their star and block some of the light it emits. Clearly, as rogue planets don’t have a parent star, they don’t have these interactions, making micro-lensing events the only way of spotting them.

“Microlensing occurs when a lensing object — a free-floating planet or star — passes between an Earth-based observer and a distant source star, its gravity may deflect and focus light from the source,” Mróz explains to ZME Science. “The observer will measure a short brightening of the source star, which we call a gravitational microlensing event.”

When the gravity of a free-floating planet deflects and focuses light from a distant star, we can observe temporary changes in star brightness. (temporary changes in star brightness.
Credit: Jan Skowron / Astronomical Observatory, University of Warsaw.)

Mróz continues by explaining that the duration of microlensing events depends on the mass of the object acting as a gravitational lens. “The less massive the lens, the shorter the microlensing event. Most of the observed events, which typically last several days, are caused by stars,” Mróz says. “Microlensing events attributed to free-floating planets usually last barely a few hours which makes them difficult to spot. We need to very frequently observe the same part of the sky to spot brief brightenings caused by free-floating planets.”

Changes of brightness of the observed star during the gravitational microlensing event by a free-floating planet. (Credit: Jan Skowron / Astronomical Observatory, University of Warsaw/ Robert Lea)

By measuring the duration of a microlensing event and shape of its light curve astronomers can estimate the mass of the lensing object. That is how the team were able to ascertain this free-floating planet is approximately Earth-sized. “Hence, we can discover very dim objects, like black holes, or free-floating planets,” says Poleski. “We found it an event, which has a timescale of 41 minutes. And it’s the shortest event ever discovered.”

Poleski explains that the lack of any other lensing body in the system told the team that it is a very strong candidate for a free-floating planet. He adds: “We know it’s a planet because of the very short timescale and we think it’s free-floating because we don’t see any star next to it.”

Going Rogue. How Free-Floating Planets Come to Wander the Universe Alone

Astronomers believe that free-floating planets actually formed in protoplanetary disks around stars in the same way that ‘ordinary’ planets are. At some point, they are ejected from their parent planetary systems, probably after gravitational interactions with other bodies, for example, with other planets in the system.

“Some low-mass planets are expected to be ejected from their parent planetary systems during the early stages of planetary system formation,” says Mróz. “According to planet formation theories, most of the ejected planets should be smaller than Earth. Theories of planet formation predict that typical masses of ejected planets should be between 0.3 and 1.0 Earth masses. Thus, the properties of this event fit the theoretical expectations.”

These free-floating rogue planets are believed to be fairly common, but researchers can’t be certain because they are so difficult to spot. “Our current studies indicate that the frequency of low-mass–in the Earth to super-Earth-mass range–free-floating or wide-orbit planets is similar to that of stars — there are about two-five such objects per each star in the Milky Way,” says Mróz. “These numbers are very uncertain because they are based on a few sightings of short-timescale microlensing events. However, if free-floating/wide-orbit planets were less frequent than stars, we would have observed much fewer short-timescale events than we do.”

The researcher adds that though these objects are relatively common, the chances of observing microlensing events caused by them are still extremely small. “Three objects — source, lens, and observer — must be nearly perfectly aligned,” Mróz says. “If we observed only one source star, we would have to wait almost a million year to see the source being microlensed.”

In fact, one of the extraordinary elements of the team’s study is that such a short duration lensing event wasn’t believed to be observable given the sensitivity of the current generation of telescopes.

“The surprise, in general, was that with current technology we could define such a short time event,” Poleski says. “It’s especially surprising if you beat the previous record by a factor of few.”

The Nancy Grace Roman Telescope and Future Rogue Reconnaissance

For Mróz, there are still questions that he would like to see answered about OGLE-2016-BLG-1928. Primarily, confirming that it definitely is a free-floating planet. 

“We aren’t fully sure whether our planet is free-floating or not. Our observations rule out the presence of stellar companions within 10 astronomical units–930 million miles–of the planet, but the planet may have a more distant companion,” Mróz says. “Let’s imagine that we’re observing microlensing events by a doppelganger of the Solar System. If Jupiter or Saturn caused a microlensing event, we would see a signature of the Sun in the microlensing event light curve. However, microlensing events by Uranus or Neptune would likely look like those of free-floating planets, because they are very far from the Sun.”

Fortunately, Mróz says that should be possible to distinguish between free-floating and wide-orbit planets. “The lens is moving relative to the source star in the sky and — a few years after the microlensing event — the lens and source should separate in the sky,” the researcher elaborates. “If the lens has a stellar companion, we will see some excess of light at its position. If it is a free-floating planet, we will not.” 

Whilst this method may seem simple, Mróz says we cannot apply it now, because the existing telescopes are not powerful enough. This includes the instrument that conducted the long-term observations that gave rise to the OGLE sky survey–the data from which the team found the micro-lensing event OGLE-2016-BLG-1928.

“[The discovery of OGLE-2016-BLG-1928] was part of the larger search for microlensing events in general, which we perform in a number of steps,” Poleski tells ZME. “In one step, I started looking at the wide orbit planets — planets similar to Uranus, or Neptune and on similar orbits. And while looking for those, I screened a list of candidate microlensing events in general and I found this one.”

Soon NASA’s Nancy Grace Roman Telescope will take over the search for microlensing events, but in the meantime, there is still data from OGLE and other projects to be examined. “We now have more data and other surveys are also collecting data. So we hope to analyze those,” Poleski says. “The longer-term future is the launch of the Nancy Grace Roman Space Telescope. It will be a telescope similar to the Hubble telescope, only with new infrared and infrared cameras and that camera field of view larger than the Hubble Space Telescope. 

“One of the main projects for the Raman telescope will be to observe galactic bulge in search for microlensing planets, including free-floating planets.”

Mroz, P., Poleski, R., Gould, A. et al., ‘A terrestrial-mass rogue planet candidate detected in the shortest-timescale microlensing event,’ Astrophysical Journal Letters, [2020] DOI: 10.3847/2041–8213/abbfad

An artist's impression of a collision between the Milky Way and a smaller dwarf galaxy, such as that which occurred about eight to 10 billion years agoV. Belokurov (Cambridge, UK) based on an image by ESO/Juan Carlos Muñoz)

A dwarf galaxy may have collided with the Milky Way 3 billion years ago

Over twenty years ago astronomers first observed an unusually high density of stars in the vicinity of the Virgo cluster with the Milky Way, but until now the cause of the so-called Virgo Overdensity was unknown. New research suggests that this overdensity was actually caused by a dwarf galaxy plugging into the heart of the Milky Way over 3 billion years ago. But unlike in folklore when a wooden stake plunges through the heart of a vampire, it was this cosmic impaler that was destroyed by the interaction. 

The gravitational influence of the Milky Way ripped the dwarf galaxy apart leaving behind telltale shell-like formations of stars as the only evidence of the violent collision. Evidence that has now been uncovered by astronomers.

Two spiral galaxies (from left) NGC 2207 and IC 2163 colliding with one another. Credit: NASA.

“When we put it together, it was an ‘aha’ moment. This group of stars had a whole bunch of different velocities, which was very strange,” says Heidi Jo Newberg, the Rensselaer Institute professor of physics, applied physics, and astronomy, who led the team that made the discovery. “Now that we see their motion as a whole, we understand why the velocities are different, and why they are moving the way that they are.”

The team’s research is published in the latest edition of The Astrophysical Journal. They detail two shell-like structures in the Virgo Overdensity and a further pair in Hercules Aquila Cloud region. Their findings are based on data provided by the Sloan Digital Sky Survey, the European Space Agency’s Gaia space telescope, and the LAMOST telescope in China.

The Virgo Overdensity: Evidence of a Cosmic Collision

The Virgo Overdensity has, until now, been something of an oddity amongst such clusters. Star surveys have revealed that some of the stars that make up the Virgo Overdensity are moving toward us, whilst others are moving away. This is behaviour that would not normally be seen in a cluster of this kind. In 2019, researchers from the Rensselaer Institute had put forward the idea that this is because the overdensity is a result of a radial — or T-bone — collision.

The shell structures described in this new study — not observed before–seems to confirm this origin for the Virgo Overdensity. These arcs of stars — curved like umbrellas — are believed by the team to be the what remains of the dwarf galaxy after it was pulled apart by the Milky Way’s overpowering gravitational influence. 

an N-body simulation of a radial merger between the Milky Way Galaxy and a dwarf galaxy. Collisions like these create what are known as “shells” in the Galaxy’s halo (Thomas Donlon)

The process caused the dwarf galaxy to ‘bounce’ through the centre of the Milky Way with its stars being gradually incorporated into our galaxy. Each time the dwarf galaxy passed through the Milky Way’s centre the stars would initially move quickly, gradually being slowed by the gravity of our galaxy, until this influence eventually pulls them back. Each time the dwarf galaxy ‘threaded back’ through the centre a new shell was created. 

Counting the number of shells allowed the team to calculate how many cycles the dwarf galaxy has undergone which in turn allows them to estimate how many years it has been since the collision — which they are naming the ‘Virgo Radial Merger’ —  took place. Thus the team dates the first passage of the dwarf galaxy through the centre of the Milky Way at 2.7 billion years ago. 

The Immigrant’s Song

Lead author Newberg believes that the majority of the stars in the Milky Way’s halo — a spherical cloud of stellar bodies that surround our galaxy’s spiral arms — appear to be ‘immigrants’ that formed in smaller galaxies and deposited by collisions different from the radial merger described above. 

The researcher, who specialises in the Milky Way’s stellar halo, says that as dwarf galaxies were absorbed into the Milky Way, ensuing tidal forces pulled their stars into long cords moving in unison through the halo. These are so-called tidal mergers which are both less violent and far more common than radial collisions. 

The fact that radial mergers are uncommon means the team was slightly taken back by the discovery of such evidence in the centre of the Milky Way. It was only as the team began to model the movement of the Virgo Overdensity that the significance of their discovery began to dawn on them. 

The Gaia Sausage–the team’s findings imply this colourfully named cluster of stars was not created in the same event that created the Virgo Overdensity, or that it is much younger than previously believed (ESA/Gaia. this image was prepared by Edmund Serpell, a Gaia Operations Engineer working in the Mission Operations Centre at ESA’s European Space Operations Centre in Darmstadt, Germany. CC by SA 3.0)

“There are other galaxies, typically more spherical galaxies, that have a very pronounced shell structure, so you know that these things happen, but we’ve looked in the Milky Way and hadn’t seen really obvious gigantic shells,” explains Thomas Donlon II, a Rensselaer graduate student and first author of the paper. “And then we realized that it’s the same type of merger that causes these big shells. It just looks different because, for one thing, we’re inside the Milky Way, so we have a different perspective, and also this is a disk galaxy and we don’t have as many examples of shell structures in disk galaxies.”

In addition to pointing towards the radial collision almost 3 billion years ago, the team’s research has potential implications for other stellar phenomena. In particular, the findings indicate that the ‘Gaia Sausage’ — a formation that astronomers believe is the result of a collision with a dwarf galaxy between 8 and 11 billion years ago — was not created by the same event that created the Virgo Overdesity, as scientists had previously believed.

The team’s findings clearly imply that the Virgo Overdensity is much younger than the Gaia Sausage meaning that the two had different origins, or that the colourfully named ‘sausage’ is fresher than previously believed. This would also mean that it could not have caused the thick central disc stars at the centre of the Milky Way.

“There are lots of potential tie-ins to this finding,” concludes Newberg. “The Virgo Radial Merger opens the door to a greater understanding of other phenomena that we see and don’t fully understand, and that could very well have been affected by something having fallen right through the middle of the galaxy less than 3 billion years ago.”

Comet NEOWISE Comes into Focus for a Close-up

Time is running out to catch a glimpse of the comet NEOWISE. The comet — the brightest object to grace the skies over the Northern Hemisphere in 25 years — will soon disappear from view. At least as far as the naked eye is concerned. Fortunately, the Hubble Space Telescope is on hand to capture stunning images of the comet — discovered on March 27th by NASA’s Wide-field Infrared Survey Explorer (WISE) space telescope during its mission to search for near-Earth objects. 

Hubble Captures a stunning close-up Comet NEOWISE (NASA, ESA, Q. Zhang (California Institute of Technology), A. Pagan (STScI))
Hubble Captures a stunning close-up Comet NEOWISE (NASA, ESA, Q. Zhang (California Institute of Technology), A. Pagan (STScI))

The image taken by Hubble on 8th August — which represents the closest ever taken of the comet since it first lit up the sky — shows NEOWISE as it sweeps past the Sun. This is the first time that astronomers have managed to capture such a bright object as it passes our star.

Hubble snapped the object as it rapidly makes its way out of the solar system, with it not scheduled to return for 6,800 years. The comet caused a stir amongst amateur star watchers and the general public as it was visible with the naked eye under the right conditions.

“Nothing captures the imagination better than actually seeing its tails stretching into the sky in person,” Qicheng Zhang,  a graduate student studying planetary science at Caltech, Pasadena, CA, who has been heavily involved in the study of NEOWISE. “The comet last came around about 4,500 years ago. This was around when the Egyptian pyramids were being built.”

“My research area covers comets and their evolution under solar heating,” Zhang explains to ZME Science. “I also like to keep track of potentially bright comets to actually see in the sky, which included this particular comet.”

Colour image of the comet taken by Hubble on 8 August 2020 within the frame of a ground-based image of the comet that was taken from the Northern Hemisphere on 18 July 2020. (NASA, ESA, Q. Zhang (California Institute of Technology), A. Pagan (STScI), and Z. Levay)
Colour image of the comet taken by Hubble on 8 August 2020 within the frame of a ground-based image of the comet that was taken from the Northern Hemisphere on 18 July 2020. (NASA, ESA, Q. Zhang (California Institute of Technology), A. Pagan (STScI), and Z. Levay)

The image shows NEOWISE’s halo of glowing gas and dust illuminated by light from the Sun surrounding the icy nucleus of the comet, too small at little more than 4.8km across to be fully resolved by the telescope. In contrast, the dust halo that surrounds the comet’s heart is too large to be fully resolved by the space telescope, with its diameter measuring an estimated 18,000 km.

Zhang points out that as NEOWISE moves past the Sun, there is a chance we could still glimpse its icy core: “As the comet recedes from the Sun, the dust with clear and reveal the solid nucleus currently buried within, providing an opportunity to directly observe the source of all the activity that made the comet impressive last month.”

Let’s Stick Together: Why NEOWISE Survived and ATLAS Didn’t

Previous attempts to capture other bright comets as they pass the Sun have failed because these objects have disintegrated as they passed too close to the star. This break-up is driven by both the incredible heat of the Sun causing the icy heart of the comets to fragment, and the powerful gravitational influence of our star further pulling the comets apart. 

The most striking example of this came shortly after the discovery of NEOWISE, with the observation of the fragmentation of the comet ATLAS in April this year. The collapse of this comet — believed at the time to offer our best look at such an icy body — in 30 separate pieces was also caught by Hubble. 

ATLAS feels the heat. The comet, discovered in December 2019 breaks up under the intense heat and gravitational influence of the Sun ( NASA/ ESA/ STScI/ D. Jewitt (UCLA))
ATLAS feels the heat. The comet, discovered in December 2019 breaks up under the intense heat and gravitational influence of the Sun ( NASA/ ESA/ STScI/ D. Jewitt (UCLA))

Unlike comet ATLAS, itself only discovered in December 2019, comet NEOWISE somehow survived its close passage to the Sun–with its solid, icy nucleus able to withstand the blistering heat of the star– enabling Hubble to capture the comet in an intact state.

As the latest image of NEOWISE shows, however, it is not going to escape its encounter with the Sun completely unscathed. Jets can clearly be seen blasting out in opposite directions from the poles of the comet’s icy nucleus. These jets represent material being sublimated–turning straight from a solid to a gas skipping a liquid stage–beneath the surface of the comet. This ultimately results in cones of gas and dust erupting from the comet, broadening out as the move away from the main body, forming an almost fan-like shape.

This animation composed of three Hubble images of NEOWISE shows clearly jets of sublimated material erupting from the comet forming fan-like shapes. (NASA, ESA, Q. Zhang (California Institute of Technology), A. Pagan (STScI), and M. Kornmesser))

Far from being just a stunning image of a comet as it passes through the inner solar system, the Hubble images stand to teach astronomers much about NEOWISE and about comets in general.

“It’s a fairly large comet that approached closer to the Sun than the vast majority of comets of its size do,” Zhangs says. “These factors contributed to its high brightness and also made it a good candidate to see how solar heating alters comets, as the effects are theoretically amplified by its close approach to the Sun.

“That information is useful for interpreting observed characteristics of other comets that don’t approach as close to the Sun, and thus where the changes are more gradual and might not be directly observable.”

In particular, the colour of the comet’s dust halo, and the way it changes as NEOWISE moves away from the Sun, gives researchers a hint as to the effect of heat on such materials. This could, in-turn, help better determine the properties of the dust and gas that form what is known as the ‘coma’ around a comet.

“We took images to show the colour and polarization of the dust released by the comet, to get a sense of what it looks like before it’s broken down by sunlight,” says Zhang. “That analysis is ongoing–and will take a while to do properly–but as the published images show, we’ve caught at least a couple of jets carrying dust out from the rotating nucleus.”

The information contained in the Hubble data will become clearer as researchers delve deeper into it. But, the investigation of NEOWISE’s cometary counterparts will benefit from future telescope technological breakthroughs. This will include spotting comets much more quickly and thus, further out from the Sun.

“When this comet was discovered by the NEOWISE mission, it was only 3 months from its close approach to the Sun and had already begun ramping up activity,” Zhang says. “More sensitive surveys, like the upcoming Legacy Survey of Space and Time (LSST) at the Rubin Observatory, will allow us to find such comets much earlier before they become active, enabling us to track them throughout their apparition from beginning to end.

“This will facilitate a more precise comparison of what changes the comets undergo during their solar encounter.”

The next step in Zhang’s research, however, will be comparing the qualities of comet NEOWISE to other such objects, particularly a recent interstellar visitor to our solar system: “This is one of three comets I have observed or have planned to observe in this manner, the others being the interstellar comet 2I/Borisov and the distant solar system comet C/2017 K2 (PANSTARRS),” the researcher concludes. “My team of collaborators and I will be evaluating all three comets to see how their differences in present location and formation/dynamical history translate into differences in physical properties.”

This artist's conception illustrates a Jupiter-like planet alone in the dark of space, floating freely without a parent star. (NASA)

Rogue Planets Could Outnumber Stars in the Milky Way

Our galaxy is teeming with rogue planets either torn from their parent stars by chaotic conditions or born separate from a star. These orphan planets could be discovered en masse by an outcoming NASA project — Nancy Grace Roman Space Telescope. 

The Milky Way is home to a multitude of lonely drifting objects, galactic orphans — with a mass similar to that of a planet — separated from a parent star. These nomad planets freely drift through galaxies alone, thus challenging the commonly accepted image of planets orbiting a parent star. ‘Rogue planets’ could, in fact, outnumber stars in our galaxy, a new study published in the Astronomical Journal indicates. 

“Think about how crazy it is that there could be an Earth, a Mars, or a Jupiter floating all alone through the galaxy. You would have a perfect view of the night sky but stuck in an eternal night,” lead author of the study, Samson Johnson, an astronomy graduate student at The Ohio State University, tells ZME Science. “Although these planets could not host life, it is quite a place to travel to with your imagination. The possibility of rogue planets in our galaxy had not occurred to me until coming to Ohio State.”

This artist’s conception illustrates a Jupiter-like planet alone in the dark of space, floating freely without a parent star. (NASA)

Up to now, very few very of these orphan planets have actually been spotted by astronomers, but the authors’ simulations suggest that with the upcoming launch of NASA’s Nancy Grace Roman Space Telescope in the mid-2020s, this situation could change. Maybe, drastically so.

“We performed simulations of the upcoming Nancy Grace Roman Space Telescope (Roman) Galactic Exoplanet Survey to determine how sensitive it is to microlensing events caused by rogue planets,” Johnson says. “Roman will be good at detecting microlensing events from any type of ‘lens’ — whether it be a star or something else — because it has a large field of view and a high observational cadence.”

The team’s simulations showed that Roman could spot hundreds of these mysterious rogue planets, in the process, helping researchers identify how they came to wander the galaxy alone and indicating how great this population could be in the wider Universe.

Rogue by Name, Rogue by Nature: Mysterious and Missing

Thus far, much mystery surrounds the process that sees these planets freed from orbit around a star. The main two competing theories suggest that these stars either are thrown free of their parent star, or form in isolation. Each process would likely lead to rogue planets with radically different qualities. 

ESO’s New Technology Telescope at the La Silla Observatory captures the rogue planet CFBDSIR J214947.2-040308.9 in infrared light–appearing as a faint blue dot near the centre of the picture. This is the closest such object to the Solar System discovered thus far. (ESO/P. Delorme)

“The first idea suggests that rogue planets form like planets in the Solar System, condensing from the protoplanetary disk that accompanies stars when they are born,” Johnson explains. “But as the evolution of planetary systems can be chaotic and messy, members can be ejected from the system leading to most likely rogue planets with masses similar to Mars or Earth.”

 Johnson goes on to offer an alternative method of rogue planet formation that would see them form in isolation, similar to stars that form from giant collapsing gas clouds. “This formation process would likely produce objects with masses similar to Jupiter, roughly a few hundred times that of the Earth.”

“This likely can’t produce very low-mass planets — similar to the mass of the Earth. These almost certainly formed via the former process,” adds co-author Scott Gaudi, a professor of astronomy and distinguished university scholar at Ohio State. “The universe could be teeming with rogue planets and we wouldn’t even know it.”

The question is if these objects are so common, why have we spotted so few of them? “The difficulty with detecting rogue planets is that they emit essentially no light,” Gaudi explains. “Since detecting light from an object is the main tool astronomers use to find objects, rogue planets have been elusive.”

Astronomers can use a method called gravitational microlensing to spot rogue planets, but this method isn’t without its challenges, as Gaudi elucidates:

“Microlensing events are both unpredictable and exceedingly rare, and so one must monitor hundreds of millions of stars nearly continuously to detect these events,” the researcher tells ZME Science. “This requires looking at very dense stellar fields, such as those near the centre of our galaxy. It also requires a relatively large field of view.”

Additionally, as the centre of the Milky Way is highly obscured by requiring us to look at it in the near-infrared region of the electromagnetic spectrum — a task that is extremely difficult as the Earth’s atmosphere makes the sky extremely bright in near-infrared light.

“All of these points argue for a space-based, high angular resolution, wide-field, near-infrared telescope,” says Gaudi. “That’s where Roman — formally the Wide Field InfraRed Survey Telescope (WFIRST) — comes in.” 

Nancy Grace Roman Space Telescope (and Einstein) to the Rescue!

The Roman telescope — named after Nancy Grace Roman, NASA’s first chief astronomer, who paved the way for space telescopes focused on the broader universe–will launch in the mid-2020s. It is set to become the first telescope that will attempt a census of rogue planets — focusing on planets in the Milky Way, between our sun and the centre of our galaxy, thus, covering some 24,000 light-years.

An artist’s redition of the Nancy Grace Roman Space Telescope, named after ‘the Mother of Hubble’ (NASA)

The team’s study consisted of simulations created to discover just how sensitive the Roman telescope could be to the microlensing events that indicate the presence of rogue planets, finding in the process, that the next generation space telescope was 10 times as sensitive as current Earth-based telescopes. This difference in sensitivity came as a surprise to the researchers themselves. “Determining just how sensitive Roman is was a real shock,” Johnson says. “It might even be able to tell us about moons that are ejected from planetary systems! We also, found a new ‘microlensing degeneracy’ in the process of the study — the subject another paper that will be coming out shortly.”

Johnson’s co-author Gaudi echoes this surprise. “I was surprised that Roman was sensitive to rogue planets with mass as low as that of Mars and that the signals were so strong,” the researcher adds. “I did not expect that before we started the simulations.”

The phenomenon that Roman will exploit to make its observations stems from a prediction made in Einstein’s theory of general relativity, that suggests that objects with mass ‘warp’ the fabric of space around them. The most common analogy used to explain this phenomenon is ‘dents’ created in a stretched rubber sheet by placing objects of varying mass upon it. The heavier the object — thus the greater the mass — the larger the dent. 

This warping of space isn’t just responsible for the orbits of planets, it also curves the paths of light rays, the straight paths curving as they pass the ‘dents’ in space. This means that light from a background source is bent by the effect of the mass of a foreground object. The effect has recently been used to spot a distant Milky Way ‘look alike’. But in that case, and in the case of many gravitational lensing events, the intervening object was a galaxy, not a rogue planet, and thus was a much less subtle, more long-lasting, and thus less hard to detect effect than ‘microlensing’ caused by a rogue planet. 

This animation shows how gravitational microlensing can reveal island worlds. When an unseen rogue planet passes in front of a more distant star from our vantage point, light from the star bends as it passes through the warped space-time around the planet. The planet acts as a cosmic magnifying glass, amplifying the brightness of the background star. (NASA’s Goddard Space Flight Center/CI Lab)

“Essentially, a microlensing event happens when a foreground object — in this case, a rogue planet — comes into very close alignment with a background star. The gravity of the foreground object focuses light from the background star, causing it to be magnified,” Gaudi says. “The magnification increases as the foreground object comes into alignment with the background star, and then decreases as the foreground object moves away from the background star.”

As Johnson points out, microlensing is an important and exciting way to study exoplanets — planets outside the solar system — but when coupled with Roman, it becomes key to spotting planetary orphans.

“Roman really is our best bet to find these objects. The next best thing would be Roman 2.0 — with a larger field of view and higher cadence,” the researcher tells ZME, stating that rogue planets are just part of the bigger picture that this forthcoming space-based telescope could allow us to see. “I’m hoping to do as much work with Roman as possible. The next big project is determining what Roman will be able to teach us about the frequency of Earth-analogs — Earth-mass planets in the habitable zones of Sun-like stars.”

Original Research

Johnson. S. A., Penny. M, Gaudi. B. S, et al, ‘Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. II. Free-floating Planet Detection Rates*,’ The Astronomical Journal, [2020]. 

The Milky Way’s halo might be thousands of times hotter than the sun’s surface

Artist’s illustration of the halo of hot gas surrounding the Milky Way galaxy (Credit: NASA)

Our home galaxy, the Milky Way, contains a halo that consists of a hazy fog of dust, gas, and dark matter. Scientists already believe the enormous halo to measure at least 300,000 light-years across (as a reference point, the Milky Way itself reaches 100,000 light-years across space).

Now, a new study out of Ohio State University (OSU) suggests that the extreme temperatures the researchers found in a previous OSU analysis — up to 10 million degrees Kelvin, or about 18 million degrees Fahrenheit — could possibly be found throughout the whole halo. Prior it was believed that only certain parts of the halo could reach the high temperature.

“We can’t say for sure that it is everywhere, because we have not analyzed the entire halo,” said Smita Mathur, professor of astronomy at OSU. “But we know now that the temperatures we saw in the first study definitely are not unique, and that is very exciting.”

A recent April study out of the University of California, Irvine found that the Milky Way could be flinging stars into the outer halo. The movements are believed to be triggered by powerful supernova explosions. Supernovas occur when stars explode and lose most of their mass.

Using “hyper-realistic” computer simulations from the Feedback in Realistic Environments 2 project (FIRE 2), the scientists modeled the disruptions in galactic rotations that would otherwise be considered orderly.

“These highly accurate numerical simulations have shown us that it’s likely the Milky Way has been launching stars in circumgalactic space in outflows triggered by supernova explosions,” study author James Bullock, dean of the university’s School of Physical Sciences and a professor of physics and astronomy. “It’s fascinating, because when multiple big stars die, the resulting energy can expel gas from the galaxy, which in turn cools, causing new stars to be born.”

The data from the Ohio State report came from an X-ray observatory telescope run by the European Space Agency. That telescope, called XMM-Newton, collects data in X-rays that would have otherwise been blocked by Earth’s atmosphere.

“It showed us that the halo was much hotter than we had known, but it didn’t show us whether that was the case throughout the galaxy, or if the telescope had picked up an aberration caused by an unknown force coming from the direction where the telescope was pointed,” Mathur said.

Anjali Gupta, a visiting astronomy researcher at OSU, analyzed data from Suzaku, a Japanese X-ray satellite telescope, which collected spectrum data from the Milky Way’s halo in four different directions. That analysis confirmed their earlier findings — that the halo is much hotter than had previously been known.

However, it still remains to be seen if the same conditions exist for halos surrounding those galaxies far, far, away.

The new findings were presented at the annual meeting of the American Astronomical Society, held online this week because of the COVID-19 pandemic.

ESO astronomers find the closest black hole to Earth

This artist’s impression shows the orbits of the objects in the HR 6819 triple system. Credit: ESO.

Researchers affiliated with the European Southern Observatory (ESO) have detected an invisible object orbiting a double-star system. Upon closer inspection, the object was revealed to be a black hole. Lying just 1,000 light-years away from Earth, this makes it the closest black hole found thus far. In fact, the stellar system that the black hole calls home can be seen with the naked eye.

The first stellar system with a black hole that can be seen with the unaided eye

Originally, the astronomers were observing HR 6819 as part of a study of double-star systems. However, after many months of observations, the researchers noticed that one of the two visible stars orbits an invisible object every 40 days, while the second star is at a relatively large distance from this object. This invisible object turned out to be a black hole, making HR 6819 a triple system.

The observation was performed using the FEROS spectrograph on the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile.

“The dullest object one can imagine”

“Because it is not being “fed” by its companion star, it does nothing, except for orbiting its companion star.  It is the dullest object one can imagine, just the dead remains of a once-powerful star.  However, that is exactly what makes this boring individual exciting because there should be many more awaiting their discovery,” Dietrich Baade, Emeritus Astronomer at ESO in Garching and co-author of the study, told ZME Science.

But this is just the tip of the iceberg. The astronomers believe that silent black holes are actually extremely abundant in the universe. Already, the researchers believe that their findings apply to another system, called LB-1, which may also be a triple system.

“Perhaps, the biggest challenge was to keep going with the observations and their analysis until our friend Stan Štefl could determine the orbital period of 40 days. We are still sad that he died in a car accident in 2014 and cannot receive his share in the recognition of our work. Already in 2010, he laid the foundation to the ‘Aha!’ we experienced at the end of 2019 when we realized that the then newly published observations of the system LB-1 were hard to distinguish from ours of HR 6819.  This prompted us to resume our work on HR 6819 which confirmed the conclusion reached back in 2010 that this system consists of 2 luminous stars and one invisible object.  LB-1 has the same architecture (contrary to the discovery announcement, which portrayed LB-1 as a binary star).  Our more in-depth analysis now demonstrates clearly the black-hole nature of the unseen object in HR 6819,” Baade said.

Unlike most black holes identified thus far, the one lurking in the HR 6819 system does not interact too much with its environment. Since it doesn’t engulf a lot of matter and energy, it doesn’t draw attention, appearing truly ‘black, which added to the challenge of discovery.

“Not only did we find one of the first 2 or 3 black holes (BHs; perhaps even the very first one) that do not shine brightly in X-rays (because they are not fed with gas from a luminous companion star) but it is also the most nearby of all types of BHs.  The latter implies that such BHs must be very common because the solar system is not at a place with special properties,” Baade told ZME Science.

Baade and colleagues believe that there may be many more black holes that are just as inconspicuous as the one lurking in the HR 6819 system.

Previous work that modeled stellar populations suggests that there should be 100 million to one billion stellar-sized black holes in the Milky Way. However, no more than two dozen black holes have been found thus far in our galaxy.

The astronomers believe that this low black hole count is due to the fact that most of them likely do not emit bright X-ray light. There may be hundreds of millions of X-ray-quiet black holes silently populating the Milky Way, according to Baade.

“Since isolated BHs are next to impossible to detect in significant numbers, multiple stars in which at least one of the luminous stars is forced into an orbit about the BH, are the best way forward towards filling in the giant gap between observations and models,” he added.

In addition to offering clues about the behavior of X-ray-quiet black holes, the findings might also explain the occurrence of certain cosmic mergers that trigger gravitational waves powerful enough to be detected from Earth. Distant outer objects, such as the distant star in the HR 6819 triple system, can gravitationally influence the merge of the inner pair (innermost star and black hole), astronomers suggest.

“We shall try to obtain more observations to find and characterize more systems so that their commonalities can identify the patterns of their formation and evolution.  In addition, we shall investigate published reports about other systems for possible alternative interpretations (cf. LB-1).  In this way, our work will contribute to further complementing our understanding of the evolution of single stars, multiple stars, and the Milky Way.  This also includes the history and the future of our Sun and the solar system,” Baade concluded.

The findings appeared today in the journal Astronomy & Astrophysics.

Runaway star ejected from the centre of the Milky Way at incredible speed

An artist’s impression of S5-HVS1’s ejection by Sagittarius A*, the black hole at the centre of the galaxy. The black hole and the captured binary partner to S5-HVS1 are seen far away in the left corner of the picture, while S5-HVS1 is in the foreground, speeding away from them. ( James Josephides (Swinburne Astronomy Productions))

Astronomers have discovered a star travelling at an incredible 6 million km/h — ten times faster than the average star — after being ejected by the supermassive black hole at the centre of the Milky Way five million years ago.

Carnegie Mellon University Assistant Professor of Physics Sergey Koposov discovered the star — named S5-HVS1 — as part of the Southern Stellar Stream Spectroscopic Survey (S5).

“The velocity of the discovered star is so high that it will inevitably leave the galaxy and never return,” said Douglas Boubert from the University of Oxford, a co-author of the study.

An artist’s impression of the ejection mechanism of a star by a supermassive black hole. Credit: James Josephides (Swinburne Astronomy Productions)

S5-HVS1 — located in the constellation of Grus — is part of a population of objects known as ‘high-velocity stars’ (HVSs). These stars sparked curiosity amongst astronomers after the first example was discovered in 2005. In the next 14 years, many more examples of HVSs have been uncovered.

But, even amongst these aptly-named stars, S5-HVS1 is exceptional for its high speed. The star’s close passage to Earth at a mere (in astronomical terms) 2.9 x 10⁴ light-years away, also makes it somewhat unique.

Armed with information about the runaway star’s blazing speed coupled with its close proximity has allowed astronomers to track its trajectory back to the centre of the Milky Way and the supermassive black hole — Sagittarius A* (Sgr A*) — which dwells there.

“This is super exciting, as we have long suspected that black holes can eject stars with very high velocities. However, we never had an unambiguous association of such a fast star with the galactic centre,” says Koposov, the lead author of this work and member of Carnegie Mellon’s McWilliams Center for Cosmology. “We think the black hole ejected the star with a speed of thousands of kilometres per second about five million years ago.

“This ejection happened at the time when humanity’s ancestors were just learning to walk on two feet.”

A bad break-up?

So how on Earth did S5-HVS1 come to be travelling at such an extraordinary speed?

Astronomers believe that the star was once part of a binary system with a companion star. It was ejected from this partnership after both stars’ orbits strayed too close to Sgr A*. Whilst its partner was captured by the incredible gravitational attraction of the supermassive black hole, the gravitational struggle tore S5-HVS1 free and launched it on its rapid journey.

The location of S5-HVS1 on the sky and the direction of its motion. The star is flying away from the galactic centre, from which it was ejected 5 million years ago. ( Sergey Koposov)

This process is known as the ‘Hills mechanism’ and was first suggested by astronomer Jack Hills thirty years ago and has long been considered as a likely mechanism for the origins of high-velocity stars.

“This is the first clear demonstration of the Hills Mechanism in action,” points out Ting Li from Carnegie Observatories and Princeton University, and leader of the S5 Collaboration. “Seeing this star is really amazing as we know it must have formed in the galactic centre, a place very different from our local environment.

“It is a visitor from a strange land.”

An exceptional observation

The astronomers made the discovery of S5-HVS1 was made with 3.9-metre Anglo-Australian Telescope (AAT) near Coonabarabran, NSW, Australia. The team was only able to assess the true speed of the star and details of its incredible journey when these observations were coupled with further data from the European Space Agency’s Gaia satellite.

“The observations would not be possible without the unique capabilities of the 2dF instrument on the AAT,” adds Daniel Zucker, an astronomer at Macquarie University in Sydney, Australia, and a member of the S5 executive committee. “It’s been conducting cutting-edge research for over two decades and still is the best facility in the world for our project.”

The team’s results are published in the journal Monthly Notices of the Royal Astronomical Society.

“I am so excited this fast-moving star was discovered by S5,” says Kyler Kuehn, at Lowell Observatory and a member of the S5 executive committee. “While the main science goal of S5 is to probe the stellar streams — disrupting dwarf galaxies and globular clusters — we dedicated spare resources of the instrument to searching for interesting targets in the Milky Way, and voila, we found something amazing for ‘free.’

“With our future observations, hopefully, we will find even more!”

Original research: https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stz3081/5612212

Credit: Skowron, Science 2019.

Another study confirms the Milky Way is warped and twisted rather than a flat disc

In February, a study challenged the long-standing idea that the Milky Way is shaped like a flat disc. Instead, researchers found that our galaxy’s outer edge is warped and twisted. Now, a second study has new evidence to back up this claim.

Credit: Skowron, Science 2019.

Credit: Skowron, Science.

It’s quite difficult to determine the shape of a galactic structure you’re a part of. Imagine being a really clever cell part of the human body — you could determine the shape of outside creatures like cats, dogs, or other people, but the shape of your own body is much more difficult as you don’t have a good vantage point. Luckily, there are some tricks that scientists can use. Just like the original research that investigated the Milky Way’s shape, researchers at the University of Warsaw in Poland also employed Cepheid variable stars as markers to identify the edges of the galaxy.

Cepheids are hot and massive stars that have five to twenty times the mass of our sun and up to 100,000 times as bright. They pulsate radially for days to months at a time — and this period of pulsation can be combined with the Cepheid’s brightness to reliably establish its distance from the sun.

Because they are so bright, Cepheids can be clearly seen millions of light-years away and can be easily distinguished from other bright stars in their vicinity. This makes them indispensable tools in astronomers’ kit. For instance, it’s thanks to Cepheids that Edwin Hubble and Milton L. Humason were able to prove that the Universe is in a state of expansion.

Credit: Science, 2019.

Credit: Science, 2019.

In their new study, astronomers at the University of Warsaw determined the position of 2,431 Cepheids found throughout the Milky Way via the Optical Gravitational Lensing Experiment (OGLE), a project that tracks the brightness of billions of stellar objects. In addition to OGLE data, the authors also tapped into other variable star catalogues such as the General Catalogue of Variable Stars (GCVS), the All Sky Automated Survey (ASAS), the All-Sky Automated Survey for Supernovae (ASAS-SN), the Asteroid Terrestrial-Impact Last Alert System (ATLAS), and Gaia Data Release 2 (Gaia DR2).

All these thousands of Cepheids act like 3-D beacons, which allowed the researchers to reconstruct the precise shape of our galaxy — a spiral galaxy which is a little warped and twisted around the edges. What’s more, this warped edge contains many young stars, which is quite unusual.

“Our map shows the Milky Way disk is not flat. It is warped and twisted far away from the galactic center,” said co-author Przemek Mroz. “This is the first time we can use individual objects to show this in three dimensions,” some, he said, “as distant as the expected boundary of the Galactic disk.”

What caused this curvature is yet unknown although there are some possible explanations, including a near-collision with another galaxy and dark matter. More studies are needed to shed light on the issue.

The findings appeared in the journal Science.

Einstein’s General Relativity passes the test at the centre of our Galaxy

Measurements of a star passing close to the supermassive black hole at the centre of the Milky Way confirms the predictions of Einstein’s theory of general relativity in a high gravity environment.

An artist visualization of the star S0–2 as it passes by the supermassive black hole at the Galactic centre. As the star gets closer to the supermassive black hole, light it emits experiences a gravitational redshift that is predicted by Einstein's General Relativity. By observing this redshift, we can test Einstein's theory of gravity (Nicole R. Fuller, National Science Foundation)

An artist visualization of the star S0–2 as it passes by the supermassive black hole at the Galactic centre. As the star gets closer to the supermassive black hole, light it emits experiences a gravitational redshift that is predicted by Einstein’s General Relativity. By observing this redshift, we can test Einstein’s theory of gravity (Nicole R. Fuller, National Science Foundation)

A detailed study of a star orbiting the supermassive black hole at the centre of our Galaxy, reveals that Einstein’s theory of general relativity is accurate in its description of the behaviour of light struggling to escape the gravity around this massive space-time event.

The analysis — conducted by Tuan Do, Andrea Ghez and colleagues — involved detecting the gravitational redshift in the light emitted by a star closely orbiting the supermassive black hole known as Sagittarius A*. The redshift was measured as the star reached the closest point in its orbit — which has a duration of 16 years — to the black hole.

Lasers from the two Keck Telescopes propagated in the direction of the Galactic centre. Each laser creates an artificial star that can be used to correct for the blurring due to the Earth’s atmosphere. (Ethan Tweedie) 

Lasers from the two Keck Telescopes propagated in the direction of the Galactic centre. Each laser creates an artificial star that can be used to correct for the blurring due to the Earth’s atmosphere. (Ethan Tweedie)

The team found that the star experienced gravitational redshift — which occurs when light is stretched to longer wavelengths and towards the red ‘end’ of the electromagnetic spectrum by the effect of gravity — as it gets closer to the black hole,  conforming to Einstein’s theory of general relativity and its predictions regarding gravity.

At the same time, the results defy predictions made by the Newtonian theory, which has no explanation for gravitational redshift.

Ghez says: “(The findings are) a transformational change in our understanding about not only the existence of supermassive black holes but the physics and astrophysics of black holes.”

The major difference between general relativity and the Newtonian calculation of gravity is, that whereas Newton envisioned gravity as a force acting between physical objects, Einstein’s theory saw gravity as a geometric phenomenon.

The presence of mass ‘curves’ space it occupies. Physical objects, including light, must then follow this curvature. As John Wheeler infamously put it: “matter tells space how to curve, space tells matter how to move.”

Testing relativity in regions of high gravity

Image of the orbits of stars around the supermassive black hole at the centre of our galaxy. Highlighted is the orbit of the star S0–2. This is the first star that has enough measurements to test Einstein’s General Relativity around a supermassive black hole. [Credit: Keck/UCLA Galactic Center Group]

Image of the orbits of stars around the supermassive black hole at the centre of our galaxy. Highlighted is the orbit of the star S0–2. This is the first star that has enough measurements to test Einstein’s General Relativity around a supermassive black hole. [Credit: Keck/UCLA Galactic Center Group]

The new research resembles an analysis conducted last year by the GRAVITY collaboration, except in this new expanded analysis, the team report novel spectra data.

Although general relativity has been thoroughly tested in relatively weak gravitational fields — such as those on Earth and in the Solar System—before last year, it had not been tested around a black hole as big as the one at the centre of the Milky Way.

Observations of the stars rapidly orbiting Sagittarius A *provide a method for general relativity to be evaluated in an extreme gravitational environment.

Do explains why these kind of tests are important:

“We need to test GR in extreme environments because that’s where we think the theory might break down.”

“If we can see which predictions from general relativity have deviations, that gives us clues as to how to build a better model of gravity.”

A figure showing the challenges the Ghez team had in processing decades of image data and spectroscopy input to follow the star S0–2. (Zina Deretsky, National Science Foundation)

A figure showing the challenges the Ghez team had in processing decades of image data and spectroscopy input to follow the star S0–2. (Zina Deretsky, National Science Foundation)

To obtain their results, the team analyzed new observations of the star S0–2 as it made its closest approach to the enormous black hole in 2018. They then combined this data with measurements Ghez and her team have made over the last 24 years.

The team has many avenues of investigations available to them from here, Tuan tells me.

He continues: “Two of them I’m excited about are testing space-time around the black hole by looking at the orbit of the star S0–2.”

“GR predicts that the orbit should precess, or rotate, meaning that it won’t come back where it started.”

The team should also be able to start using more stars other than S0–2 for these tests as the time baseline of observations increase and technology improves

Do concludes: “ These measurements open a new era of GR tests at the Galactic centre so it’s very exciting.”

This research appears in the 26 July 2019 issue of Science.

Astronomers map the Local Void – the huge “nothingness” surrounding the Milky Way

The local void — a vast cosmic structure surrounding our Milky Way galaxy — has been mapped in a new study, suggesting why our galaxy doesn’t travel with the expansion of the universe.

The large-scale structure of the universe is a tapestry of congregations of galaxies and vast voids. Applying same tools from an earlier study, Brent Tully from the University of Hawaii and his international team of astronomers have been able to map the size and shape of an extensive empty region they called the Local Void that borders the Milky Way galaxy.

Shaded grey contours outline the extent of the Local Void, while blue dots show major mass constituents (large galaxies, galaxy groups, and clusters). The curved blue lines show the derived motions of these massive objects, after removing the overall expansion of the universe. The most important galaxy congregations are given special symbols, like the red ball identifying the Virgo Cluster. The dominant pattern of motions revealed by the orbits is a flow away from the Local Void. (University of Hawaii)

Shaded grey contours outline the extent of the Local Void, while blue dots show major mass constituents (large galaxies, galaxy groups, and clusters). The curved blue lines show the derived motions of these massive objects, after removing the overall expansion of the universe. The most important galaxy congregations are given special symbols, like the red ball identifying the Virgo Cluster. The dominant pattern of motions revealed by the orbits is a flow away from the Local Void. Credits: University of Hawaii.

Tully and his team were able to measure the motions of 18,000 galaxies in the Cosmicflows-3 compendium of galaxy distances. This allowed them to build a 3D cosmographic map that highlighting the boundary between the collection of matter and the absence of matter. This boundary defining the edge of the Local Void.

The team used the same technique to identify the full extent of our home supercluster of over one hundred thousand galaxies in 2014, giving it the name Laniakea — or “immense heaven” in Hawaiian.

For 30 years, astronomers have been trying to identify why the motions of the Milky Way, our nearest large galaxy neighbour Andromeda, and their smaller neighbours deviate from the overall expansion of the Universe by over 600 km/s (1.3 million mph).

This new study — published in The Astrophysical Journal — shows that roughly half of this motion is generated “locally” from the combination of a pull from the massive nearby Virgo Cluster and our participation in the expansion of the Local Void as it becomes ever emptier.

Studying the local void

Galaxies not only move with the overall expansion of the universe — or the Hubble Flow — but they also respond to the gravitational influence of their neighbours and regions with an abundance of mass.

A smoothed rendition of the structure surrounding the Local Void. Our Milky Way galaxy lies at the origin of the red-green-blue orientation arrows (each 200 million lightyears in length). We are at a boundary between a large, low-density void, and the high-density Virgo cluster. (University of Hawaii)

A smoothed rendition of the structure surrounding the Local Void. Our Milky Way galaxy lies at the origin of the red-green-blue orientation arrows (each 200 million lightyears in length). We are at a boundary between a large, low-density void, and the high-density Virgo cluster. (University of Hawaii).

 As a consequence, relative to the Hubble Flow they are moving towards the densest areas and away from regions with little mass — the voids.

The knowledge that our Milky Way galaxy is at the edge of an extensive empty region that they called the Local Void dates back to research spearheaded by Tully and Richard Fisher in 1987.

Despite the fact that the existence of the Local Void has been widely accepted, it has remained poorly studied until now, because it lies behind the centre of our galaxy. This means it heavily obscured from our view by gas and dust lying in the galaxy’s equatorial plane.

In addition to a video showing the simulations the team created, the astronomers have also provided the public with a resource that enables them to manipulate their view of the local void.

You can find that program here.

Credit: Pixabay.

The Milky Way once devoured a dwarf galaxy 10 billion years ago

Credit: Pixabay.

Credit: Pixabay.

Four billion years from now, the Milky Way and Andromeda galaxies will collide in an epic clash of the titans that will light the sky in other worlds that are far away enough from the mayhem. This wouldn’t be the first time our galaxy is involved in a galactic merger. According to astronomers at the Instituto de Astrofisica de Canarias (IAC), about 10 billion years ago the Milky Way devoured a dwarf galaxy called Gaia-Enceladus. The remnants of the dwarf galaxy are believed to now form the Milky Way’s famous halo.

Big fish get bigger

Astronomers used to believe that the Milky Way is formed of two separate sets of stars. The disparities between some stars eventually turned out to reflect a much more complex story. Spanish researchers at IAC used the Gaia space telescope to measure the position, brightness, and distance of roughly one million stars.  The research team also analyzed the density of metals found in stars, which revealed the disparities between the two sets — one “bluer” containing less metal, one “redder” containing more.

Remarkably, the researchers found that both sets of stars are about the same age only that the “blue” one was set into a “chaotic motion” — a telltale sign of a violent galactic collision.

“The novelty of our work is that we have been able to assign precise ages to the stars that belong to the galaxies that merged and, by knowing these ages, when the merger took place,” Carme Gallart, lead author of the study published in Nature Astronomy, said in a statement.

The Milky Way is made of at least 100 billion stars, held together by the immense gravity of a supermassive black hole lying at its center, called Sagittarius A*. The astronomers say that the collision between proto-Milky Way and Gaia-Enceladus promoted star formation for four billion years, after which gas from these formations settled into the Milky Way’s thin disk while remnants of the dwarf galaxy formed our galaxy’s halo.

It’s quite impressive that scientists were able to identify the very first stars that were part of the early Milky Way and how our galaxy was modified by this merger. Studies such as this will not only inform scientists how the Milky Way formed but also how galaxies evolve in general.

Artist impression of a warped Milky Way. Credit: CHEN Xiaodian.

The Milky Way’s outer disk may actually be warped and twisted

For thousands of years, humans thought that Earth was at the center of the universe. It was only in recent centuries that it became an established scientific fact that Earth orbits the sun, and later that the solar system — along with hundreds of billions of other stars — orbits a common galactic center. Until very recently, it has been very difficult for scientists to visualize what the Milky Way looks like given that we are embedded inside it.

Artist impression of a warped Milky Way. Credit: CHEN Xiaodian.

Artist impression of a warped Milky Way. Credit: CHEN Xiaodian.

Thanks to observation, reconstruction, and comparison to other galaxies, researchers have a fairly accurate idea of what our galaxy looks like. If you were to travel outside the galaxy and look down upon it from above, what you’d see is a barred spiral galaxy with two spiral arms called Scutum–Centaurus and Carina–Sagittarius. But, the spiral disk is anything but stable, new research from China shows. The new study found that the farther away you travel from the galactic core, this disk becomes increasingly warped and twisted.

The Milky Way’s S-like appearance at its edges is due to the fact that gravity becomes weaker the farther you are from the galaxy’s inner regions. Since hydrogen atoms in the far outer disk are no longer confined to a thin plane, they get warped.

There were many challenges in this study. One of them is establishing distances from the sun to the Milky Way’s outer disk when you don’t know what the disk actually looks like yet. The research team at the National Astronomical Observatories of Chinese Academy of Sciences (NAOC), led by Chen Xiaodian, had to employ a new catalog of variable stars known as classical Cepheids. Such stars are hot and massive – five to twenty times the mass of our sun and up to 100,000 times as bright. They also pulsate radially for days to months at a time — and this period of pulsation can be combined with the Cepheid’s brightness to reliably establish its distance from the sun.

A 3D distribution of the classical Cepheids in the Milky Way's warped disk. Credit: CHEN Xiaodian.

A 3D distribution of the classical Cepheids in the Milky Way’s warped disk. Credit: CHEN Xiaodian.

Because they are so bright, Cepheids can be clearly seen millions of light years away and can be easily distinguished from other bright stars in their vicinity, making them indispensable tools in any astronomers’ kit. For instance, it’s thanks to Cepheids that Edwin Hubble and Milton L. Humason were able to prove that the Universe is in a state of expansion. Now, Cepheids have proven their worth once more, establishing an important physical characteristic of the Milky Way’s disk.

“Somewhat to our surprise, we found that in 3D our collection of 1339 Cepheid stars and the Milky Way’s gas disk follow each other closely. This offers new insights into the formation of our home galaxy,” says Prof. Richard de Grijs from Macquarie University in Sydney, Australia, and senior co-author of the paper. “Perhaps more importantly, in the Milky Way’s outer regions, we found that the S-like stellar disk is warped in a progressively twisted spiral pattern.”

The same twisted spiral patterns have been seen before in more than a dozen other galaxies. Combined with these observations, the study’s results suggest that the likely culprit for the Milky Way’s warped spiral pattern is torque from the massive inner disk.

“This new morphology provides a crucial updated map for studies of our galaxy’s stellar motions and the origins of the Milky Way’s disk,” says Dr. DENG Licai, senior researcher at NAOC and co-author of the study published in Nature Astronomy.

Credit: C. O'Hare; NASA/Jon Lomberg.

The Earth is smack in the middle of a ‘dark matter hurricane’

According to an international team of astronomers, our solar system is in the path of a “dark matter hurricane” — but there’s no need to panic. The whole event is totally harmless and, what’s more, might actually help scientists finally detect this elusive phenomenon.

Credit: C. O'Hare; NASA/Jon Lomberg.

Credit: C. O’Hare; NASA/Jon Lomberg.

Dark matter makes up roughly 27% of the universe, whereas “regular” matter accounts for only 5% — the rest being accounted for by dark energy. despite its ubiquity, nobody knows what dark matter really is or how it works. At the same time, nothing other than dark matter can explain the motion of stars and galaxies, which are expanding more than can be accounted for by regular, visible matter.

Although the evidence for the existence of dark matter is very strong, identifying it has proven extremely challenging — but we may now have a good shot. Researchers from Universidad de Zaragoza, King’s College London and the Institute of Astronomy in the U.K. have been studying a stellar stream left behind by a dwarf spheroidal galaxy that was devoured by the Milky Way aeons ago. The S1 stream, as it was called, was discovered just last year by a team studying data from the Gaia satellite.

Other such streams have been observed before, but this is the first to cross paths with our own solar system. Luckily, none of the 30,000 stars that comprise S1 will collide with us. However, the dark matter that’s moving along with this stream might be picked up by detectors on Earth.

According to several models showing the distribution of the dark matter and its density, the dark matter hurricane is traveling at a staggering 500 km/s. The analysis also allowed the researchers to predict which possible signatures of the stream scientists ought to look for to find dark matter. For instance, the results suggest that WIMP detectors have a slim chance of picking up anything. Weakly interacting massive particles (WIMPs) are hypothetical particles that are thought to constitute dark matter and, by virtue of their weak-scale interaction, WIMPs should be able to be observed by directly detecting their interactions with ordinary matter.

On the other hand, axion detectors may actually have a fighting chance, the authors write in the journal Physical Review DAxions are hypothetical particles that have a small mass in the milli-electronvolt (eV) range, making them 500 million times lighter than an electron. Additionally, an axion should have no spin. Detectors such as the Axion Dark Matter Experiment might be able to pickup axions from S1 due to possible bumps in the broad spectrum of axions. In the presence of a strong magnetic field, axions should be converted into photons, which we can see, according to a previous estimate.

While there are over 30 such streams known in our galaxy, S1 is the only one to directly interact with our solar system. What’s more, our paths will intersect for millions of more years. So, even if our technology is not advanced enough to detect dark matter particles, there is still plenty of time for more sensitive detectors to be built.

The final stage of a union between two galactic nuclei in the messy core of the merging galaxy NGC 6240. Credit: NASA, ESA, W. M. Keck Observatory, Pan-STARRS and M. Koss.

Supermassive black hole pairs imaged in merging galactic cores

For the first time, astronomers have observed pairs of galaxies in the final stages of merging into a single, larger galactic body. The findings suggest that such events are more common than astronomers used to think.

The final stage of a union between two galactic nuclei in the messy core of the merging galaxy NGC 6240. Credit: NASA, ESA, W. M. Keck Observatory, Pan-STARRS and M. Koss.

The final stage of a union between two galactic nuclei in the messy core of the merging galaxy NGC 6240. Credit: NASA, ESA, W. M. Keck Observatory, Pan-STARRS and M. Koss.

Astronomers suspect that supermassive black holes lurk at the heart of every sizable galaxy, holding the galactic fiber together. For instance, at the galactic core of the Milky Way, there should be a supermassive black hole called Sagittarius A*, a staggering 4.5 million solar masses in size.

Black holes likely reach this sort of dizzying size through the merger of galaxies. However, evidence has been conflicting due to a lack of direct imaging of the process, which is obscured by swirling clouds of gas and dust. What’s more, simulations show that the more these galaxies progress in their merger, the greater the concealment effect.

To peer through all the matter that obscures supermassive black holes, scientists combed through a huge catalog of 10 years’ worth of X-ray measurements taken by the Burst Alert Telescope (BAT) aboard NASA’s Neil Gehrels Swift Observatory.

“The advantage to using Swift’s BAT is that it observes high-energy, ‘hard’ X-rays,” said study co-author Richard Mushotzky, a professor of astronomy at the University of Maryland. “These X-rays penetrate through the thick clouds of dust and gas that surround active galaxies, allowing the BAT to see things that are literally invisible in other wavelengths.”

Next, the research team analyzed another catalog of galaxies from NASA’s Hubble Space Telescope and the Keck Observatory in Hawaii whose X-ray signatures matched the Swift readings.

Various colliding galaxies along with closeup views of their coalescing nuclei in the bright cores. The images were taken in near-infrared light by the Keck Observatory in Hawaii. Credit: Keck images: W. M. Keck Observatory and M. Koss.

Various colliding galaxies along with closeup views of their coalescing nuclei in the bright cores. The images were taken in near-infrared light by the Keck Observatory in Hawaii. Credit: Keck images: W. M. Keck Observatory and M. Koss.

The breakthrough lied with the Keck Observatory’s adaptive optics technology whose deformable mirrors controlled by a computer enable a phenomenal increase in resolution. Using this tech, researchers were able to produce extremely sharp, near-infrared images of X-ray-producing black holes not found in the Hubble archive.

“People had conducted studies to look for these close interacting black holes before, but what really enabled this particular study were the X-rays that can break through the cocoon of dust,” explained Koss. “We also looked a bit farther in the universe so that we could survey a larger volume of space, giving us a greater chance of finding more luminous, rapidly-growing black holes.”

In total, 96 galaxies were observed with the Keck telescope and 385 galaxies from the Hubble archive. According to the results, 17 percent of these galaxies host a pair of black holes at their center, which are locked in the late stages of a galactic merger. This was surprising to learn since previously simulations suggested that black hole pairs spend very little time in this phase.

“Seeing the pairs of merging galaxy nuclei associated with these huge black holes so close together was pretty amazing,” said Michael Koss, co-author of the new study published in the journal Nature. “In our study, we see two galaxy nuclei right when the images were taken. You can’t argue with it; it’s a very ‘clean’ result, which doesn’t rely on interpretation.”

The findings suggest that galactic mergers may indeed be a key process by which black holes grow to stupendous masses. And the stages that the astronomers have mapped out in this study likely foretell the fate of our very own galaxy. In about 6 billion years, scientists estimate that the Milky Way will merge with the Andromeda galaxy into one big galaxy.

There is still much to learn about black hole mergers, though. Right now, hardware is a huge limitation, but, once the James Webb Space Telescope is deployed in 2021, scientists will be able to measure masses, growth rates, and other physical parameters of black hole pairs.

Illustration of hot clumps of gas that orbit the black hole at the center of the Milky Way. Credit: ESO/Gravity Consortium/L. Calçada.

Astronomers image plasma flares on the bleeding edge of Milky Way’s supermassive black hole

Illustration of hot clumps of gas that orbit the black hole at the center of the Milky Way. Credit: ESO/Gravity Consortium/L. Calçada.

Illustration of hot clumps of gas that orbit the black hole at the center of the Milky Way. Credit: ESO/Gravity Consortium/L. Calçada.

Scientists have known for a long time that at the very heart of the Milky Way lies a supermassive black hole, about four million times more massive than the Sun. As its name suggests, we can’t image a black hole directly, but cutting-edge telescopes can tease out the infrared light emitted by interstellar gas as it swirls into the black hole. Now, an international team of researchers led by the Max Planck Institute for Extraterrestrial Physics reported evidence of knots of gas that appear to orbit the galactic center. This remarkable observation is the closest look yet at our galactic supermassive black hole and, at the same time, offers new opportunities to test the laws of physics.

The point of no return

To image things in the vicinity of Sagittarius A*, the Milky Way’s supermassive black hole, researchers looked to the GRAVITY project. Using a special technique called interferometry, four eight-meter-wide telescopes at the European Southern Observatory’s Very Large Telescope in Chile were combined to produce images that only a hypothetical telescope as large as entire countries could produce. By the same technique, in the future, a ‘planet-sized’ instrument called the Event Horizon Telescope hopes to produce an actual image of a black hole.

The new observations measured the brightness and position of infrared flares in the vicinity of Sagittarius A*. These flares actually trace a tiny circle in the night’s sky, the researchers found, moving clockwise.

Yepun telescope, part of the European Southern Observatory's (ESO's) Very Large Telescope. Credit: ESO.

Yepun telescope, part of the European Southern Observatory’s (ESO’s) Very Large Telescope. Credit: ESO.

These kinds of outbursts had been detected before. However, this was the first time that astronomers precisely measured the flares’ positions and motions before they dissipated. Each flare moved at about 30% light speed in a 45-minute orbit around what we can only suppose is a black hole.

Earlier this year, the same team measured the relativistic distortion of light from a star, S2, during its closest approach to Sagittarius A*.

These hot spots might be produced by shock waves in magnetic fields, much as solar flares erupt from the sun. Due to the immense gravitational forces present in the vicinity of the black hole, space-time itself is twisted into something resembling a lens, which causes these hot spots, circling at 30% the speed of light, to flash beacons of light across the cosmos. And by further studying these flares, researchers hope to tease out the black hole’s spin or rotation.

All of this, of course, assuming Einstein’s general theory of relativity is correct, which implies that the orbits of objects around a black hole are determined solely by the black hole’s mass and spin. If not, then the theory might need some refinement to accommodate for any observed inconsistencies.

Researchers reported their findings in the journal Astronomy & Astrophysics.