Tag Archives: Virgo

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.”

Artist’s impression of binary black holes about to collide. Image credit: Mark Myers, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

Gravitational waves reveal largest black hole merger and the first intermediate-mass black hole

Gravitational waves have been detected from what appears to be the largest black hole merger ever observed. The powerful and previously unobserved hierarchical merger resulted in  an intermediate-mass black hole, an object never before detected.

A massive burst of gravitational waves equivalent to the energy output of eight Suns has been detected by the LIGO laser interferometer. Researchers at LIGO and its sister project VIRGO believe that the waves originate from a merger between two black holes. But, this isn’t your average black hole merger (if there is such a thing). The merger — identified as gravitational wave event GW190521 — is not only the largest ever detected in gravitational waves — but it is also the first recorded example of what astrophysicists term a ‘hierarchical merger’ occurring between two black holes of different sizes, one of which was born from a previous merger.

“This doesn’t look much like a chirp, which is what we typically detect,” says Virgo member Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS), comparing the signal to LIGO’s first detection of gravitational waves in 2015. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”

GW190521 facts courtesy of LIGO-VIRGO (R. Ewing, R. Huxford, D. Singh
The Pennsylvania State University)
GW190521 facts courtesy of LIGO-VIRGO (R. Ewing, R. Huxford, D. Singh
The Pennsylvania State University)

Even more excitingly, it seems that black hole birthed in the event has a mass of between 100–1000 times that of the Sun, putting it in the mass range of an intermediate-mass black hole (IMBH). Something that researchers have theorised about for decades, but up until now, have failed to detect.

The gravitational wave signal–spotted by LIGO on 21st May 2019–appears to the untrained eye as little more than four short squiggles that lasted little more than one-tenth of a second, but for Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration, whose group focuses on the development of highly-accurate waveform models, it holds a wealth of information. “It’s amazing, but from about four gravitational-wave cycles, we could extract unique information about the astrophysical source,” she tells ZME Science.

“The waves are fingerprints of the source that has produced them.”

Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration.

As well as containing vital information about black holes and a staggering merger event, as the signal originated 17 billion light-years from Earth and at a time when the Universe was half its current age, it is also one of the most distant gravitational wave sources ever observed. The incredible distance the signal has travelled may initially seem at odds with the fact that the Universe is only 14.8 billion years old, but the disparity arises from the fact our universe is not static but is expanding.

A still from a numerical relativity simulation for GW190521 showing the gravitational waves emitted just before merger, overlaid with the signal as observed by the detectors. This is the largest binary system yet detected as shown by the horizons of this event compared to several previous events. (EPO)
A still from a numerical relativity simulation for GW190521 showing the gravitational waves emitted just before merger, overlaid with the signal as observed by the detectors. This is the largest binary system yet detected as shown by the horizons of this event compared to several previous events. (Deborah Ferguson, Karan Jani, Deirdre Shoemaker, Pablo Laguna, Georgia Tech, MAYA Collaboration)

Details of the international team’s important findings are featured in a series of papers publishing in journals such as Physical Review Letters, and The Astrophysical Journal Letters, today.

Missing Intermediete-Mass Black Holes

Thus far, the black holes discovered by astronomers have either been those with a mass inline with that of larger stars–so-called stellar-mass black holes, or supermassive black holes, with masses far exceeding this. Black holes that exist between these masses have remained, frustratingly hidden. Until now.

“The LIGO and Virgo collaborations detected a gravitational wave corresponding to a very interesting black hole merger. This was named GW190521 and corresponds to two large black holes during the final orbit and merger,” Pedro Marronetti, program director for gravitational physics at the National Science Foundation (NSF) responsible for the oversight of LIGO, tells ZME Science.

“What makes GW190521 extraordinary in comparison to other gravitational wave events is the mass of the black holes involved, the product of the merger is a 142 solar mass black hole and the first object of its kind with mass above 100 solar masses but below a million solar masses to be discovered.”

Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)

Thus, the resultant black hole of 142 solar masses  exists in that crucial, thus far undetected, mass range indicating an intermediate-mass black hole (IMBH).

“These black holes, heavier than 100 solar masses but much lighter than the supermassive black holes at the centre of galaxies — which can be millions and billions of solar masses — have eluded detection until now,” Marronetti says. “Additionally, the heavier of the original black holes with 85 solar masses also presents an enigma.”

Pair Instability and Hierarchical Black Hole Mergers

The enigma that Marronetti refers to is the fact that heavier of the two black holes that entered the merger, is of a size that suggests it too must have been created by a merger event between two, even smaller, black holes. “The most common channel of formation of black holes involves heavy stars that end their lives in supernova explosions,” the NSF program director points out. “However, this formation channel prevents the creation of black holes heavier than 65 solar masses but lighter than 130 solar masses due to a phenomenon called ‘pair-instability’.”

This graphic shows the masses of black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and neutron stars detected through gravitational waves (orange). GW190521 is highlighted in the middle of the graphic as the merger of two black holes that produced a remnant that is the most massive black hole observed yet in gravitational waves. [Image credit: LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]
This graphic shows the masses of black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and neutron stars detected through gravitational waves (orange). GW190521 is highlighted in the middle of the graphic as the merger of two black holes that produced a remnant that is the most massive black hole observed yet in gravitational waves. [Image credit: LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

As nuclear fusion ceases, there is no longer enough outward radiation pressure to prevent gravitational collapse. “The star suddenly starts producing photons that are energetic enough to create electron-positron pairs,” Marronetti explains. “These photons, in turn, create an outward pressure that is not strong enough to stop the star from collapsing violently due to its self-gravitational pull.”

This results in a difference in gravitational pressure between the star’s core and its outer layers. As a massive shock travels through these ‘puffed out’ outer layers they are blown away in a massive explosion. With smaller stars, this leaves behind an exposed core that becomes a stellar remnant such as a white dwarf, neutron star or black hole. But if the star is a range above 130 solar masses, but below 200 solar masses, the result is more disastrous.

“The resulting supernova explosion completely obliterates the star, leaving nothing behind– no black hole or neutron star is produced,” Marronetti says. “It will take stars heavier that 200 solar masses to collapse into black hole fast enough to avoid this complete disintegration.”

LIGO and Virgo have observed their largest black hole merger to date, an event called GW190521, in which a final black hole of 142 solar masses was produced. This chart compares the event to others witnessed by LIGO and Virgo and indicates that the remnant of the GW190521 merger falls into a category known as an intermediate-mass black hole – and is the first clear detection of a black hole of this type. Intermediate-mass black holes, which have previously been predicted theoretically, would have masses between those of stellar-mass black holes and the supermassive ones at the hearts of galaxies. Image credit: LIGO/Caltech/MIT/R. Hurt (IPAC)
LIGO and Virgo have observed their largest black hole merger to date, an event called GW190521, in which a final black hole of 142 solar masses was produced. This chart compares the event to others witnessed by LIGO and Virgo and indicates that the remnant of the GW190521 merger falls into a category known as an intermediate-mass black hole – and is the first clear detection of a black hole of this type. Intermediate-mass black holes, which have previously been predicted theoretically, would have masses between those of stellar-mass black holes and the supermassive ones at the hearts of galaxies. Image credit: LIGO/Caltech/MIT/R. Hurt (IPAC)

As Marronetti points out; this means that the 85-solar mass back hole could only be formed by the merger of two smaller black holes, as at these masses, collapsing stars can’t form black holes. “This is a quite unusual event that can only occur in regions of dense black hole population such as globular clusters,” the researcher adds. “GW190521 is the first detection that is likely to be due to this ‘hierarchical merger’ of black holes.”

Marronetti continues by explaining that a hierarchical merger consists of one or more black holes that were produced by a previous black hole merger. This hierarchy of mergers allows for the formation of progressively heavier and heavier black holes from an original population of small ones.

“We don’t really know how common these hierarchical mergers are since this is the first time we have direct evidence of one. We can only say that they are not very common.”

Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)

LIGO Delivering Discoveries and Surprises

The team uncovered the unusual nature of this particular merger by assessing the gravitational wave signal with a powerful state of the art computational models. Not only did this reveal that GW190251 originates from the most massive black hole merger ever observed and that this was no ordinary merger but a hierarchical merger, but also crucial information about the black holes involved in the event.

“The signal carries information about the masses and spins of the original back holes as well as their final product,” Marronetti adds, alluding to the fact that the LIGO -VIRGO team were able to measure that spin and determine that as the black holes circled together, they were also spinning around their own axes. The angles of these axes appeared to have been out of alignment with the axes of their orbit. This misaligned spin caused the black holes to ‘wobble’ as they moved together.

Artistic interpretation of the binary black hole merger responsible for GW190521. The space-time, figured by a fabric on which a view of the cosmos is printed, is distorted by the GW190521 signal. The turquoise and orange mini-grids represent the dragging effects due to the individually rotating black holes. The estimated spin axes, or self-rotations, of the black holes are indicated with the corresponding colored arrows. The background suggests a star cluster, one of the possible environments where GW190521 could have occurred. Credits: Raúl Rubio / Virgo Valencia Group / The Virgo Collaboration.
An artistic interpretation of the binary black hole merger responsible for GW190521. The space-time, figured by a fabric on which a view of the cosmos is printed, is distorted by the GW190521 signal. The turquoise and orange mini-grids represent the dragging effects due to the individually rotating black holes. The estimated spin axes, or self-rotations, of the black holes, are indicated with the corresponding coloured arrows. The background suggests a star cluster, one of the possible environments where GW190521 could have occurred. Credits: Raúl Rubio / Virgo Valencia Group / The Virgo Collaboration.)

“Our waveform models were used to detect GW190521 and also to interpret its nature, extracting the properties of the source, such as masses, spins, sky location, and distance from Earth. For the first time, the waveform models included new physical effects, notably the precession of the spins of the black holes and higher harmonics,” Buonanno says. “What we mean when we say higher harmonics is like the difference in sound between a musical duet with musicians playing the same instrument versus different instruments.

“The more substructure and complexity the binary  has — for example, black holes with different masses or spins—the richer is the spectrum of the radiation emitted.”

Alessandra Buonanno, Principal Investigator of the LIGO Scientific Collaboration.

Unanswered Questions and Future Investigations

Even with the staggering amount of information the team has been able to collect about the merger that gave rise to the signal GW190251, there are still some unanswered questions and details that must be confirmed.

The LIGO-VIRGO detectors use two very distinct methods to search the Universe for gravitational waves, an algorithm to pick out a specific wave pattern most commonly produced by compact binary mergers, and more general ‘burst’ searches. The latter searches for any signal ‘out of the ordinary’ and it’s the mechanism via which the researchers found GW190215.

Morronetti expresses some surprise that the methods used by the team were able to unlock these secrets, believing that this result demonstrates the versatility of LIGO. “My main surprise was that this event was detected using a search algorithm that was not specifically created to find merger signals,” says the NSF director. “This is the first detection of its kind and shows the capability of LIGO to detect phenomena beyond compact mergers.”

 “This is of tremendous importance since it showcases the instrument’s ability to detect signals from completely unforeseen astrophysical events. LIGO shows that it can also observe the unexpected.”

Pedro Marronetti, program director for gravitational physics, the National Science Foundation (NSF)


This leaves open the small chance that the signal was created by something other than a hierarchical merger. Perhaps something entirely new. The authors hint at the tantalising prospect of some new phenomena, hitherto unknown, in their paper, but Marronetti is cautious: “By far, the most likely cause is the merger of two black holes, as explained above. However, this is not as certain as with past LIGO/Virgo detections.

“There is still the small chance that the signal was caused by a different phenomenon such as a supernova explosion or an event during the Big Bang. These scenarios are possible but highly unlikely.”

Confirming the nature of the event that gave rise to the GW190251 signal is something that the LIGO team will be focusing on in the future as the interferometer also searches for similar events via the gravitational waves they emit. “

With GW190521, we have seen the tip of the iceberg of a new population of black holes,” Buonanno says, adding that LIGO’s next operating run (O4) will explore a volume of space 3 times larger than the current run (O3). “Having access to a larger number of events, which were too weak to be observed during O3, will allow us to shed light on the formation scenario of binary black holes like GW150921.”

Scientists to announce more details about gravitational waves

The discovery of gravitational waves took the scientific world by storm, confirming a theory first proposed by Einstein. Now, researchers representing LIGO, Virgo, and some 70 observatories are set to announce more details about this intriguing phenomenon.

It was no surprise that this year’s Nobel Prize in Physics was awarded for contributions in detecting gravitational waves. The prize was awarded to three LIGO physicists (Laser Interferometer Gravitational-Wave Observatory), but LIGO itself is a project involving more than a thousand people. The Virgo interferometer also includes over 300 scientists and technicians, and tens of smaller observatories are additionally offering valuable contributions. All in all, many people are working on gravitational waves, and we’re starting to see results.

The first detection of gravitational waves, made Sept. 14, 2015, and announced Feb. 11, 2016. Great claims require great evidence, and physicists wanted to be sure they’ve got these solid claims. Finding these waves is no easy feat — we can only detect them from interactions between massive objects such as neutron stars or black holes, and even these interactions produce incredibly small effects. The interferometers are basically mirrors placed 4 kilometers apart, and they barely distort by 10−18 m, which is less than one-thousandth the charge diameter of a proton.

Binary systems made up of two massive objects orbiting each other are an important source for gravitational-wave astronomy, but even these create incredibly small distortions. Credit: NASA/Dana Berry, Sky Works Digital.

Since 2015, gravitational waves have been detected three more times, the last occasion being a joint report from both LIGO and Virgo. We can say, with a confidence level of over 99.99999%, that we’ve spotted gravitational waves — but this is in no way the end of the story. If anything, it’s the beginning of a new field of science. Gravitational-wave astronomy is witnessing its baby years. The solid theoretical foundation has been there for a century, but the practical part is just beginning — and there’s really no telling how far this field can take us. This is why this announcement is so exciting. Monday, Oct. 16, at 10 a.m. EDT at the National Press Club in Washington, D.C., scientists representing almost everybody who’s working in gravitational wave detection will hold a press conference.

We have really no idea what they are about to announce, but since it’s such a massive participation, it could be something big. Since they were first published, the papers on gravitational waves have been cited more than 1,700 times total, so the scientific world will be listening, and so will we.