Tag Archives: dwarf galaxy

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.”

Astrophysicists find more evidence of ‘wandering’ black holes

Artist’s conception of a dwarf galaxy, its shape distorted, most likely by a past interaction with another galaxy, and a massive black hole in its outskirts (pullout). The black hole is drawing in material that forms a rotating accretion disc and generates jets of material propelled outward. Image credit: Sophia Dagnello, NRAO/AUI/NSF

Dwarf galaxies have traditionally been considered too small to host massive black holes, but new research emerging from Montanna State University (MSU) has revealed dozens of examples. The research, published in the Astrophysical Journal has delivered another surprise, these black holes aren’t located where scientists usually expect to find them.

“All of the black holes I had found before were in the centres of galaxies,” says Amy Reines, an assistant professor in the Department of Physics in the College of Letters and Science. “These were roaming around the outskirts. I was blown away when I saw this.”

Reines and her team searched 111 dwarf galaxies within a radius of a billion-light-years of Earth using the National Science Foundation’s Karl G. Jansky Very Large Array at the National Radio Astronomy Observatory, Albuquerque, New Mexico. During the course of their search, they identified 13 galaxies that very probably host black holes, the majority of which were not centralised. 

Reines is also a researcher in the MSU’s eXtreme Gravity Institute, which unites astronomers and physicists in order to study phenomena in which the gravitational influence is so powerful that it blurs the separation of space and time. This includes events and objects such as neutron stars, black holes, mergers and collisions between the two and even, the initial extreme period of rapid expansion of the universe — the big bang. 

The researcher explains that whilst stellar-mass black holes — those with a mass of up to 10 times that of our Sun — form as large stars undergo gravitational collapse, we are, thus far, uncertain how supermassive black holes form. This class of black hole which can have masses of up to billions of times that of the Sun is most commonly found in the centre of galaxies. 

This is certainly the case with our galaxy, the Milky Way, which hosts the supermassive black hole Sagittarius A* (SgrA*) at its centre. Dwarf galaxies are smaller than spiral galaxies like the Milky Way, containing a few billion stars rather than 100–400 billion as spiral galaxies tend to.

The results collected by Reines confirm computer simulations generated by Jillian Bellovary, assistant professor at Queensborough Community College, New York and Research Associate at the American Museum of Natural History. 

How black holes get lost

Bellovary’s computer simulations suggested that black holes could be disturbed from the centre of dwarf galaxies by interactions they undergo as they travel through space. This result coupled with Reines’ study have the potential to change the way we look for black holes in dwarf galaxies going forward. This change in thinking could also impact theories of how both dwarf galaxies and supermassive black holes form. 

“We need to expand searches to target the whole galaxy, not just the nuclei where we previously expected black holes to be,” Reines adds.

No stranger for the search for black holes, Reines has been hunting these events for a decade, ever since she was a graduate student at the University of Virginia. Whilst she initially focused on star formation in dwarf galaxies, her research led her to something else that captured her interest: a massive black hole “in a little dwarf galaxy where it wasn’t supposed to be.”

Henize 2–10: a dwarf galaxy that hides a massive secret ( Reines et al. (2011))
Henize 2–10: a dwarf galaxy that hides a massive secret ( Reines et al. (2011))

The little dwarf galaxy she refers to is Heinze 2–10, located 30-million-light-years from Earth, which had previously been believed too small to host a massive black hole. “Conventional wisdom told us that all massive galaxies with a spheroidal component have a massive black hole and little dwarf galaxies didn’t,” Reines explains, adding that when she discovered such a relationship it was a “eureka” moment. After publishing these findings in the journal Nature she continued searching for further black holes in dwarf galaxies. “Once I started looking for these things on purpose, I started finding a whole bunch,” Reines says.

Visible-light images of galaxies that VLA observations showed to have massive black holes. Center illustration is artist’s conception of the rotating disk of material falling into such a black hole, and the jets of material propelled outward. Image credit: Sophia Dagnello, NRAO/AUI/NSF; DECaLS survey; CTIO
Visible-light images of galaxies that VLA observations showed to have massive black holes. Center illustration is artist’s conception of the rotating disk of material falling into such a black hole, and the jets of material propelled outward. Image credit: Sophia Dagnello, NRAO/AUI/NSF; DECaLS survey; CTIO

Changing her tactics by shifting from visual data from radio signals, Reines uncovered over 100 possible black holes in her first search of a sample that included 40,000 dwarf galaxies. In current search, as described in the latest paper, Reines returned to radio searches, hunting for radio signatures with that sample. This, she says, should allow her to find massive black holes in star-forming dwarf galaxies, even though she has only found one thus far. 

“When new discoveries break our current understanding of the way things work, we find even more questions than we had before,” comments Yves Idzerda, head of the Department of Physics at MSU.

As for Reines, the search continues. 

“There are lots of opportunities to make new discoveries because studying black holes in dwarf galaxies is a new field,” she said. “People are definitely captivated by black holes. They’re mysterious and fascinating objects.”

Original research: https://iopscience.iop.org/article/10.3847/1538-4357/ab4999

NGC1569 is a star-forming galaxy. Galaxies such as this could see their star formation rates affected by strong winds emanating from a central black hole. (HST/NASA/ESA)

Black Holes could stunt the growth of dwarf galaxies

NGC1569 is a star-forming galaxy. Galaxies such as this could see their star formation rates affected by strong winds emanating from a central black hole. (HST/NASA/ESA)
NGC1569 is a star-forming galaxy. Galaxies such as this could see their star formation rates affected by strong winds emanating from a central black hole. (HST/NASA/ESA)

Black holes at the centre of small dwarf galaxies could slow or even halt the formation of stars via the powerful winds they produce, researchers from University of California, Riverside, have discovered. This suppression of star-formation could have a marked influence on the evolution of such galaxies.

The result seems to confirm the long-held suspicion that supermassive black holes at the centre of galaxies can influence that galaxy’s evolution — including how they grow and the way that they age. But, the research also delivers a surprise; the winds that the astronomers measured coming from the black hole were more powerful than the team reckoned for. This means that models of star formation in dwarf galaxies may require a rethink.

“We expected we would need observations with much higher resolution and sensitivity, and we had planned on obtaining these as a follow-up to our initial observations,” said Gabriela Canalizo, a professor of physics and astronomy at UC Riverside who led the research team. “But we could see the signs strongly and clearly in the initial observations.

“The winds were stronger than we had anticipated.”

Gabriela Canalizo

Thus meaning that black holes don’t just influence the development of larger galaxies, but also play a role in the evolution of smaller dwarf galaxies — galaxies containing anywhere from a few thousand to a few billion stars.

Canalizo continues: “Our findings now indicate that their effect can be just as dramatic, if not more dramatic, in dwarf galaxies in the universe.”

The study — the results of which are discussed in the Astrophysical Journal — used data collected in the Sloan Digital Sky Survey (SDSS), a project which maps 35% of the sky above Earth. In doing so, the survey has been able to identify 50 dwarf galaxies — 29 of which demonstrated clear characteristics of possessing black holes at their centres. A further six of these showed evidence of high-velocity outflows of ionised gas — the powerful winds in question.

The next step for the researchers was to use the Keck telescopes — based in Hawaii — to both detect and measure the properties of these winds, marking the first time this has been achieved.

Discussing what her team found, Canalizo adds: “We found some evidence that these winds may be changing the rate at which the galaxies are able to form stars.”

Studying dwarf galaxies could be the key to understanding how galaxies in general evolve

The study of these smaller galaxies could help scientists answer lingering about galactic evolution in general.

“Larger galaxies often form when dwarf galaxies merge together,” explains Christina Manzano-King, a doctoral student in Canalizo’s lab and the first author of the paper. As a consequence of this, she continues, dwarf galaxies are particularly useful in understanding how galaxies evolve.

Dwarf galaxies hosting active galactic nuclei that have spatially extended outflows. (SDSS)
Dwarf galaxies hosting active galactic nuclei that have spatially extended outflows. (SDSS)

“Dwarf galaxies are small because after they formed, they somehow avoided merging with other galaxies,” she adds. “Thus, they serve as fossils by revealing what the environment of the early universe was like.

“Dwarf galaxies are the smallest galaxies in which we are directly seeing winds — gas flows up to 1,000 kilometres per second — for the first time.”

Christina Manzano-King

Explaining what causes these powerful winds, Manzano-KIng points to material being fed into the black hole. This material — usually gas and dust — forms an accretion disc around the black hole. In this disc — which gradually feeds the black hole — conditions are so violent that friction and tremendous tidal forces heats the material. This releases radiative energy which shoves gas out of the galaxy’s centre and into intergalactic space.

This negatively affects the amount of gas available for star formation.

Manzano-King continues: “What’s interesting is that these winds are being pushed out by active black holes in the six dwarf galaxies rather than by stellar processes such as supernovae.

“Typically, winds driven by stellar processes are common in dwarf galaxies and constitute the dominant process for regulating the amount of gas available in dwarf galaxies for forming stars.”

Astronomers believe that winds emanating from black holes can compress gas and thus aid the gravitational collapse of gas clouds, kick-starting star-formation. But, if the wind is too strong and thus expels gas from the galaxy’s centre, rather than aiding the star formation process, gas becomes unavailable and hinders the process.

This is exactly what appears to be happening in the six galaxies that the team’s research highlighted. In these cases, the wind has had a clear detrimental impact on star formation rates.

Rethinking the relationship between black holes and star formation rates

This research may result in a rethinking of models of star formation and the evolution of galaxies. Current models do not take into account the impact of black holes in dwarf galaxies.

From left to right: Laura Sales, Christina Manzano-King, and Gabriela Canalizo. The team’s research could force a rethinking of star formation rates in dwarf galaxies ( Stan Lim, UC Riverside)
From left to right: Laura Sales, Christina Manzano-King, and Gabriela Canalizo. The team’s research could force a rethinking of star formation rates in dwarf galaxies ( Stan Lim, UC Riverside)

“Our findings show that galaxy formation models must include black holes as important, if not dominant, regulators of star formation in dwarf galaxies,” points out Laura V. Sales, assistant professor of physics and astronomy at UC Riverside.

As for the future of this research, the team next plans to investigate characteristics of gas outflows such as mass and momentum.

“This would better inform theorists who rely on such data to build models,” concludes Manzano-King. “These models, in turn, teach observational astronomers just how the winds affect dwarf galaxies.

“We also plan to do a systematic search in a larger sample of the Sloan Digital Sky Survey to identify dwarf galaxies with outflows originating in active black holes.”

Original research: ‘AGN-Driven Outflows in Dwarf Galaxies’ Christina M. Manzano-King, Gabriela Canalizo, and Laura V. Sales.

On the left is Pisces A, 19 million light-years away. On the right we can see Pisces B, which is around 30 million light-years way. Credit: NASA, ESA, and E. Tollerud (STScI)

Two dwarf-galaxies have left the wilderness to join a galactic party

On the left is Pisces A, 19 million light-years away. On the right we can see Pisces B, which is around 30 million light-years way. Credit: NASA, ESA, and E. Tollerud (STScI)

On the left is Pisces A, 19 million light-years away. On the right we can see Pisces B, which is around 30 million light-years way. Credit: NASA, ESA, and E. Tollerud (STScI)

Hubble just spotted two dwarf galaxies leave the galactic wilderness for a much more crowded region, drawn in by gravity. The pair previously inhabited a region of the Universe sparsely populated with galaxies, the 150 million light-years across Local Void. Astronomers say the galaxies are now ready to seriously nurse many new stars as they enter a more welcoming breeding ground rich in gas and dust.

“These Hubble images may be snapshots of what present-day dwarf galaxies may have been like at earlier epochs,” said lead researcher Erik Tollerud of the Space Telescope Science Institute in Baltimore, Maryland. “Studying these and other similar galaxies can provide further clues to dwarf galaxy formation and evolution.”

A dwarf galaxy is a small galaxy composed of about 100 million up to several billion stars. Though impressive, it’s still a trifle even when compared to a medium-sized galaxy like the Milky Way which hosts 200–400 billion stars. But dwarf galaxies, which are extremely faint and hard to detect, are extremely interesting for astronomers.

First of all, they’re the most numerous kind of galaxies in the universe. Secondly, dwarfs are the Milky Way’s closest neighbours allowing high-quality data to be gathered by telescopes. Dwarfs were also the building blocks of larger galaxies which formed billions of years ago — after all, everything started out small. We owe a lot of what we know about how galaxies form to dwarfs.

These recent dwarfs spotted by the Hubble Telescope, Pisces A and B, each contain only about 10 million stars — very sparsely populated even by galactic dwarven standards.

“These galaxies may have spent most of their history in the void,” Tollerud explained. “If this is true, the void environment would have slowed their evolution. Evidence for the galaxies’ void address is that their hydrogen content is somewhat high relative to similar galaxies. In the past, galaxies contained higher concentrations of hydrogen, the fuel needed to make stars. But these galaxies seem to retain that more primitive composition, rather than the enriched composition of contemporary galaxies, due to a less vigorous history of star formation. The galaxies also are quite compact relative to the typical star-forming galaxies in our galactic neighborhood.”

Astronomers found the dwarf pair while surveying hydrogen content in the Milky Way using radio telescopes. Thousands of small blobs packed with dense hydrogen gas were discovered within our galaxy, but some 30 to 50 of these blobs seemed to be located outside of it. Tollerud and colleagues then selected a couple which seemed like worthy candidates for nearby galaxies and asked permission to use Hubble’s Advanced Camera for Surveys to analyse them.

For this purpose, Hubble is particularly well-suited thanks to its sharp vision which can resolve individual stars and reliably estimate a galaxy’s distance. The distance is particularly important for determining brightness, as well as for calculating how far a galaxy is from a void.

The scientists found Pisces A is 19 million light-years away from Earth while Pisces B is roughly 30 million light-years away. Each galaxy has 20 to 30 bright blue stars, signifying they’re still very young — less than 100 million years old. At some point, the galaxies doubled their star formation rate as they closed in a more crowded sector. This star formation rate may slow down if the dwarfs draw too near a much larger galaxy and become satellites.

“The galaxies could even probably stop forming stars altogether, because they will stop getting new gas to make stars,” Tollerud said. “So they will use up their existing gas. But it’s hard to tell right now exactly when that would happen, so it’s a reasonable guess that the star formation will ramp up at least for a while.”

Findings appeared in the Astrophysical Journal.


Gamma Ray Signal Might Help Scientists Zoom in on Dark Matter

At the core of a newly found dwarf galaxy, astronomers discovered a mysterious source of gamma rays that may signal the presence of the mysterious dark matter. If this is confirmed, then it would be the first time we see dark matter through anything else than its gravitational pull.

dwarf galaxy dark matter

Artistic representation of a dwarf galaxy. Image via Stanford University.

Dark matter is a hypothetical kind of matter that cannot be seen with telescopes but accounts for most of the matter in the Universe – we know this because we observe its gravitational pull. But dark matter neither emits nor absorbs light, and we can’t see it, basically. Whatever it may be, we don’t know anything about it, other than we can see its effects. Understanding it would enable us to see the Universe in a new way, unlocking one of its biggest mysteries.

Now astronomers have reported that a small, newly discovered galaxy orbiting the Milky Way is emitting a surprising amount of electromagnetic radiation in the form of gamma rays. What does this have to do with dark matter? Well, dwarf galaxies have so little atomic (“normal”) matter that astronomers use them as hunting grounds for dark matter. If the gamma-ray signal is confirmed, this would first confirm the existence of dark matter (something which some astrophysicists still doubt) and provide some information about it. But while this evidence is “tantalizing” Alex Geringer-Sameth of Carnegie Mellon University and colleagues from Brown and Cambridge Universities say that it is still premature to draw conclusions.

“They are very quiet systems, just containing some old stars and a lot of dark matter,” Dr. Geringer-Sameth said in an email. “If you see any excess gamma rays coming from them, something intriguing is going on.”

Neal Weiner, a dark matter theorist at New York University, agreed, saying that this study has quite a lot of potential.

“If you see gamma rays in a dwarf galaxy, it would be a good way to make a case that you are seeing dark matter.”

It’s not the first time astronomers hoped they have found dark matter though, and no one wants to get too excited. In December last year, astronomers believed they found X-Ray signals of dark matter, while in April 2014, a different team found other interesting signals. But for now, nothing is confirmed. Dark matter, one of the biggest Universal mysteries, still has to wait.

Source: Carnegie Mellon University.