What happens when galaxies collide? Well, if any humans are around in about a billion years, they might find out. That’s when our Milky Way galaxy is scheduled to collide with our neighbour the Andromeda galaxy. That event will be an epic, titanic, collision. The supermassive black holes at the center of both galaxies will feast on new material and flare brightly as the collision brings more gas and dust within reach of their overwhelming gravitational pull. Where massive giant stars collide with each other, lighting up the skies and spraying deadly radiation everywhere. Right?
Maybe not. In fact, there might be no feasting at all, and hardly anything titanic about it.
Galaxy mergers are beautiful sights, but ultimately deadly. In the midst of the collision, the combined galaxy will shine brighter than it ever has before. But that glory comes with a price: all those new stars use up all the available fuel, and star formation grinds to a halt.
In 2005 astronomers found a dense grouping of stars in the Virgo constellation. It looked like a star cluster, except further surveys showed that some of the stars are moving towards us, and some are moving away. That finding was unexpected and suggested the Stream was no simple star cluster.
A 2019 study showed that the grouping of stars is no star cluster at all; instead, it’s the hollowed-out shell of a dwarf spheroidal galaxy that merged with the Milky Way. It’s called the Virgo Overdensity (VOD) or the Virgo Stellar Stream.
A new study involving some of the same researchers shows how and when the merger occurred and identifies other shells from the same merger.
Modern professional astronomers aren’t much like astronomers of old. They don’t spend every suitable evening with their eyes glued to a telescope’s eyepiece. You might be more likely to find them in front of a super-computer, working with AI and deep learning methods.
One group of researchers employed those methods to find a whole new collection of stars in the Milky Way; a group of stars which weren’t born here.
NGC 6240 is a puzzle to astronomers. For a long time, astronomers thought the galaxy is a result of a merger between two galaxies, and that merger is evident in the galaxy’s form: It has an unsettled appearance, with two nuclei and extensions and loops.
Astronomers have spotted three supermassive black holes (SMBHs) at the center of three colliding galaxies a billion light years away from Earth. That alone is unusual, but the three black holes are also glowing in x-ray emissions. This is evidence that all three are also active galactic nuclei (AGN,) gobbling up material and flaring brightly.
This discovery may shed some light on the “final parsec problem,” a long-standing issue in astrophysics and black hole mergers.
Galaxy mergers are not particularly rare, but they are important events. Not only for the galaxies involved, but for scientists trying to piece together how galaxies evolve. Now, astronomers using ALMA have found the earliest example yet of merging galaxies.
On a clear night, you can make out the band of the Milky Way in the night sky. For millennia, astronomers looked upon it in awe, slowly coming to the realization that our Sun was merely one of billions of stars in the galaxy. Over time, as our instruments and methods improved, we came to realize that the Milky Way itself was merely one of billions of galaxies that make up the Universe.
Thanks to the discovery of Relativity and the speed of light, we have also come to understand that when we look through space, we are also looking back in time. By seeing an object 1 billion light-years away, we are also seeing how that object looked 1 billion years ago. This “time machine” effect has allowed astronomers to study how galaxies came to be (i.e. galactic evolution).
The process in which galaxies form and evolve is characterized by steady growth over time, which began shortly after the Big Bang. This process, and the eventual fate of galaxies, remain the subject of intense fascination, and is still fraught with its share of mysteries.
Illustration of the depth by which Hubble imaged galaxies in prior Deep Field initiatives, in units of the Age of the Universe. Credit: NASA and A. Feild (STScI)
Galaxy Formation:
The current scientific consensus is that all matter in the Universe was created roughly 13.8 billion years ago during an event known as the Big Bang. At this time, all matter was compacted into a very small ball with infinite density and intense heat called a Singularity. Suddenly, the Singularity began expanding, and the Universe as we know it began.
After rapidly expanding and cooling, all matter was almost uniform in distribution. Over the course of the several billion years that followed, the slightly denser regions of the Universe began to become gravitationally attracted to each other. They therefore grew even denser, forming gas clouds and large clumps of matter.
These clumps became primordial galaxies, as the clouds of hydrogen gas within the proto-galaxies underwent gravitational collapse to become the first stars. Some of these early objects were small, and became tiny dwarf galaxies, while others were much larger and became the familiar spiral shapes, like our own Milky Way.
Galactic Mergers:
Once formed, these galaxies evolved together in larger galactic structures called groups, clusters and superclusters. Over time, galaxies were attracted to one another by the force of their gravity, and collided together in a series of mergers. The outcome of these mergers depends on the mass of the galaxies in the collision.
Small galaxies are torn apart by larger galaxies and added to the mass of larger galaxies. Our own Milky Way recently devoured a few dwarf galaxies, turning them into streams of stars that orbit the galactic core. But when large galaxies of similar size come together, they become giant elliptical galaxies.
When this happens, the delicate spiral structure is lost, and the merged galaxies become large and elliptical. Elliptical galaxies are some of the largest galaxies ever observed. Another consequence of these mergers is that the supermassive black holes (SMBH) at their centers become even larger.
Not all mergers will result in elliptical galaxies, mind you. But all mergers result in a change in the structure of the merged galaxies. For example, it is believed that the Milky Way is experiencing a minor merger event with the nearby Magellanic Clouds; and in recent years, it has been determined that the Canis Major dwarf galaxy has merged with our own.
While mergers are seen as violent events, actual collisions are not expected to happen between star systems, given the vast distances between stars. However, mergers can result in gravitational shock waves, which are capable of triggering the formation of new stars. This is what is predicted to happen when our own Milky Way galaxy merges with the Andromeda galaxy in about 4 billion years time.
Galactic Death:
Ultimately, galaxies cease forming stars once they deplete their supply of cold gas and dust. As the supply runs out, star forming slows over the course of billions of years until it ceases entirely. However, ongoing mergers will ensure that fresh stars, gas and dust are deposited in older galaxies, thus prolonging their lives.
At present, it is believed that our galaxy has used up most of its hydrogen, and star formation will slow down until the supply is depleted. Stars like our Sun can only last for 10 billion years or so; but the smallest, coolest red dwarfs can last for a few trillion years. However, thanks to the presence of dwarf galaxies and our impending merger with Andromeda, our galaxy could exist even longer.
However, all galaxies in this vicinity of the Universe will eventually become gravitationally bound to each other and merge into a giant elliptical galaxy. Astronomers have seen examples of these sorts of “fossil galaxies”, a good of which is Messier 49 – a supermassive elliptical galaxy.
These galaxies have used up all their reserves of star forming gas, and all that’s left are the longer lasting stars. Eventually, over vast lengths of time, those stars will wink out one after the other, until the whole thing is the background temperature of the Universe.
After our galaxy merges with Andromeda, and goes on to merge with all other nearby galaxies in the local group, we can expect that it too will undergo a similar fate. And so, galaxy evolution has been occurring over billions of years, and it will continue to happen for the foreseeable future.
The Anglo-Australian Telescope in New South Wales has been watching how lazy giant galaxies gain size – and it isn’t because they create their own stars. In a research project known as the Galaxy And Mass Assembly (GAMA) survey, a group of Australian scientists led by Professor Simon Driver at the International Centre for Radio Astronomy Research (ICRAR) have found the Universe’s most massive galaxies prefer “eating” their neighbors.
According to findings published in the journal “Monthly Notices of the Royal Astronomical Society”, astronomers studied more than 22,000 individual galaxies to see how they grew. Apparently smaller galaxies are exceptional star producers, forming their stellar members from their own gases. However, larger galaxies are lazy. They aren’t very good at stellar creation. These massive monsters rarely produce new stars on their own. So how do they grow? They cannibalize their companions. Dr. Aaron Robotham, who is based at the University of Western Australia node of the International Centre for Radio Astronomy Research (ICRAR), explains that smaller ‘dwarf’ galaxies were being consumed by their heavyweight peers.
“All galaxies start off small and grow by collecting gas and quite efficiently turning it into stars,” he said. “Then every now and then they get completely cannibalized by some much larger galaxy.”
So how does our home galaxy stack up to these findings? Dr. Robotham, who led the research, said the Milky Way is at a tipping point and is expected to now grow mainly by eating smaller galaxies, rather than by collecting gas.
“The Milky Way hasn’t merged with another large galaxy for a long time but you can still see remnants of all the old galaxies we’ve cannibalized,” he said. “We’re also going to eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about four billion years.” Robotham also added the Milky Way wouldn’t escape unscathed. Eventually, in about five billion years, we’ll encounter the nearby Andromeda Galaxy and the tables will be turned. “Technically, Andromeda will eat us because it’s the more massive one,” he said.
This simulation shows what will happen when the Milky Way and Andromeda get closer together and then collide, and then finally come together once more to merge into an even bigger galaxy.
Simulation Credit: Prof Chris Power (ICRAR-UWA), Dr Alex Hobbs (ETH Zurich), Prof Justin Reid (University of Surrey), Dr Dave Cole (University of Central Lancashire) and the Theoretical Astrophysics Group at the University of Leicester. Video Production Credit: Pete Wheeler, ICRAR.
What exactly is going on here? Is it a case of mutual attraction? According to Dr. Robotham when galaxies grow, they acquire a heavy-duty gravitational field allowing them to suck in neighboring galaxies with ease. But why do they stop producing their own stars? Is it because they have exhausted their fuel? Robotham said star formation slow downs in really massive galaxies might be “because of extreme feedback events in a very bright region at the center of a galaxy known as an active galactic nucleus.”
“The topic is much debated, but a popular mechanism is where the active galactic nucleus basically cooks the gas and prevents it from cooling down to form stars,” Dr. Robotham said.
Will the entire Universe one day become just a single, large galaxy? In reality, gravity may very well cause galaxies groups and clusters to congeal into a limited number of super-giant galaxies, but that will take many billions of years to occur.
“If you waited a really, really, really long time that would eventually happen, but by really long I mean many times the age of the Universe so far,” Dr. Robotham said.
While the GAMA survey findings didn’t take billions of years, it didn’t happen overnight either. It took seven years and more than 90 scientists to complete – and it wasn’t a single revelation. From this work there have been over 60 publications and there are still another 180 in progress!
In a new study led by University of Central Lancashire astronomer Dr. Victor Debattista, researchers are looking into the mystery of how black holes grow and evolve. For many years, astronomers surmised black holes took on mass when their host galaxies merged, but now new modeling techniques show that black holes in spiral galaxies are forced to take on mass.
“Recent Hubble Space Telescope (HST) observations have revealed that a majority of active galactic nuclei (AGN) are resident in isolated disk galaxies, contrary to the usual expectation that AGN are triggered by mergers.” says Debattista. “Here we develop a new test of the cosmic evolution of supermassive black holes (SMBHs) in disk galaxies by considering the local population of SMBHs. We show that substantial SMBH growth in spiral galaxies is required as disks assemble.”
Weighing in a range of one million to one billion times that of the Sun, the black holes located at the core of most galaxies would appear to be gaining at much quicker rates than expected. These are not just exceptions – more like rules. Even the Milky Way’s quiescent black hole might be gaining as much mass as the Sun every 3,000 years. Past observations have shown growth during collision events, when huge amounts of gas around the black hole become intensely hot and shine as an active galactic nucleus. This is a process which can be spotted as far back as the first formations in our Universe. However, these new simulations are giving insight into large scale growth without the need for violence.
“The X-ray-selected sample of moderate luminosity AGN consists of more than 50% disk galaxies, with ongoing mergers evident no more frequently than in nonactive galaxies.” explains the research team. “Some show that even heavily obscured quasars are hosted largely by disks, not by mergers. Studies of star-formation using Herschel find that the specific star formation rates of X-ray selected AGN hosts are no different from those of inactive galaxies, also indicating that AGN hosts are not undergoing fundamentally different behaviors”
These modeling techniques, combined with current observations done with the Hubble Space Telescope, give credence to the theory that black holes can gain significant mass even in “quiet” spiral galaxies. As a matter of fact, there is a strong possibility that AGNs present in some spiral galaxies may even outnumber galaxy mergers. To make this concept even more exciting, astronomers are anticipating an event later this year in our own galaxy – an event where a gas cloud near the Milky Way’s nucleus will encounter our own central black hole. According to predictions, our black hole may take on as much as 15 Earth masses in a period of 10 years from this cloud.
This concept of black hole growth isn’t entirely new, though. According to other research done with the Hubble Space Telescope and led by Dr. Stelios Kazantzidis of Ohio State University and Professor Frank C. van den Bosch of Yale University, they had previously pinpointed mass properties of black holes – making size predictions which utilized the speed of stars residing in the galaxies. In this instance, the team disproved previous assumptions that black holes were unable to grow while the host galaxy grew. Their comparison of spiral and elliptical galaxies “found there is no mismatch between how big their black holes are.” This means black holes would be gaining in mass – growing along at the same rate as the galaxy itself.
“These simulations show that it is no longer possible to argue that black holes in spiral galaxies do not grow efficiently. ” comments Debattista on this new research. ” Our simulations will allow us to refine our understanding of how black holes grew in different types of galaxies.”
