Galaxy Mergers: Gravity’s Ultimate Dance
What takes place when some of the most massive objects in the universe collide? The images that we see of merging and colliding galaxies are some of the most extraordinary that exist in the realm of astronomy. They represent a gravitational dance on scales far greater than we can begin to comprehend. This being so, our observation of them is limited: lasting for billions of years, they exist on timescales longer than humanity’s entire lifetime (1); we only get to see them frozen in time — mere flashes of what they were and will be. So, what exactly happens when galaxies collide? How do they affect what goes on within the galaxies? What is left behind?
Though they were far more common in the early universe, galaxy collisions are still a commonplace process. Between 5–25% of all galaxies are currently undergoing a merger or collision (2). We know that smaller dwarf galaxies colliding with massive ones are the most common, but these don’t disrupt the larger galaxy in a very significant manner. It is more interesting and controversial to investigate what goes on in galaxies during the massive collisions: how these mergers affect the solar systems, the star formation rate, black holes, and how they ultimately alter a galaxy’s evolution.
Before we investigate this, however, we need some background on galaxies. How do they come to be? The most elementary model for galaxy formation begins with a cloud of gas. Just after the Big Bang the universe was filled nearly uniformly with hydrogen, helium, and dark matter, but due to gravity, this uniformity didn’t last for long. Minute differences in density began to clump together into what we call protogalactic clouds (3).
As these clouds collapse, stars begin to form in their centers where stellar formation conditions exist. These stars later reside in the halo of the spiral galaxy. As it collapses further, the cloud spins more quickly due to the conservation of angular momentum, and — like throwing pizza dough — the cloud begins to flatten. Stars begin to form in this disk, and in billions of years, we have what we consider today to be a spiral galaxy.
The second important thing to recognize is the universe is expanding very rapidly — somewhere along the lines of 70 km/s per additional megaparsec we look away from Earth (4). This means, for example, that a galaxy 1 megaparsec away will be traveling away from us at 70 km/s, while a galaxy 2 megaparsecs away will be traveling away at 140 km/s. That adds up very quickly — so much so that it can exceed the speed of light. Thus, it begs the question of how two galaxies could even come close to colliding if everything is spreading out.
We know that gravity dominates on large scales compared to its colleague forces, and if gravity is strong enough, it can simply overcome the expansion of the universe. So, while nearly all galaxies are redshifted (i.e., they are heading away from us), a galaxy can appear blueshifted (i.e., it is heading toward us). We can see this directly with the Andromeda galaxy, which is heading towards our own, the Milky Way. We are destined for a collision ourselves!
In approximately four billion years, our massive neighbor Andromeda will begin to merge with the Milky Way. Will the Earth be done for? Remarkably, no. We must remember that practically speaking, galaxies are mostly empty space. To illustrate this, consider our Sun, which rests in an extraordinarily average part of the Milky Way, not in the outskirts nor the dense center. Yet, the nearest star to our own is Proxima Centauri, which is still 4.2 light years away (5) — meaning it takes light over four years to make that journey.
Let’s put that into perspective: 4.2 light years is approximately 40,000,000,000,000 kilometers or 265,000 astronomical units. An astronomical unit is the distance between the Earth and the Sun, but these are still very large and abstract numbers that, intuitively speaking, tell us very little. More intuitively, traveling to the nearest star would be equivalent to traveling around the Earth about 1 billion times. For us from the United States, that is about 20,000,000,000,000,000 bald eagle wingspans or 6,500,000,000,000,000 Ford F-150s bumper-to-bumper. Everything is incredibly spread out.
What does this mean for galaxy mergers? Quite simply, solar systems are not destroyed in the process. The orbits of stars in the galaxy are certainly changed but not the solar systems themselves. There will not be humans on Earth in 4 billion years — our oceans will have long since evaporated — but when the Milky Way and Andromeda collide, our Sun and Earth will not be destroyed, just severely displaced (6). If humans are somehow still walking around in 4 billion years, they will certainly get quite the spectacle.
If solar systems are not affected by the merging process, what is? Star formation rate may seem like a good candidate. Stars are formed from collapsing molecular clouds of gas that heat up and radiate until sufficiently hot and pressurized for fusion to ignite in their cores. Therefore, stirring up the gas in galaxies during a merger must impact this process, right? One would expect turbulence to increase the star formation rates (SFRs) dramatically in what is called a “starburst”, but it turns out that the impact might not be as significant as we once took it to be. Like everything related to galaxy mergers, the impact on SFRs is not well-understood and is heavily debated. Merging galaxies tend to see an increase in SFRs, but how much it increases is still being determined.
A study from 2019, however, collected data from a large sample of galaxies — over 200,000 — across a vast set of redshifts, from z = 0 to z = 4, and found some more conclusive but counterintuitive results. The paper states that there was in fact an influence on SFR in galaxies that were in the process of merging, as we expected, but “the resulting change in SFR is small, typically a factor of ∼1.2” (7). That is only a 20% increase, which is rather surprising for an event that is so astronomical, for lack of a better term. There can, of course, be merging galaxies with large starbursting regions, particularly when the merging galaxies have a large amount of gas, but as a rule, “merger-induced starbursts are found in the minority of merging systems” (7). It depends more on whether the merger is “wet” (gas-rich) or “dry” (gas-poor).
What about the supermassive black holes at the centers of galaxies? Do they merge into an even more massive supermassive black hole? We have never observed the merger of two supermassive black holes, but we believe that they would spiral into each other and merge, as expected. This process would release extraordinary amounts of gravitational waves (i.e., ripples in the fabric of spacetime) as well as X-rays, which are believed to come from the hot gas surrounding and falling into the black holes (8).
This relates closely to the formation of active galactic nuclei (AGNs). Of course, supermassive black holes become all that much more massive during the merging process, but if more gas begins to accrete to the final black hole, it can begin to glow as it falls in. The result is unbelievably bright — far brighter than we would expect from the stellar populations (9). There is, in fact, a direct correlation between galaxy mergers and AGNs: one paper studied galaxies up to a redshift of z = 0.6 and confirmed that “mergers play a significant role in triggering AGNs” (10).
For a long time, astronomers have believed that galaxy mergers played a fundamental role in galaxy evolution. We originally thought that galaxies start out elliptical — i.e., gas-poor, lacking star formation or any definitive structure — and eventually turn into spiral galaxies. But in fact, the opposite is turning out to be true: we see galaxies becoming more massive, redder, and more disk-shaped over time, not the other way around.
Collisions can profoundly alter this evolution. Astronomers have found that elliptical galaxies are most common near the dense centers of galaxy clusters, where we would imagine mergers happen most frequently (3). The morphologies of elliptical galaxies perhaps point to a turbulent past, which is how they may have lost their uniform angular momentum that once made them spirals.
Astronomers have found massive elliptical galaxies within the first 3–4 billion years of the universe’s existence (11), which doesn’t seem like enough time for these elliptical galaxies to come to be. One explanation is that two massive, gas-rich spiral galaxies collided, causing a starburst that made the galaxies very dry very quickly — leaving behind a massive, gas-poor, elliptical galaxy in the early universe. This is just one explanation, however, and the role of galaxy mergers in galactic evolution remains controversial; we still don’t know exactly how important they are to the evolution of galaxies.
There is much science to be done, and we are still working to understand the exact effect galaxy mergers have on all aspects of the galaxies involved. Because of this, much of what is written here will likely be out of date in no time. But that is actually a good thing, for it shows we are learning and building our knowledge of the universe!
As we continue to learn, it is important to keep everything in perspective: consider that early in the universe, protogalaxies were far closer together, so multiple galaxies often formed in the same region. The conditions of the early universe were denser than they are today because everything is spreading out. This meant that mergers were very common in this early universe, and consequently, galaxies gained mass more quickly. We can extrapolate that as the universe continues to expand, mergers will become less and less common.
But at this point in human history, we get to say that we have witnessed them firsthand, before everything expanded to the extent that all the galaxies are beyond each other’s horizons. We can imagine someone in a civilization in the distant future, living on a planet, around a star, in a galaxy who looks out into the night sky to find nothing but their own galaxy. They would think that they are alone. But we, living at this point in cosmic history where galaxies are still close enough to collide and interact, know that we are not.
We get to see these ephemeral dances while they are still on display…
Written by Curran Collier
2022–02–26
Sources:
1. “GMS: JWST Science Simulation: Galaxy Collision.” NASA, NASA, 29 Oct. 2010, https://svs.gsfc.nasa.gov/10687.
2. “Astronomers Pin Down Galaxy Collision Rate.” HubbleSite.org, 27 Oct. 2011, https://hubblesite.org/contents/news-releases/2011/news-2011-30.html.
3. Bennett, Jeffrey O., Megan Donahue, Nicholas Schneider, Mark Voit. The Cosmic Perspective, 9th Edition. Pearson, 20190104. VitalBook file.
4. Warren, Sasha. “The Hubble Constant, Explained.” What Is the Hubble Constant? University of Chicago News,https://news.uchicago.edu/explainer/hubble-constant-explained.
5. “The Nearest Neighbor Star.” NASA, NASA, https://imagine.gsfc.nasa.gov/features/cosmic/nearest_star_info.html.
6. Dunbar, Brian. “NASA’s Hubble Shows Milky Way Is Destined for Head-on Collision.” NASA, NASA, 15 Mar. 2021,https://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html.
7. Pearson, W. J., et al. “Effect of Galaxy Mergers on Star-Formation Rates.” Astronomy & Astrophysics, vol. 631, 2019,https://doi.org/10.1051/0004-6361/201936337.
8. “What Happens When Two Supermassive Black Holes Merge?” ESA, 23 May 2019,https://www.esa.int/ESA_Multimedia/Images/2019/05/What_happens_when_two_supermassive_black_holes_merge.
9. “Brightest.” NASA, NASA, 16 Oct. 2012, https://asd.gsfc.nasa.gov/blueshift/index.php/2012/10/16/alexes-est-blog-brightest/.
10. Gao, F., et al. “Mergers Trigger Active Galactic Nuclei out to z ∼ 0.6.” Astronomy & Astrophysics, vol. 637, 2020,https://doi.org/10.1051/0004-6361/201937178.
11. “Rare Merger Reveals Secrets of Galaxy Evolution.” ESA, 22 May 2013,https://www.esa.int/Science_Exploration/Space_Science/Herschel/Rare_merger_reveals_secrets_of_galaxy_evolution.
