From the March 2019 issue

How galaxies interact

Today’s galaxies represent billions of years of mergers — some quiet, others not so much.
By | Published: February 22, 2019 | Last updated on December 6, 2023
NGC 2623 in the constellation Cancer the Crab is the product of the merger of two spiral galaxies some 250 million light-years away. The merger, which triggered many rounds of star formation, has resulted in the two cores combining to create one active galactic nucleus.
ESA/Hubble & NASA
Drive out to the country on a clear summer night and find the darkest place you can. Find the constellations Scorpius and Sagittarius, and look for dark regions amid the brilliance of the stars. What you’re seeing is the trail of dust circling our galactic center. That dust didn’t just start there — that’s dust from numerous small galaxies drawn together into the Milky Way we know today.

In fact, every large galaxy in the sky came from an assemblage of small galaxies — and sometimes even from two like-size galaxies. The basic building blocks of the smallest galaxies are globular clusters, groups of stars sharing a gravitational influence. Once they attract each other, several globular clusters begin to form a dwarf galaxy. And it snowballs from there.

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“That thing will merge with another similarly sized thing to create a slightly larger galaxy, over and over and over,” says Amanda J. Moffett, a postdoctoral researcher at Vanderbilt University who studies galaxy evolution.

Today’s galaxies are the products of billions of years of mergers, some quieter than others. Astronomers are able to group most nearby galaxies into the three main classifications: elliptical, spiral, and irregular. But further back in time, almost all were irregular. These galaxies, the ones at high redshift (more distant and moving away from us faster), take on strange, unfamiliar forms.

“At high redshift, we do see galaxies that are just a spheroid-looking component; all we really see is a simple ball of stars,” Moffett says. It’s those simple balls of stars that got the ball rolling into the galaxies we see today.

A wide disk contains most of the Milky Way’s stars not located in its core. A central stripe called the old thin disk, about 1,000 light-years thick, contains most of them. It lies within a more diffuse gaseous zone called the thick disk, which is about three times thicker, or 3,000 light-years.
Astronomy: Roen Kelly
Our home

The Milky Way’s recent merger history has been quiet, Moffett says. But like all galaxies, it’s a product of multiple building blocks, and it’s still picking on some of its neighbors, dragging them toward it in an inevitable dance between satellite dwarf galaxies and their larger host galaxies.

For instance, the Milky Way is bullying two neighbors, the Large and Small Magellanic Clouds, irregular galaxies easily seen from the Southern Hemisphere. The same thing is happening to both: The Milky Way is drawing in filaments of gas, slowly siphoning matter off. This happens as the two satellite galaxies are also pushing and pulling at each other.

“They’re interacting with each other, but then our galaxy is very clearly bringing the pair in as well with these streams of gas that are coming out of the system,” says Mary Putman, a Columbia University astronomy professor. Some satellite galaxies end up in a sort of death spiral with the larger galaxy. “They will eventually fall in due to dynamical friction and the large gravitational pull of our galaxy,” she says. “The timescales can be very, very long though, depending on their orbit.”

This image of dwarf galaxy NGC 4449 in Canes Venatici reveals a dense stream of stars in its halo (outer regions). Astronomers point to this as evidence of NGC 4449 merging with an even smaller dwarf galaxy. Researchers have captured many mergers involving massive galaxies, but this is one of the first images to show two dwarfs merging.
Subaru Telescope
Putman is part of a group investigating what happens with small galaxy mergers. “We’ve been tracking down [answers to questions like] how often do we see them? What are their properties? How do they differ from larger galaxies?” she says. “The idea here is that this is kind of the first step in forming a galaxy, and we still can observe it in the local universe.”

Two things happen when a dwarf galaxy gets tugged on by a larger galaxy. Dust is pulled out in a process known as ram pressure stripping, leaving a “naked” dwarf. Gravity also pulls the stars inward, and the dwarf begins to look like the Sagittarius Stream, a group of stars ripped from the Sagittarius Dwarf Elliptical Galaxy and currently spiraling into the Milky Way.

At one point, we may have even merged with a big galaxy. “The last possibility for a major merger was quite a long time ago, when our thick disk formed,” Putman says.

The Milky Way has two disks of stars moving at different velocities. The thin disk is from the beginning of the galaxy, while the thick disk was bulked up during a merger — though Putman says it’s not yet well understood what the merging galaxy was like.

The Large Magellanic Cloud (upper right) and the Small Magellanic Cloud are satellite galaxies of the Milky Way. Recent research has revealed a bridge of stars between them that formed as a result of their interaction. But an even greater interaction, due to our galaxy’s gravity, will occur as the Clouds are cannibalized and their stars added to the Milky Way millions of years into the future.
ESO/S. Brunier
Mergers and acquisitions

So what happens inside a bigger merger? While the two groups of stars and dust may have a lot of mass, there’s also a lot of space between objects in a galaxy. This means that while the effects of a merger can be extreme for the structure of the galaxies themselves, they don’t necessarily greatly affect the stars within.

“Most of the time in these mergers, stars move right between each other and really only interact gravitationally,” Moffett says. “Your orbits might get disrupted a little bit because of extra gravitational pull from other things that are around them, but they don’t typically collide.”

But things do get weird — really weird. Just look at the Antennae Galaxies (NGC 4038 and NGC 4039), which are in the process of becoming one. Trails of stars are thrown across thousands of light-years as they move inward, and the galactic cores are moving closer together. Eventually the two objects will settle into a more regular shape, but that’s not today.

In typical mergers between two large galaxies, they first slingshot past each other. This disrupts the dust of the galaxies. After the near miss, they move in close and then move apart once again, but that close pass is enough to start to destabilize their structures. The galaxies then begin the official merger, often appearing as a cloud of stars and dust that settles over time, their former central regions blending and creating a new, more powerful center of gravity.

This color composite image shows the Hubble Ultra Deep Field. Green circles mark “high-z” galaxies, which have a redshift around 8. Red circles show the locations of even higher redshift candidates. About 20 to 30 percent of these high-z galaxy candidates are close to foreground galaxies, consistent with the prediction that a significant fraction of galaxies at high redshifts are gravitationally lensed by individual foreground galaxies.
NASA/ESA/S. Wyithe (University of Melbourne)/H. Yan (Ohio State University)/R. Windhorst (Arizona State University)/ S. Mao (Jodrell Bank Centre for Astrophysics and National Astronomical Observatories of China)

Past encounters

Putman says the Andromeda Galaxy (M31) may have already had one merger in the past. The galaxy has two supermassive black holes (SMBHs) at its center, something that would be unusual if not for mergers. But having two supermassive black holes merge? That’s quite a different subject.

“What simulations in computers tell, or have been telling for a while, is that you can only bring two [supermassive] black holes to a certain distance and then they’ll stall,” says Salvatore Vitale, an MIT professor who works on the Laser Interferometer Gravitational-wave Observatory (LIGO) project. Strangely, if they did merge, the gravitational waves produced would be far, far outside of LIGO’s detectability. We’d be more likely to see the fireworks show from infalling gas.

But standing between us and a SMBH merger is the Final Parsec Problem, that physical barrier that holds two supermassive black holes from merging, instead locking into an orbit lasting billions upon billions of years.
Once galaxies settle in, there aren’t many ways of teasing out their merger history without examining them in detail. That’s hard with them being so far off. But every large galaxy has come together through a merger — and other, bigger mergers may be just over the horizon.

Editor’s note: The quote from Salvatore Vitale has been adjusted for clarification. An earlier version of the quote did not specify the two black holes involved were supermassive.