History of Andromeda’s galactic cannibalism

In a cosmic detective story of interstellar proportions, astronomers sifting through faint signatures in the halo of stars surrounding the Andromeda galaxy are unearthing a 10-billion-year history of galactic cannibalism.

Andromeda is the closest large galaxy to our own, often viewed as the Milky Way’s twin. That makes it a convenient place for trying to understand the history of galaxies like our own, which is harder to study because we are inside it. 

In particular, says Dougal Mackey, an astronomer at Australian National University, Canberra, that makes it a good place to look to see how galaxies grow and evolve, often by “eating” other, smaller galaxies. 

If dwarf galaxies come to close to large ones like Andromeda, Mackey says, “tidal forces from the larger system shred them [and] pull them apart [until] their mass gets assimilated”.

But these galactic feeding frenzies leave clues: remnants of destroyed galaxies that build up in a “halo” around the main galaxy.

“It’s been known for 10 to 15 years that Andromeda has a vigorous history of accumulating and destroying its neighbours,” Mackey says. In fact, he says, “It seems to have a much more intense history of that than the Milky Way.”

But even though Andromeda is nearby, by galactic standards, it’s still 2.5 million light-years away.

At that distance, Mackey says, “The problem is that the destroyed remains are extremely faint.”

But, he says, it turns out that some of these remnants are globular clusters—compact balls of thousands or hundreds of thousands of stars that are common parts of many galaxies. “They are quite bright,” Mackey says.

This allowed his team to use them as beacons, bright enough to allow the measurement of their motions as they orbited the Andromeda Galaxy as a whole, as published in the journal Nature.

Based on this, he says, his team found that many of them fell into stellar streams that were clearly the remnants of recently destroyed galaxies. “They had to have been destroyed fairly recently,” he says, “because we can still see the stretched-out streams.”

But others orbited differently, and appear to have originated from galaxies that were destroyed very long ago, perhaps as far back as 10 to 12 billion years ago.

“That tells us there were two main events that formed the halo of Andromeda,” Mackey says. “One occurred very long ago. The other must have happened relatively recently.”

Not that Andromeda couldn’t also have eaten innumerable smaller galaxies. ”We can’t trace them with galactic clusters, because they didn’t have any to begin with,” Mackey says.

In the future, he adds, it may be possible to use globular clusters to calculate the total mass of the Andromeda galaxy more accurately than the current best estimates, which make it roughly the same size as the Milky Way, with no clear indication which is larger. 

That will help scientists better understand the future of our corner of the Universe, including not only how the Milky Way and Andromeda will interact with other, smaller, nearby by galaxies, but how they will eventually collide and merge, billions of years in the future.

“If we want to understand the future merger of the Milky Way and Andromeda, it’s easier if we have better mass estimates of the two protagonists,” Mackey says.

Allison Kirkpatrick, an astronomer at the University of Kansas, Lawrence, who was not part of the study team, calls the new paper “a sophisticated analysis in stellar archaeology,” a type of study that is important for galactic evolution models, “since we think most of the stellar mass in galaxies comes from mergers with smaller galaxies”.

Paul Mason, an astrophysicist at New Mexico State University, Las Cruces, adds that the study shows galactic evolution is an ongoing matter, not just something that occurred early in the history of the Universe. 

In fact, he says, it is becoming increasingly clear that the future will involve Andromeda’s merger not only with several additional small galaxies, but also the Milky Way, a process in which our “Local Group” of galaxies will evolve into smaller and smaller numbers of increasingly large galaxies until, “finally” it will become “a single mega-galaxy.”

Beyond that? “After roughly 100 billion years,” he says, “we will fall into the heart of the Virgo Supercluster,” which includes dozens of galactic clusters similar to our own Local Group.