Andromeda’s galaxy-gobbling past
4 Oct 2019
Our neighbouring galaxy Andromeda has engulfed several smaller galaxies, likely within the last few billion years.
An international team of astronomers found traces of small galaxies that Andromeda engulfed perhaps as far back as 10 billion years ago when it was first forming, traces of which can still be observed.
Andromeda, or Messier 31, is around 2.5 million light-years from Earth, making it the closest major galaxy to the Milky Way.
Dr Dougal Mackey at the Australian National University, said: "By tracing the faint remains of these smaller galaxies with embedded star clusters, we've been able to recreate the way Andromeda drew them in and ultimately enveloped them at the different times.
"Andromeda has a much bigger and more complex stellar halo than the Milky Way, which indicates that it has cannibalised many more galaxies, possibly larger ones.”
It is possible to trace a galaxy’s growth history by studying the properties of its stellar halo, a spherical distribution of stars.
The international team of researchers found two distinct populations of globular clusters – dense groups of stars that circulate galaxies – rotating perpendicular to each other. From this, they deduced that two galaxies had been consumed in different directions.
“We interpret these separate cluster populations as arising from two major accretion epochs, probably separated by billions of years,” the research paper, published in Nature, reads. “Stellar substructures from the first epoch are gone, but those from the more recent second epoch still remain.”
Large galaxies grow through the accumulation of dwarf galaxies, like raindrops running down a windowpane. Andromeda is currently approaching our galaxy at a speed of about 110 kilometres per second, and it will eventually engulf the Milky Way – in about four billion years.
“Knowing what kind of a monster our galaxy is up against is useful in finding out the Milky Way's ultimate fate,” Dr Mackey said.
Professor Geraint Lewis, co-author of the paper and astrophysicist at the Sydney Institute for Astronomy, the University of Sydney, told Laboratory News: "The collision between Andromeda and the Milky Way will be a violent event for both, with them both being significantly disrupted in the collision.
"When galaxies collide, the chances of stars colliding is almost zero, as they are very small compared to the huge distances between them, so the sun and solar system is likely to survive. But there is a reasonable chance that the sun will be ejected from the collision and end up in intergalactic space."
The research project used imaging data from the Pan-Andromeda Archaeological Survey from the Canada-France-Hawaii Telescope, as well as the Keck and Gemini telescopes for estimating spectroscopy and velocities.
