The violent history of the big galaxy next door

Astronomers have pieced together the cannibalistic past of the neighbouring large galaxy Andromeda, which has set its sights on our Milky Way as the main course.

The galactic detective work found that Andromeda has eaten several smaller galaxies, likely within the last few billion years, with left-overs found in large streams of stars.

This study was led by Dr Dougal Mackey from the Australian National University (ANU) and Professor Geraint Lewis from the University of Sydney, and involves a team of researchers from across the globe, including Professor Annette Ferguson and Professor Jorge Penarrubia from the School’s Institute for Astronomy. 

The international team also found evidence of more small galaxies that Andromeda gobbled up even earlier, perhaps as far back as during its first phases of formation about 10 billion years ago.

Dr Mackey from the ANU Research School of Astronomy and Astrophysics reported:

The Milky Way is on a collision course with Andromeda in about four billion years, so knowing what kind of a monster our galaxy is up against is useful in finding out its ultimate fate. 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.

The signs of ancient feasting are written in the stars orbiting Andromeda, with the team studying dense clusters of stars, known as globular clusters, to reveal the ancient mealtimes.  By tracing the faint remains of these smaller galaxies with embedded star clusters, they team have been able to recreate the way Andromeda drew them in and ultimately enveloped them at the different times. The discovery presents new mysteries however, with the two bouts of galactic feeding coming from completely different directions.

Professor Lewis said:

This is very weird and suggests that the extragalactic meals are fed from what’s known as the ‘cosmic web’ of matter that threads the universe. More surprising is the discovery that the direction of the ancient feeding is the same as the bizarre ‘plane of satellites’, an unexpected alignment of dwarf galaxies orbiting Andromeda.

Professor Annette Ferguson commented:

This is a very interesting result that we will have to work quite hard to understand.  It demonstrates how much information about the past lives of galaxies lies buried in their remote outer parts.

 The study, published in Nature, analysed data from the Pan-Andromeda Archaeological Survey, known as PAndAS.

The team involved institutions from Australia, New Zealand, the United Kingdom, Netherlands, Canada, France and Germany.