ANDROMEDA CRASH TRIGGERED A MASS GALACTIC MIGRATION 2 BILLION YEARS AGO

The influx of stars into our neighbouring galaxy reflects a violent event in the Milky Way's past

- Robert Lea

Astronomers have discovered new evidence that Andromeda, the galaxy next door to our own, grew by merging with another galaxy. The event triggered a mass migration of stars into the galaxy. This event suggests that the migration of stars to Andromeda – also known as Messier 31 – and the galaxy’s growth history is very similar to that of the Milky Way. That means the findings have implications for our understanding of both galaxies. The evidence came in the form of observations of the individual motions of almost 7,500 stars in the inner halo of Andromeda, showing these stars had begun their lives as part of another galaxy that merged with Andromeda around 2 billion years ago.

Scientists have long predicted that large galaxies have grown to their current sizes via collisions and mergers throughout their history, but the patterns in the motions of stars that could confirm this have been elusive. The investigation was conducted by an international team of astronomers using the Dark Energy Spectroscopic Instrument (DESI) on the Nicholas U. Mayall Telescope at Kitt Peak National Observatory, operated by the National Optical-Infrared Astronomy Research Laboratory (NOIRLab). “Our new observations of the Milky Way’s nearest large galactic neighbour, the Andromeda Galaxy, reveal evidence of a galactic immigration event in exquisite detail,” lead researcher and NOIRLab astronomer Arjun Dey, said. “Although the night sky may seem unchanging, the universe is a dynamic place. Galaxies like Messier 31 and our Milky Way are constructed from the building blocks of many smaller galaxies over cosmic history. “