Scientists hope disc of older stars found in Andromeda will reveal the building blocks of our own galaxy



A thick stellar disc of older stars in the Andromeda Galaxy has been identified by astronomers for the first time.

The discovery of the disc in Messier 31 - the nearest large spiral galaxy to our own Milky Way - will help scientists better understand the processes involved in the formation and evolution of such galaxies.

In a five-year study researchers using the Keck Telescope in Hawaii analysed the velocities of individual bright stars within the Andromeda Galaxy.

How a volley of stars map out a thick disc structure. Astronomers have found a thick disc in the Andromeda Galaxy for the first time

They were able to observe a group of stars tracing a thick disc - distinct from those comprising the galaxy's already-known thin disc - and assessed how these stars differ from thin-disc stars in height, width and chemistry.

Approximately 70 per cent of Andromeda's stars are contained in the galaxy's thin stellar disc.

This disc structure contains the spiral arms traced by regions of active star formation, and it surrounds a central bulge of old stars at the core of the galaxy.

Lead researcher Michelle Collins, from the University of Cambridge's Institute of Astronomy, said: 'From observations of our own Milky Way and other nearby spirals, we know that these galaxies typically possess two stellar discs, both a 'thin' and a 'thick' disc.'

The thick disc consists of older stars whose orbits take them along a 'thicker' path - one that extends both above and below the galaxy's thin disc.

Ms Collins said: 'The classical thin stellar discs that we typically see in Hubble imaging result from the accretion of gas towards the end of a galaxy's formation, whereas thick discs are produced in a much earlier phase of the galaxy's life, making them ideal tracers of the processes involved in galactic evolution.'

The Andromeda Galaxy is the nearest large spiral galaxy to our own Milky Way. During a five-year study researchers analysed the velocities of individual bright stars within the galaxy

The formation process of thick discs is not yet well understood. Previously, the best hope for understanding this structure was by studying the thick disc present in our own Milky Way.

However, much of our galaxy's thick disc is obscured from view.

The discovery of a similar thick disc in Andromeda presents a much clearer view of spiral structure.

Astronomers will be able to determine the properties of the disc across the galaxy and will search for signatures of the events related to its formation, the researchers said.

'Our initial study of this component already suggests that it is likely older than the thin disc, with a different chemical composition,' said Michael Rich, from the University of California Los Angeles and the lead researcher at the Keck Observatory for the observations.



'Future, more detailed observations should enable us to unravel the formation of the disc system in Andromeda, with the potential to apply this understanding to the formation of spiral galaxies throughout the universe.'

Scott Chapman, from the Institute of Astronomy at Cambridge, added: 'This result is one of the most exciting to emerge from the larger parent survey of the motions and chemistry of stars in the outskirts of Andromeda.