Thick Stellar Disk Isolated in Andromeda

A team of astronomers from the UK, the US and Europe have identified a thick stellar disk in the nearby Andromeda galaxy for the first time.

The discovery and properties of the thick disk will constrain the dominant physical processes involved in the formation and evolution of large spiral galaxies like our own Milky Way.


By analysing precise measurements of the velocities of individual bright stars within the Andromeda galaxy using the Keck telescope in Hawaii, the team have managed to separate out stars tracing out a thick disk from those comprising the thin disc, and assess how they differ in height, width and chemistry.

Spiral structure dominates the morphology of large galaxies at the present time, with roughly 70% of all stars contained in a flat stellar disc.

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

“From observations of our own Milky Way and other nearby spirals, we know that these galaxies typically possess two stellar disks, both a ‘thin’ and a ‘thick’ disc,” explains the leader of the study, Michelle Collins, a PhD student at Cambridge’s Institute of Astronomy.


The thick disk consists of older stars whose orbits take them along a path that extends both above and below the more regular thin disk.

“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.”

Currently, the formation process of the thick disc is not well understood. Previously, the best hope for comprehending this structure was by studying the thick disc of our own Galaxy, but much of this is obscured from our view.


The discovery of a similar thick disk in Andromeda presents a much cleaner view of spiral structure. Andromeda is our nearest large spiral neighbour, close enough to be visible to the unaided eye, and can be seen in its entirety from the Milky Way.


Astronomers will be able to determine the properties of the disk across the full extent of the galaxy and look for signatures of the events connected to its formation. It requires a huge amount of energy to stir up a galaxy’s stars to form a thick disk component, and theoretical models proposed include accretion of smaller satellite galaxies, or more subtle and continuous heating of stars within the galaxy by spiral arms.

Thick Stellar Disk Isolated in Andromeda