Andromeda galaxy had more violent history than Milky Way

A new study has revealed that Andromeda had more violent history, as scientists found a more disordered stellar population than our Milky Way, suggesting that it may have been bombarded by smaller galaxies.

Puragra Guhathakurta, professor of astronomy and astrophysics at the University of California, Santa Cruz and his team combined data from two large surveys of stars in Andromeda, one conducted at the W. M. Keck Observatory in Hawaii and the other using the Hubble Space Telescope.

A detailed study of the motions of different stellar populations in the disk of the Andromeda galaxy has found striking differences from our own Milky Way, suggesting a more violent history of mergers with smaller galaxies in Andromeda's recent past.

The researchers said that if one could look at the disk edge on, the stars in the well-ordered, coherent population would lie in a very thin plane, whereas the stars in the disordered population would form a much puffier layer.

The findings revealed a clear trend related to stellar age, with the youngest stars showing relatively ordered rotational motion around the center of the Andromeda galaxy, while older stars displayed much more disordered motion. Stars in a "well ordered" population are all moving coherently, with nearly the same velocity, whereas stars in a disordered population have a wider range of velocities, implying a greater spatial dispersion.

The researchers considered different scenarios of galactic disk formation and evolution that could account for their observations. One scenario involves the gradual disturbance of a well-ordered disk of stars as a result of mergers with small satellite galaxies.

An alternate scenario involves the formation of the stellar disk from an initially thick, clumpy disk of gas that gradually settled. The oldest stars would then have formed while the gas disk was still in a puffed up and disordered configuration. Over time, the gas disk would have settled into a thinner configuration with more ordered motion, and the youngest stars would then have formed with the disk in a more ordered configuration.