By studying the motions of different stellar populations in the disk of the Andromeda galaxy, researchers have deduced that Andromeda has had a more violent past than our Milky Way. 

The structure and internal motions of the stellar disk of a spiral galaxy hold important keys to understanding the galaxy's formation history. The Andromeda galaxy, called M31 by almost no one except the 2 percent of astronomers who like to insist on renaming and reclassifying things, is the closest spiral galaxy to the Milky Way and the largest in what is called the Local Group of galaxies in our cosmic address. 

The new study 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. The Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey has used the Keck/DEIMOS multi-object spectrograph to measure radial velocities of more than 10,000 individual bright stars in Andromeda.

The recently completed Panchromatic Hubble Andromeda Treasury (PHAT) survey provides high-resolution imaging at six different wavelengths for more than half of these stars.

Dots show locations of stars in the spectroscopic survey superimposed on an image of the Andromeda galaxy (M31). Dots are color coded according to their velocity relative to the Milky Way, as measured using the DEIMOS spectrograph on the Keck II telescope. The center of M31 is moving towards us at 300 km/s, whereas stars to the northeast of the center have more positive velocities, indicating that they are moving away from us relative to M31's center.
Credit: Claire Dormanm European Space Agency

"In the Andromeda galaxy we have the unique combination of a global yet detailed view of a galaxy similar to our own. We have lots of detail in our own Milky Way, but not the global, external perspective," said Puragra Guhathakurta, professor of astronomy and astrophysics at the University of California, Santa Cruz.  

"The high resolution of the Hubble images allows us to separate stars from one another in the crowded disk of Andromeda, and the wide wavelength coverage allows us to subdivide the stars into sub-groups according to their age," said UC Santa Cruz graduate student Claire Dorman, who presented the findings today at the winter meeting of the American Astronomical Society in Seattle. The study presents the velocity dispersion of young, intermediate-age, and old stars in the disk of Andromeda, the first such measurement in another galaxy.

Dorman's analysis 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.

"If you 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," Dorman explained.

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. Previous studies have found evidence of such mergers in tidal streams of stars in the extended halo of Andromeda, which appear to be remnants of cannibalized dwarf galaxies. Stars from those galaxies can also accrete onto the disk, but accretion alone cannot account for the observed increase in velocity dispersion with stellar age, Dorman said.

This Hubble image of a crowded star field in the disk of the Andromeda galaxy shows that stars of different ages can be distinguished from one another on basis of temperature (as indicated by color) and brightness. Credit: Ben Williams, PHAT collaboration

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.

According to Dorman, a combination of these mechanisms could account for the team's observations. "Our findings should motivate theorists to carry out more detailed computer simulations of these scenarios," she said.

The comparison to the Milky Way revealed substantial differences suggesting that Andromeda has had a more violent accretion history in the recent past. "Even the most well ordered Andromeda stars are not as well ordered as the stars in the Milky Way's disk," Dorman said.

In the currently favored "Lambda Cold Dark Matter" paradigm of structure formation in the universe, large galaxies such as Andromeda and the Milky Way are thought to have grown by cannibalizing smaller satellite galaxies and accreting their stars and gas. Cosmologists predict that 70 percent of disks the size of Andromeda's and the Milky Way's should have interacted with at least one sizable satellite in the last 10,000 years. The Milky Way's disk is much too orderly for that to have happened, whereas Andromeda's disk fits the prediction much better.

"In this context, the motion of the stars in Andromeda's disk is more normal, and the Milky Way may simply be an outlier with an unusually quiescent accretion history," Guhathakurta said.

Other researchers who collaborated with Dorman and Guhathakurta on this study include Anil Seth at the University of Utah; Daniel Weisz, Julianne Dalcanton, Alexia Lewis, and Benjamin Williams at the University of Washington; Karoline Gilbert at the Space Telescope Science Institute; Evan Skillman at the University of Minnesota; Eric Bell at the University of Michigan; and Katherine Hamren and Elisa Toloba at UC Santa Cruz. This research was funded by the National Science Foundation and NASA.