Andromeda Galaxy has a history of violence

Chuck Bednar for – Your Universe Online

Detailed analysis of different stellar populations in the disk of Andromeda suggests the galaxy may have had a far more violent history of mergers with smaller galaxies than the Milky Way, researchers from the University of California, Santa Cruz report in a new study.

As UC Santa Cruz graduate student Claire Dorman and professor Puragra Guhathakurta explain, the structure and internal motions of a spiral galaxy’s stellar disk can hold vital clues to better understand how its formed. Andromeda, which is also known as M31, is the closest spiral galaxy to the Milky Way, as well as the largest one in the local group of galaxies.

“In the Andromeda Galaxy we have the unique combination of a global yet detailed view of a galaxy similar to our own,” explained Guhathakurta, a professor of astronomy and astrophysics. “We have lots of detail in our own Milky Way, but not the global, external perspective.”

The researchers combined data from two large-scale surveys of stars in Andromeda: the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo (SPLASH) and the Panchromatic Hubble Andromeda Treasury (PHAT).

SPLASH, which was conducted at the W. M. Keck Observatory in Hawaii, measured the radial velocities of over 10,000 individual bright stars in Andromeda using a multi-object spectrograph. PHAT, on the other hand, was obtained using the Hubble Space Telescope and provided high-resolution imaging at six different wavelengths for more than half of these stars.

“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 Dorman, who presented her findings Thursday at the winter meeting of the American Astronomical Society in Seattle.

The research presents the velocity dispersion of young, intermediate-age, and old stars in the disk of Andromeda, marking the first time such measurements were collected in another galaxy. In addition, it reveals a noticeable trend linked to stellar age.

Younger stars demonstrated relatively ordered movement around the center of the galaxy, while older stars display far more disordered motion, the study authors found. Stars in a well-ordered population all move coherently and at approximately the same velocity, while those in a disordered population have a wider range of velocities, suggesting 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,” said Dorman. She and Guhathakurta considered different possible scenarios of galactic disk formation and evolution which could help explain their observations.

One such scenario centers around the gradual disturbance of a well-ordered disk of stars as a result of mergers with small satellite galaxies. Previous research uncovered evidence suggesting these types of mergers in tidal streams of stars occurred in the extended halo of Andromeda.

Stars there appear to be the remnants of cannibalized dwarf galaxies, the researchers explained, and stars from those galaxies can also accrete onto the disk. However, Dorman noted that accretion also cannot account for the observed increase in velocity dispersion with stellar age/

A second scenario suggests that the stellar disk formed as an initially thick and clumpy disk of gas gradually settled. The oldest starts there would have formed before the gas disk entered a thin, orderly configuration. After it settled down and its motion become more ordered, then the younger starts would have formed with the disk in a more ordered configuration.

Dorman suggested that a combination of these mechanisms could account for their observations, and that their findings should encourage theoretical researchers to conduct detailed simulations of these different scenarios.


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