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 (M31) is the closest spiral galaxy to the Milky Way and the largest 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. We have lots of detail in our own Milky Way, but not the global, external perspective,” said Puragra Guhathakurta from the University of California, Santa Cruz.
The new study, led by Claire Dorman from UC and Guhathakurta, 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.
“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 subgroups according to their age,” said Dorman. 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.
