Department of Physics, Engineering Physics & Astronomy

Department of Physics, Engineering Physics & Astronomy
Department of Physics, Engineering Physics & Astronomy

The Stellar Populations of the Andromeda Spiral Galaxy

Dr. Jason Kalirai

Monday, October 30, 2006
12:30 PM @ Stirling 201


Simulations of hierarchical galaxy formation suggest that large galaxies such as the Milky Way and M31 should contain extended stellar halos that are chemically distinct (more metal poor) from the inner bulge. Yet, two decades of intensive effort had failed to reveal any such population in our nearest neighbor, M31. We report on the results from a photometric and spectroscopic survey of red giant branch (RGB) stars over a large expanse in the Andromeda spiral galaxy (M31). Using a combination of photometric and spectroscopic diagnostics, we isolate bonafide M31 RGB stars in its bulge, disk, and halo at projected distances of R = 12 - 160 kpc from the center of M31. Along the major axis out to 30 kpc, we find clear evidence for a kinematically cold, metal-rich disk-like population. Out to 30 kpc along the minor axis, we confirm earlier studies and find that M31 is dominated by a metal-rich, R^1/4 surface brightness (de Vaucouleur) profile. However, beyond this distance, the brightness profile of M31 RGB stars lies well above an outward extrapolation of the inner bulge-like profile and is consistent with an R^-2.5 surface brightness profile (i.e., an extended, power-law halo). We measure both spectroscopic and photometric metallicities for this new population and find that, in fact, the outer halo of M31 is metal-poor relative to the inner bulge. Taken together, these results suggest that the bulge to halo ratio of M31 is much larger than the Milky Way.