The splash survey: Kinematics of Andromeda's inner spheroid

Abstract

The combination of large size, high stellar density, high metallicity, and Sérsic surface brightness profile of the spheroidal component of the Andromeda galaxy (M31) within R proj 20 kpc suggests that it is unlike any subcomponent of the Milky Way. In this work we capitalize on our proximity to and external view of M31 to probe the kinematical properties of this "inner spheroid." We employ a Markov chain Monte Carlo (MCMC) analysis of resolved stellar kinematics from Keck/DEIMOS spectra of 5651 red giant branch stars to disentangle M31's inner spheroid from its stellar disk. We measure the mean velocity and dispersion of the spheroid in each of five spatial bins after accounting for a locally cold stellar disk as well as the Giant Southern Stream and associated tidal debris. For the first time, we detect significant spheroid rotation (v rot 50 km s-1) beyond R proj 5 kpc. The velocity dispersion decreases from about 140 km s-1 at R proj = 7 kpc to 120 km s-1 at R proj = 14 kpc, consistent to 2σ with existing measurements and models. We calculate the probability that a given star is a member of the spheroid and find that the spheroid has a significant presence throughout the spatial extent of our sample. Lastly, we show that the flattening of the spheroid is due to velocity anisotropy in addition to rotation. Though this suggests that the inner spheroid of M31 more closely resembles an elliptical galaxy than a typical spiral galaxy bulge, it should be cautioned that our measurements are much farther out (2-14r eff) than for the comparison samples. © 2012. The American Astronomical Society. All rights reserved..