Kinematics Of M31's Inner Spheroid Using SPLASH and PHAT Data

International audienceOur proximity to and external view of the Andromeda galaxy (M31) make it an excellent testbed for probing the relationship between structural subcomponents (e.g., disk and spheroid) of a large spiral galaxy. The combination of large size, high metallicity, and Sersic surface brightness profi le (with index n_Sersic 2-4) of M31's spheroid suggest that it is unlike any subcomponent of the Milky Way. We use resolved stellar kinematics from Keck/DEIMOS spectra of 5600 red giant branch stars to disentangle M31's spheroid from its stellar disk. Kinematical evidence is presented for the northern extension of the Giant Southern Stream. We show that accounting for the stream and associated tidal debris, in addition to a locally cold stellar disk, yields a systematic rotation pattern in the spheroid. We measure the mean velocity and dispersion of the spheroid in each of five spatial bins. We detect significant rotation beyond r_proj 10 kpc. This is the first time that rotation has been detected in the spheroid past r_proj 1 kpc. The velocity dispersion decreases from 150 km/s at r_proj = 4 kpc to 115 km/s at r_proj = 14 kpc, consistent to 2sigma with previous measurements as well as with a bulge/halo model. We also calculate the probability that a given star is a member of the spheroid and that the spheroid has a signi ficant contribution at all radii in our sample along both the major and minor axes. Lastly, we show that the v_rot/sigma more closely resembles that of a so-called "fast rotating" elliptical galaxy than of a rotationally-flattened spiral galaxy bulge, but it should be cautioned that our M31 spheroid measurements are much further out (in units of effective radius) than for the comparison samples