Internal Stellar Kinematics of M32 from the SPLASH Survey: Dark Halo Constraints

As part of the SPLASH survey of the Andromeda (M31) system, we have obtained Keck/DEIMOS spectra of the compact elliptical (cE) satellite M32. This is the first resolved-star kinematical study of any cE galaxy. In contrast to most previous kinematical studies that extended out to r≾30" ~ 1 r^(eff) I ~ 100 pc, we measure the rotation curve and velocity dispersion profile out to r ~ 250" and higher order Gauss-Hermite moments out to r ~ 70". We achieve this by combining integrated-light spectroscopy at small radii (where crowding/blending are severe) with resolved stellar spectroscopy at larger radii, using spatial and kinematical information to account statistically for M31 contamination. The rotation curve and velocity dispersion profile extend well beyond the radius (r ~ 150") where the isophotes are distorted. Unlike NGC 205, another close dwarf companion of M31, M32's kinematics appear regular and symmetric and do not show obvious sharp gradients across the region of isophotal elongation and twists. We interpret M31's kinematics using three-integral axisymmetric dynamical equilibrium models constructed using Schwarzschild's orbit superposition technique. Models with a constant mass-to-light ratio can fit the data remarkably well. However, since such a model requires an increasing tangential anisotropy with radius, invoking the presence of an extended dark halo may be more plausible. Such an extended dark halo is definitely required to bind a half-dozen fast-moving stars observed at the largest radii, but these stars may not be an equilibrium component of M32

Keck Spectroscopy of M31's HST/MCT Region. I: Stellar Kinematics of the Disk and Bulge

International audienceTrends in stellar kinematics in the inner regions of the Andromeda galaxy (M31) offer insight into the structure and formation of spiral galaxies. The synthesis of the SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) and PHAT (Panchromatic Hubble Andromeda Treasury) collaborations offers an unprecedented opportunity to understand the interplay between the assembly, star formation, and chemical enrichment histories of large disk galaxies

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

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