The SPLASH Survey: Kinematics of Andromeda's Inner Spheroid

TL;DR

This study uses stellar kinematics from Keck/DEIMOS spectra to analyze the inner spheroid of Andromeda, revealing significant rotation and velocity anisotropy, which suggests it resembles an elliptical galaxy more than a typical spiral bulge.

Contribution

First detailed kinematic analysis of M31's inner spheroid using MCMC on resolved stellar spectra, revealing rotation and anisotropy beyond 5 kpc.

Findings

Detection of spheroid rotation (~50 km/s) beyond 5 kpc

Velocity dispersion decreases from 140 km/s to 120 km/s between 7 and 14 kpc

Spheroid's flattening due to velocity anisotropy and rotation

Abstract

The combination of large size, high stellar density, high metallicity, and Sersic surface brightness profile of the spheroidal component of the Andromeda galaxy (M31) within R_proj ~ 20 kpc suggest 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) beyond R_proj ~ 5 kpc. The velocity dispersion decreases from about 140 km/s at R_proj = 7 kpc to 120 km/s at R_proj = 14 kpc, consistent to 2 sigma 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 - 14 r_eff) than for the comparison samples.