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 } \sim 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 The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology , the University of California , and NASA .
The Observatory was made possible by the generous financial support of the W. M. Keck Foundation . /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 } \sim 50 ~ { } km~ { } s ^ { -1 } ) beyond R _ { proj } \sim 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 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 .