A Kinematically Selected, Metal‐poor Stellar Halo in the Outskirts of M31

We present evidence for a metal-poor, [Fe/H]$\sim-1.4$ $\sigma$=0.2 dex,stellar halo component detectable at radii from 10 kpc to 70 kpc, in ournearest giant spiral neighbor, the Andromeda galaxy. This metal-poor sampleunderlies the recently-discovered extended rotating component, and has nodetected metallicity gradient. This discovery uses a large sample of 9861radial velocities of Red Giant Branch (RGB) stars obtained with the Keck-IItelescope and DEIMOS spectrograph, with 827 stars with robust radial velocitymeasurements isolated kinematically to lie in the halo component primarily bywindowing out the extended rotating component which dominates the photometricprofile of Andromeda out to $<$50 kpc (de-projected). The stars lie in 54spectroscopic fields spread over an 8 square degree region, and are expected tofairly sample the halo to a radius of $\sim$70 kpc. The halo sample shows nosignificant evidence for rotation. Fitting a simple model in which the velocitydispersion of the component decreases with radius, we find a central velocitydispersion of $152\kms$ decreasing by $-0.90\kms/\kpc$. By fitting acosmologically-motivated NFW halo model to the halo stars we constrain thevirial mass of M31 to be greater than $9.0 \times 10^{11} \msun$ with 99%confidence. The properties of this halo component are very similar to thatfound in our Milky Way, revealing that these roughly equal mass galaxies mayhave led similar accretion and evolutionary paths in the early Universe.Comment: 13 pages, 12 figures, accepted in ApJ. substantially revised versio