We present the discovery of a kinematically-cold stellar population along the southeastern minor axis of the Andromeda galaxy ( M31 ) that is likely the forward continuation of M31 ’ s giant southern stream .
This discovery was made in the course of an on-going spectroscopic survey of red giant branch ( RGB ) stars in M31 using the DEIMOS instrument on the Keck II 10-m telescope Data presented herein were obtained at the W. M. Keck Observatory , which is operated as a scientific partnership among the California Institute of Technology , the University of California and the National Aeronautics and Space Administration .
The Observatory was made possible by the generous financial support of the W. M. Keck Foundation . .
Stellar kinematics are investigated in eight fields located 9 – 30 kpc from M31 ’ s center ( in projection ) .
A likelihood method based on photometric and spectroscopic diagnostics is used to isolate confirmed M31 RGB stars from foreground Milky Way dwarf stars : for the first time , this is done without using radial velocity as a selection criterion , allowing an unbiased study of M31 ’ s stellar kinematics .
The radial velocity distribution of the 1013 M31 RGB stars shows evidence for the presence of two components .
The broad ( hot ) component has a velocity dispersion of \sigma ^ { sph } _ { v } =129 km s ^ { -1 } and presumably represents M31 ’ s virialized spheroid .
A significant fraction ( 19 % ) of the population is in a narrow ( cold ) component centered near M31 ’ s systemic velocity with a velocity dispersion that decreases with increasing radial distance , from \sigma ^ { sub } _ { v } = 55.5 km s ^ { -1 } at R _ { proj } = 12 kpc to \sigma ^ { sub } _ { v } = 10.6 km s ^ { -1 } ( an intrinsic velocity dispersion of 9.5 km s ^ { -1 } after accounting for velocity measurement error ) at R _ { proj } = 18 kpc .
The spatial and velocity distribution of the cold component matches that of the “ Southeast shelf ” predicted by the Fardal et al .
( 21 ) orbital model of the progenitor of the giant southern stream .
The metallicity distribution of the cold component matches that of the giant southern stream , but is about 0.2 dex more metal rich on average than that of the hot spheroidal component .
We discuss the implications of our discovery on the interpretation of the intermediate-age spheroid population found in this region in recent ultra-deep HST imaging studies .