Paper: astro-ph/0512161
Date: Tue, 6 Dec 2005 21:11:44 GMT (843kb)

Title: Kinematics and Metallicity of M31 Red Giants: The Giant Southern Stream
and Discovery of a Second Cold Component at R = 20 kpc

Authors: J. S. Kalirai, P. Guhathakurta, K. M. Gilbert (UCSC), D. B. Reitzel
(UCLA), S. R. Majewski (UVa), R. M. Rich (UCLA), & M. C. Cooper (UCB)

Comments: 32 pages, 13 figures, 1 table. Accepted for publication in ApJ
\\
We present spectroscopic observations of red giant branch (RGB) stars in the
Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck
II 10-m telescope. The three fields targeted in this study are in the M31
spheroid, outer disk, and giant southern stream. In this paper, we focus on the
kinematics and chemical composition of RGB stars in the stream field located at
a projected distance of R = 20 kpc from M31's center. A mix of stellar
populations is found in this field. M31 RGB stars are isolated from Milky Way
dwarf star contaminants using a variety of spectral and photometric
diagnostics. The radial velocity distribution of RGB stars displays a clear
bimodality -- a primary peak centered at v = -513 km/s and a secondary one at v
= -417 km/s -- along with an underlying broad component that is presumably
representative of the smooth spheroid of M31. Both peaks are found to be
dynamically cold with intrinsic velocity dispersions of sigma(v) = 16 km/s. The
mean metallicity and metallicity dispersion of stars in the two peaks is also
found to be similar: [Fe/H] = -0.45 and sigma([Fe/H]) = 0.2. The observed
velocity of the primary peak is consistent with that predicted by dynamical
models for the stream, but there is no obvious explanation for the secondary
peak. The nature of the secondary cold population is unclear: it may represent:
(1) tidal debris from a satellite merger event that is superimposed on, but
unrelated to, the giant southern stream; (2) a wrapped around component of the
giant southern stream; (3) a warp or overdensity in M31's disk at R > 50 kpc
(this component is well above the outward extrapolation of the smooth
exponential disk brightness profile).

\\ ( http://arXiv.org/abs/astro-ph/0512161 , 843kb)