Paper: astro-ph/0605170
Date: Sat, 6 May 2006 04:52:36 GMT (223kb)

Title: The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)

Authors: J. S. Kalirai, K. M. Gilbert, P. Guhathakurta, S. R. Majewski, J. C.
Ostheimer, R. M. Rich, M. C. Cooper, D. B. Reitzel, R. J. Patterson

Comments: Accepted for publication in ApJ on May 4th, 2006 (submitted on Jan
30, 2006). 16 pages, 13 figures, 3 tables
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We present spectroscopic observations of red giant branch (RGB) stars over a
large expanse in the halo of the Andromeda spiral galaxy (M31), acquired with
the DEIMOS instrument on the Keck II 10-m telescope. Using a combination of
five photometric/spectroscopic diagnostics -- (1) radial velocity, (2)
intermediate-width DDO51 photometry, (3) Na I equivalent width (surface gravity
sensitive), (4) position in the color-magnitude diagram, and (5) comparison
between photometric and spectroscopic [Fe/H] estimates -- we isolate over 250
bona fide M31 bulge and halo RGB stars located in twelve fields ranging from R
= 12-165kpc from the center of M31 (47 of these stars are halo members with R >
60 kpc). We derive the photometric and spectroscopic metallicity distribution
function of M31 RGB stars in each of these fields. The mean of the resulting
M31 spheroid (bulge and halo) metallicity distribution is found to be
systematically more metal-poor with increasing radius, shifting from <[Fe/H]> =
-0.47+/-0.03 (sigma = 0.39) at R < 20 kpc to <[Fe/H]> = -0.94+/-0.06 (sigma =
0.60) at R ~ 30 kpc to <[Fe/H]> = -1.26+/-0.10 (sigma = 0.72) at R > 60 kpc,
assuming [alpha/Fe] = 0.0. These results indicate the presence of a metal-poor
RGB population at large radial distances out to at least R = 160 kpc, thereby
supporting our recent discovery of a stellar halo in M31: its halo and bulge
(defined as the structural components with R^{-2} power law and de Vaucouleurs
R^{1/4} law surface brightness profiles, respectively) are shown to have
distinct metallicity distributions. If we assume an alpha-enhancement of
[alpha/Fe] = +0.3 for M31's halo, we derive <[Fe/H]> = -1.5+/-0.1 (sigma =
0.7). Therefore, the mean metallicity and metallicity spread of this newly
found remote M31 RGB population are similar to those of the Milky Way halo.

\\ ( http://arXiv.org/abs/astro-ph/0605170 , 223kb)
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Paper: astro-ph/0605171
Date: Mon, 8 May 2006 17:55:15 GMT (579kb)

Title: The Outer Halo of M31: A New Method for Isolating Red Giant Stars and a
Measurement of the Brightness Profile and Metallicity Distribution

Authors: Karoline M. Gilbert, Puragra Guhathakurta, Jasonjot S. Kalirai, R.
Michael Rich, Steven R. Majewski, James C. Ostheimer, David B. Reitzel, A.
Javier Cenarro, Michael C. Cooper, Carynn Luine, and Richard J. Patterson

Comments: Submitted to the Astrophysical Journal: 55 pages, including 4
embedded tables and 21 figures
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We present a method for isolating a clean sample of red giant branch stars in
the outer regions of the Andromeda spiral galaxy (M31) from an ongoing
spectroscopic survey using the DEIMOS instrument on the Keck~II 10-m telescope.
The survey aims to study the kinematics, global structure, substructure, and
metallicity of M31's halo. Although most of our spectroscopic targets were
photometrically screened to reject foreground Milky Way dwarf star
contaminants, the latter class of objects still constitutes a substantial
fraction of the observed spectra in the sparse outer halo. Our likelihood-based
method for isolating M31 red giants uses multiple criteria: (1) radial
velocity, (2) intermediate-width band photometry through the DDO51 filter
centered on the surface-gravity sensitive MgH/Mg b absorption features, (3)
strength of the Na I 8190 Angstrom absorption line doublet, (4) location within
an (I, V-I) color-magnitude diagram, and (5) comparison of photometric versus
spectroscopic metallicity estimates. Training sets consisting of 462 definite
M31 red giants and 169 Galactic dwarf stars are used to derive empirical
probability distribution functions for each diagnostic. These functions are
used to calculate the likelihood that a given star is a red giant branch star
in M31 versus a Milky Way dwarf star. The ability to identify individual stars
as M31 red giants gives us an unprecedented level of sensitivity in studying
the properties of the galaxy's outer halo. We present the surface brightness
profile and metallicity distribution of the clean sample of outer halo M31 red
giants.

\\ ( http://arXiv.org/abs/astro-ph/0605171 , 579kb)
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Paper: astro-ph/0605172
Date: Mon, 8 May 2006 03:32:21 GMT (450kb)

Title: Discovery of an Extended Halo of Metal-poor Stars in the Andromeda
Spiral Galaxy

Authors: Puragra Guhathakurta, James C. Ostheimer, Karoline M. Gilbert, R.
Michael Rich, Steven R. Majewski, Jasonjot S. Kalirai, David B. Reitzel,
Michael C. Cooper, and Richard J. Patterson

Comments: Submitted to ApJL on May 5 2006, 3 figures
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We report here on the discovery of an extended halo of metal-poor red giant
branch (RGB) stars in the Andromeda spiral galaxy (M31). Our ongoing survey of
M31 includes wide-field deep optical images in the intermediate-width DDO51
band and Washington system M and T_2 bands obtained with the Kitt Peak National
Observatory 4-m telescope and Mosaic camera. The DDO51 band allows us to screen
M31 RGB star candidates from foreground Galactic dwarf star contaminants. The
imaging is followed up with spectroscopy using the Keck II 10-m telescope and
DEIMOS. A combination of photometric and spectroscopic diagnostics is used to
reliably isolate M31 RGB stars; these stars are seen in all of our fields out
to a projected distance of 165 kpc from M31's center. These newly discovered
RGB stars represent the hitherto elusive stellar halo of M31. The surface
brightness of M31 beyond r > 30 kpc is characterized by a power law, r^{-2.6
+/- 0.3}, distinct from the Se'rsic profile that characterizes its extended
bulge. Our data show that M31 is 3-5 times larger than any of its previously
mapped spheroidal/disk components. Together, the Galactic and M31 halos span >
1/3 of the distance between them, suggesting that stars occupy a substantial
volume fraction of our Local Group, and possibly most galaxy groups.

\\ ( http://arXiv.org/abs/astro-ph/0605172 , 450kb)