AAS 205th Meeting, 9-13 January 2005
Session 141 Our Friendly Neighbors: M31 and M33
Poster, Thursday, January 13, 2005, 9:20am-4:00pm, Exhibit Hall

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[141.10] The Subdominant Halo of M31

G. Worthey, A. España, L. A. MacArthur, S. Courteau ()

Hubble Space Telescope imaging of 11 fields in M31 sampling all galactocentric radii to 50 kpc (~9 disk scale lengths, or >99.9% of the total light enclosed) were reduced to abundance distributions with errors of order 0.1 dex from red giant colors. The radially sampled abundance distributions are all about the same width, but show a mild abundance gradient that flattens to a level of [M/H]~-0.5 outside ~20 kpc. This is a factor of ten more metal-rich than the Milky Way halo, ten times more metal-rich than the M31 dwarf spheroidals, and more metal-rich than most of the M31 globular clusters at similar galactocentric radii. Difficulties of interpretation are greatly eased if we posit that the M31 disk dominates over the halo at all radii out to 50 kpc.

In fact, scaling from current density models of the Milky Way, one should not expect to see halo stars dominating over disk stars until beyond our 50 kpc limit. Quite possibly, halo stars are less numerous than disk stars at all radii. Additional support for disk dominance comes from literature studies of planetary nebula kinematics and star counts. Corollary conclusions are that most published studies of the M31 ``halo'' are actually studies of its disk, and that halo stars will have to be isolated the old-fashioned way: via kinematics of individual stars.

This work was supported by Space Telescope Science Institute.

The author(s) of this abstract have provided an email address for comments about the abstract: gworthey@wsu.edu

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