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D.B. Reitzel (University of California, Irvine), P. Guhathakurta (UCO/Lick Observatory)
Deep KPNO UBRI photometry and a color-based screening technique, in conjunction with morphological star-galaxy separation using high resolution Keck I-band images, enable us to isolate M31 halo red giant stars in a field located at a projected distance of 19~kpc from the galaxy's center along the minor axis. After screening for stars, this field displays a clear excess population relative to a comparison field, plausibly red giants at the distance of M31. Follow-up Keck LRIS spectra have been gathered for about 99 M31 halo giant candidates. Kinematical information allow for the elimination of residual contaminants (foreground Galactic dwarfs, M31 disk stars and background galaxies). The velocity distribution is well fit by an equal mix of foreground dwarfs drawn from a standard Galactic model with and giants in M31's halo represented by a gaussian of width ~150 km s-1 centered on its systemic velocity. Metallicity measurements are made on a star-by-star basis using two independent methods: the strength of the CaII absorption lines and the location in the B-I vs I color-magnitude diagram. The photometric and spectroscopic [Fe/H] estimates are in good agreement. The sample mean is \rm\langle[Fe/H]\rangle=-1.5 with stars spanning the full range of the abundance measurement calibrations, \rm-2.3 < [Fe/H] < 0.0. While there does not appear to be a strong radial abundance gradient in M31's halo, the spread of at least 2 dex in [Fe/H] even for the spectrosacopically-selected sample of secure M31 halo red giants is comparable to the [Fe/H] spread amongst Local Group dwarf satellite galaxies and M31 globular clusters. Four secure M31 stars, v<-220 km s-1, have red B-I colors and exceptionally strong CaII absorption lines indicating solar or super-solar metallicities. These stars lie within a narrow velocity range at \approx -340 km s-1. This combination of high metallicity and velocity close to M31's systemic velocity is what might be expected for M31 disk giants.
The author(s) of this abstract have provided an email address for comments about the abstract: dreitzel@uci.edu