Isolating Red Giant Stars in M31's Elusive Outer Spheroid

Abstract

Deep UBRI images of a 15'x15' field in the M31 galaxy's outer spheroid obtained with the KPNO 4-m telescope, and Keck 10-m telescope+LRIS I-band images, are used to isolate candidate red giant branch stars located at R=19 kpc (projected) along the minor axis. These stars are distinguished from the more numerous distant field galaxies by isolating objects whose colors are consistent with the long but narrow locus occupied by RGB stars in UBRI color space and whose angular sizes are consistent with the stellar PSF (FWHM~0.6-1.5). We carry out the same analysis of data on a comparison field with a similar Galactic latitude to the M31 halo field. The color-magnitude diagram of comparison field objects is well described by a superposition of foreground Galactic dwarf stars against a backdrop of faint blue field galaxies, while the M31 halo field contains a clear excess of faint red objects (I~20-23, B-I~2-3.5) in addition to these two components. The CMD location of this population of faint red objects is as would be expected for RGB stars at the distance of M31. The surface density of RGB candidates in the R=19 kpc M31 halo field is consistent with the findings of two recent HST studies. The data indicate that M31's stellar halo is much denser/larger than that of the Galaxy: (\rho_M31/\rho_MW) (\Lambda/1.5)^(-\nu) ~ 10, where \Lambda is the ratio of M31's and the Galaxy's radial scale lengths and \nu=-3.8 is the assumed power law index of the density profile. The color and slope of the RGB in M31's outer halo are suggestive of a relatively metal-rich population, [Fe/H] >~ -1, in agreement with the HST measurements.