  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.
