Deep HST/ACS Photometry of the M81 Halo
  We present a deep color-magnitude diagram for individual stars in the halo of
the nearby spiral galaxy M81, at a projected distance of 19 kpc, based on data
taken with the Advanced Camera for Surveys on the Hubble Space Telescope (HST).
The color magnitude diagram reveals a red giant branch that is narrow and
fairly blue, and a horizontal branch that has stars that lie mostly redward of
the RR Lyrae instability strip. We derive a mean metallicity of [M/H] = -1.15
+\- 0.11 and age of 9 +\- 2 Gyr for the dominant population in our field, from
the shape of the red giant branch, the magnitude of the red clump, and the
location of the red giant branch bump. We compare our metallicity and age
results with those found previously for stars in different locations within
M81, and in the spheroids of other nearby galaxies.
