The Resolved Stellar Halo of NGC 253
  We have obtained Magellan/IMACS and HST/ACS imaging data that resolve red
giant branch stars in the stellar halo of the starburst galaxy NGC 253. The HST
data cover a small area, and allow us to accurately interpret the ground-based
data, which cover 30% of the halo to a distance of 30 kpc, allowing us to make
detailed quantitative measurements of the global properties and structure of a
stellar halo outside of the Local Group. The geometry of the halo is
significantly flattened in the same sense as the disk, with a projected axis
ratio of b/a ~ 0.35 +/- 0.1. The total stellar mass of the halo is estimated to
be M_halo ~ 2.5 +/- 1.5 x 10^9 M_sun, or 6% of the total stellar mass of the
galaxy, and has a projected radial dependence that follows a power law of index
-2.8 +/- 0.6, corresponding to a three-dimensional power law index of ~ -4. The
total luminosity and profile shape that we measure for NGC 253 are somewhat
larger and steeper than the equivalent values for the Milky Way and M31, but
are well within the scatter of model predictions for the properties of stellar
halos built up in a cosmological context. Structure within the halo is seen at
a variety of scales: there is small kpc-scale density variation and a large
shelf-like feature near the middle of the field. The techniques that have been
developed will be essential for quantitatively comparing our upcoming larger
sample of observed stellar halos to models of halo formation.
