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 \approx 0.35 \pm 0.1 .
The total stellar mass of the halo is estimated to be M _ { \mathrm { halo } } \sim 2.5 \pm 1.5 \times 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 \pm 0.6 , corresponding to a three-dimensional power law index of \sim - 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 .