Deep HST/ACS photometry of the M81 halo

Abstract

We present a deep color-magnitude diagram (CMD) 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 CMD reveals a red giant branch (RGB) 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 ± 2Gyr for the dominant population in our field, from the shape of the RGB, the magnitude of the red clump, and the location of the RGB 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. © 2010. The American Astronomical Society. All rights reserved.