Deep HST/ACS Photometry of the M81 Halo

TL;DR

This study uses deep HST/ACS imaging to analyze the stellar populations in the halo of galaxy M81, revealing details about its age and metallicity distribution and comparing it with other galactic halos.

Contribution

First deep color-magnitude diagram of M81's halo obtained with HST/ACS, providing detailed stellar population analysis and comparison with other galaxies.

Findings

Mean metallicity of [M/H] = -1.15 ± 0.11

Dominant population age of 9 ± 2 Gyr

Red giant branch is narrow and fairly blue

Abstract

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.