We study the properties of the stellar populations in M81 ’ s outermost part , which hereafter we will term the stellar halo , using Hubble Space Telescope ( HST ) Advanced Camera for Surveys observations of 19 fields from the GHOSTS survey .
The observed fields probe the stellar halo out to a projected distance of \sim 50 kpc from the galactic center .
Each field was observed in both F 606 W and F 814 W filters .
The 50 % completeness levels of the color magnitude diagrams ( CMDs ) are typically at 2 mag below the tip of the red giant branch ( TRGB ) .
Fields at distances closer than 15 kpc show evidence of disk-dominated populations whereas fields at larger distances are mostly populated by halo stars .
The RGB of the M81 ’ s halo CMDs is well matched with isochrones of \sim 10 Gyr and metallicities [ \mathrm { Fe / H } ] \sim - 1.2 dex , suggesting that the dominant stellar population of M81 ’ s halo has a similar age and metallicity .
The halo of M81 is characterized by a color distribution of width \sim 0.4 mag and an approximately constant median value of ( F 606 W - F 814 W ) \sim 1 mag measured using stars within the magnitude range 23.7 \lesssim F 814 W \lesssim 25.5 .
When considering only fields located at galactocentric radius R > 15 kpc , we detect no color gradient in the stellar halo of M81 .
We place a limit of 0.03 \pm 0.11 mag difference between the median color of RGB M81 halo stars at \sim 15 and at 50 kpc , corresponding to a metallicity difference of 0.08 \pm 0.35 dex over that radial range for an assumed constant age of 10 Gyr .
We compare these results with model predictions for the colors of stellar halos formed purely via accretion of satellite galaxies .
When we analyze the cosmologically motivated models in the same way as the HST data , we find that they predict no color gradient for the stellar halos , in good agreement with the observations .