For a few years, our group has been studying the stellar halos and satellites of nearby Milky Way-mass galaxies, with the goal of understanding the diversity in their merger histories, the effects of those mergers on their host galaxies, and the diversity of their satellite systems. We have been working for years on a beautiful dataset for the M81 group, and I’m really proud to report that Adam Smercina and the rest of the team just had a paper accepted to the Astrophysical Journal where we studied the diffuse outskirts of the M81 group using resolved stars.

We used Subaru’s Hyper Suprime-Cam to resove stellar populations around M81 in its interaction with M82 and NGC 3077, revealing M81’s stellar halo in never-before-seen detail. Using careful star-galaxy separation techniques and artificial star tests, calibrated with HST data in the survey area, we resolved the halo to unprecedented V-band equivalent surface brightnesses of 33 mag/sq. arcsec., and produced the first-ever global stellar mass density map for a Milky Way-mass stellar halo outside of the Local Group. Using the minor axis, we confirm M81’s halo as one of the lowest mass and metal-poorest known, with a stellar mass of ~1.1×10^9 solar masses, and a metallicity [Fe/H] ~ -1.2 – indicating a quiet past merger/accretion history. Our global halo census finds that tidally unbound material from M82 and NGC 3077 is currently adding significantly to M81’s halo, providing an extra 5×10^8 solar masses of material with [Fe/H] ~-0.9. We further show that, in a few Gyr, following the accretion of its massive satellite M82 (and the LMC-like NGC 3077), M81 will host one of the most massive and metal-rich stellar halos in the nearby universe, similar in mass and metallicity to the massive, metal-rich stellar halo of M31. This illustrates an essential feature of stellar halo formation and evolution – it is possible, with one ‘accretion event’ (a merger with M82 and NGC 3077) to leapfrog from a low-mass, anemic halo similar to the MW directly into a metal-rich behemoth rivaled only by systems such as M31. This dramatic transformation indicates that the observed diversity in stellar halo properties is primarily driven by diversity in the largest mergers these galaxies have experienced.

Density image of RGB stars, with intensity mapped to stellar density, where each ‘channel’ represents stars in three bins of metallicity: [Fe/H] ∼ −0.8 (red), [Fe/H] ∼ −1.2 (green), and [Fe/H] ∼ −1.8 (blue). Each channel was smoothed using first a tophat filter of size ∼20 kpc (to bring out substructure), and then a Gaussian filter of width ∼1 kpc. The interiors of M81, M82 and NGC 3077 have been filled with images from HST (credit: NASA, ESA, and the Hubble Heritage Team) – Fig. 14 from Smercina et al. 2020 (https://arxiv.org/pdf/1910.14672.pdf)