2015ApJ...809L...1O

We present the first results of a wide-field mapping survey of the M81 group conducted with Hyper Suprime-Cam on the Subaru Telescope. Our deep photometry reaches ∼2 mag below the tip of the red giant branch (RGB) and reveals the spatial distribution of both old and young stars over an area of kpc at the distance of M81. The young stars (∼30-160 Myr old) closely follow the neutral hydrogen distribution and can be found in a stellar stream between M81 and NGC 3077 and in numerous outlying stellar associations, including the known concentrations of Arp's Loop, Holmberg IX, an arc in the halo of M82, BK3N, and the Garland. Many of these groupings do not have counterparts in the RGB maps, suggesting they may be genuinely young systems. Our survey also reveals for the first time the very extended () halos of RGB stars around M81, M82, and NGC 3077, as well as faint tidal streams that link these systems. The halos of M82 and NGC 3077 exhibit highly disturbed morphologies, presumably a consequence of the recent gravitational encounter and their ongoing disruption. While the halos of M81 and NGC 3077 and the inner halo of M82 have similar colors, the outer halo of M82 is significantly bluer indicating it is more metal poor. Remarkably, our deep panoramic view of the M81 group demonstrates that the complexity long known to be present in HI is equally matched in the low surface brightness stellar component.