A Hyper Suprime-Cam View of the Interacting Galaxies of the M81 Group

TL;DR

This study uses Hyper Suprime-Cam imaging to map the M81 group, revealing detailed stellar distributions, young star formations, and extended halos, highlighting complex interactions and structures within the galaxy group.

Contribution

First wide-field deep photometric survey of the M81 group showing detailed stellar distributions, young star associations, and extended halos linked to recent gravitational interactions.

Findings

Young stars follow HI gas distribution and are found in streams and associations.

Extended halos of RGB stars around M81, M82, and NGC 3077 are revealed.

Disturbed morphologies of halos indicate recent gravitational interactions.

Abstract

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 $\sim2$ magnitudes below the tip of the red giant branch (RGB) and reveals the spatial distribution of both old and young stars over an area of $\sim 100\times115$ kpc at the distance of M81. The young stars ($\sim30-160$ 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 ($\geq 2\times \rm{R_{25}}$) 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, NGC\,3077 and the inner halo of M82 have the similar $(g-i)_{0}$ 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.