Galaxy Archaeology of M81 group

Sakurako Okamoto

Over the last decade, resolved star studies of nearby galaxies have led to the discovery of vast stellar envelopes that are often rich in substructure. These components are naturally predicted in models of hierarchical galaxy assembly, and their observed properties place important constraints on the amount, nature, and time history of satellite accretion to large galaxies. Using wide-field cameras equipped to 8m class telescopes, it has recently become possible to extend these studies to systems beyond the Local Group. I present the results from the state-of-the-art wide-field survey of the M81 galaxy group that we are conducting with Hyper Suprime-Cam on Subaru Telescope. Our photometry reaches about 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 5 sq-deg around the M81. The young main-sequence stars closely follow the HI distribution and can be found in a stellar stream between M81 and NGC3077 and in numerous outlying stellar associations. Our survey also reveals for the first time the very extended (> 2*R25) halos of RGB stars around M81, M82, and NGC 3077, as well as faint tidal streams that link these systems. The gravitational interactions between M81, M82 and NGC 3077 galaxies induced star formation in tidally stripped gas, and also significantly perturbed the older stellar components leading to disturbed halo morphologies.