Galactic Leftovers within the M81 Group: Understanding the Past Interactions using the Resolved Helium Burning Stellar Population

We present a new perspective on the past interaction and extra-tidal star formation within the M81 Group using a resolved star study from the Subaru Telescope Hyper Suprime-Cam. Through the use of the one-to-one age-luminosity relation of helium burning populations and theoretical Padova isochrones, the intermediate-aged red helium burning population can be used as a chronometer to map the past distribution of extra-tidal star formation between M81, M82, and NGC 3077 over the last ~300Myr. Through visualizing how these time slices of the M81 Group have evolved, we reach three conclusions. We first find that extra-tidal star formation has been occurring throughout the M81 Group over the past 300 Myrs, placing a lower limit on the group interaction timescale and the total mass of ‘in-situ’ stars formed in M81’s stellar halo in that time. We secondly find that the past distributions of stars are different from the current star formation represented by the main sequence and HI gas mapping, showing that the positions of the most intense extra-tidal star formation vary during a group interaction. Finally, there is a tidal bridge rich in red helium burning stars between M82 and NGC 3077. There is no present-day HI component or present-day star formation in this bridge. In analogy to our current understanding of the infall of the Magellanic Clouds to the Milky Way, we suggest that the most natural interpretation of this bridge is that M82 and NGC 3077 were previously interacting and fell into the M81 group as a pair.