Intergalactic Stellar Distributions in the Interacting M81/M82 Galaxy Group

Previous H I observations of the M81/M82/NGC 3077 galaxy group clearly show a widespread H I distribution within this galaxy group. While the gas is vulnerable to tidal disruption from a galaxy encounter, are there also stars embedded in this H I distribution? Our deep, 1 deg2 exposures of the M81/M82 group in 10 optical bands using the Beijing-Arizona-Taipei-Connecticut (BATC) filter set clearly reveal widespread stellar distributions that coincide with the atomic hydrogen clouds—considered to be the relics of the merging process of the galaxies—splayed over the region. The spectral energy distributions of the stellar groups to the east and west of M81 (including the Arp Loop) are similar to that measured at the southeast edge of the optical disk of M82. This similarity in stellar radiation, combined with the observed peculiar rotational velocity of M82, suggests that the diffuse stellar population in the intergalactic space around M81 is possibly a relic of the tidally disrupted disk of M82 during the last close encounter. Alternately, the stars could have formed in situ in the H I as it was drawn out of the galaxies. Recent measurements of distances to and radial velocities of M81 (3.63 Mpc and 48 km s-1, respectively) and M82 (3.9 Mpc and 296 km s-1) lend further support to the notion of a close passage between these two galaxies several hundred million years ago.