THE H I ENVIRONMENT OF THE M101 GROUP

We present a wide (8 degrees.5 x 6 degrees.7, 1050 x 825 kpc), deep (sigma(NHI) = 10(16.8)-10(17.5) cm(-2)) neutral hydrogen (H I) map of the M101 galaxy group. We identify two new Hi sources in the group environment, one an extremely low surface brightness (and hitherto unknown) dwarf galaxy, and the other a starless Hi cloud, possibly primordial in origin. Our data show that M101' s extended Hi envelope takes the form of a similar to 100 kpc long tidal loop or plume of Hi extending to the southwest of the galaxy. The plume has an Hi mass of similar to 10(8) M-circle dot and a peak column density of N-HI = 5 x 10(17) cm(-2), and while it rotates with the main body of M101, it shows kinematic peculiarities suggestive of a warp or flaring out of the rotation plane of the galaxy. We also find two new H I clouds near the plume with masses similar to 10(7) M-circle dot, similar to H I clouds seen in the M81/M82 group, and likely also tidal in nature. Comparing to deep optical imaging of the M101 group, neither the plume nor the clouds have any extended optical counterparts down to a limiting surface brightness of mu(B) = 29.5. We also trace Hi at intermediate velocities between M101 and NGC 5474, strengthening the case for a recent interaction between the two galaxies. The kinematically complex Hi structure in the M101 group, coupled with the optical morphology of M101 and its companions, suggests that the group is in a dynamically active state that is likely common for galaxies in group environments.