2012ApJ...761..186M

We present a wide (8.°5x6.°7, 1050x825 kpc), deep (σ_H1 = 10^16.8 - 10^17.5/cm2) neutral hydrogen (H I) map of the M101 galaxy group. We identify two new H I sources in the group environment, one an extremely low surface brightness (and hitherto unknown) dwarf galaxy, and the other a starless H I cloud, possibly primordial in origin. Our data show that M101's extended H I envelope takes the form of a ∼100 kpc long tidal loop or plume of H I extending to the southwest of the galaxy. The plume has an H I mass of ∼10⁸ M_☉ and a peak column density of iN_H1 = 5x10¹⁷/cm2, 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 ∼10⁷ M_☉, 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 µ_B= 29.5. We also trace H I at intermediate velocities between M101 and NGC 5474, strengthening the case for a recent interaction between the two galaxies. The kinematically complex H I 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.