The HI environment of the M101 group
  We present a wide (8.5x6.7 degree, 1050x825 kpc), deep
(sigma(N_HI)=10^(16.8-17.5) cm^-2) neutral hydrogen (HI) 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 (Huchtmeier & Witzel 1979) takes the form of a ~100
kpc long tidal loop or plume of HI extending to the southwest of the galaxy.
The plume has an HI mass ~ 10^8 Msun and a peak column density of N_HI=5x10^17
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 HI clouds near the plume with masses ~ 10^7 Msun,
similar to HI 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.
