HI Clouds in the M81 Filament as Dark Matter Minihalos--A Phase-Space
  Mismatch
  Cosmological galaxy formation models predict the existence of dark matter
minihalos surrounding galaxies and in filaments connecting groups of galaxies.
The more massive of these minihalos are predicted to host HI gas that should be
detectable by current radio telescopes such as the GBT. We observed the region
including the M81/M82 and NGC 2403 galaxy groups, searching for observational
evidence of an HI component associated with dark matter halos within the "M81
Filament", using the Robert C. Byrd Green Bank Telescope (GBT). The map covers
an 8.7 degree x 21.3 degree (480 kpc x 1.2 Mpc) region centered between the
M81/M82 and NGC 2403 galaxy groups. Our observations cover a wide velocity
range, from -890 to 1320 km/s, which spans much of the range predicted by
cosmological N-body simulations for dark matter minihalo velocities. Our search
is not complete in the velocity range -210 to 85 km/s, containing Galactic
emission and the HVC Complex A. For an HI cloud at the distance of M81, with a
size < 10 kpc, our average 5-sigma mass detection limit is 3.2 x 10^6 M_Sun,
for a linewidth of 20 km/s. We compare our observations to two large
cosmological N-body simulations and find that the simulation predicts a
significantly greater number of detectable minihalos than are found in our
observations, and that the simulated minihalos do not match the phase space of
observed HI clouds. These results place strong constraints on the HI gas that
can be associated with dark-matter halos. Our observations indicate that the
majority of extragalactic HI clouds with a mass greater than 10^6 M_Sun are
likely to be generated through tidal stripping caused by galaxy interactions.
