2011AJ....141....9C

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 H I gas that should be detectable by current radio telescopes such as the Robert C. Byrd Green Bank Telescope (GBT). We observed the region including the M81/M82 and NGC 2403 galaxy groups, searching for observational evidence of an H I component associated with dark matter halos within the "M81 Filament," using the GBT. The map covers an 8°.7x21°.3 (480 kpcx1.2 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 H I cloud at the distance of M81, with a size ≤ 10 kpc, our average 5σ mass detection limit is 3.2x10⁶ M_☉, 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 H I clouds. These results place strong constraints on the H I gas that can be associated with dark matter halos. Our observations indicate that the majority of extragalactic H I clouds with a mass greater than 10⁶ M_☉ are likely to be generated through tidal stripping caused by galaxy interactions.