H I CLOUDS IN THE M81 FILAMENT AS DARK MATTER MINIHALOS—A PHASE-SPACE MISMATCH

Abstract

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 87 × 213 (480 kpc × 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−1, 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−1, 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 3.2 × 106 M☉, for a linewidth of 20 km s−1. 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.