HI Clouds in the M81 Filament as Dark Matter Minihalos--A Phase-Space Mismatch

TL;DR

This study used the GBT to search for HI gas in dark matter minihalos within the M81 Filament, finding fewer detectable clouds than simulations predict, suggesting most HI clouds are likely due to galaxy interactions.

Contribution

First observational test of dark matter minihalos in the M81 Filament using GBT, comparing results with cosmological simulations to constrain HI gas in dark matter halos.

Findings

Fewer HI clouds detected than predicted by simulations.

Most extragalactic HI clouds with >10^6 M_Sun likely result from tidal interactions.

Simulated minihalos do not match observed phase space distributions.

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 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.