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.Comment: 15 pages, 7 figures. Accepted for publication in the AJ with minor revision

The Origin of Neutral Hydrogen Clouds in Nearby Galaxy Groups: Exploring the Range Of Galaxy Interactions

We combine high resolution N-body simulations with deep observations of neutral hydrogen (HI) in nearby galaxy groups in order to explore two well-known theories of HI cloud formation: HI stripping by galaxy interactions and dark matter minihalos with embedded HI gas. This paper presents new data from three galaxy groups, Canes Venatici I, NGC 672, and NGC 45, and assembles data from our previous galaxy group campaign to generate a rich HI cloud archive to compare to our simulated data. We find no HI clouds in the Canes Venatici I, NGC 672, or NGC 45 galaxy groups. We conclude that HI clouds in our detection space are most likely to be generated through recent, strong galaxy interactions. We find no evidence of HI clouds associated with dark matter halos above M_HI = 10^6 M_Sun, within +/- 700 km/s of galaxies, and within 50 kpc projected distance of galaxies.Comment: 35 pages, 10 figures, AJ accepte

Search for Diffuse Neutral Hydrogen and HI Clouds in the NGC 2403 Group

We have observed the NGC 2403 group of galaxies using the Robert C. Byrd Green Bank Telescope (GBT) in a search for faint, extended neutral hydrogen clouds similar to the clouds found around the M81/M82 group, which is located approximately 250 kpc from the NGC 2403 group along the same filament of galaxies. For an HI cloud with a size < 10 kpc within 50 kpc of a group galaxy, our 7-sigma mass detection limit is 2.2 x 10^6 M_sun for a cloud with a linewidth of 20 km/s, over the velocity range from -890 to 1750 km/s. At this sensitivity level we detect 3 new HI clouds in the direction of the group, as well as the known galaxies. The mean velocity of the new clouds differs from that of the group galaxies by more than 250 km/s, but are in the range of Milky Way High Velocity Clouds (HVCs) in that direction. It is most likely that the clouds are part of the Milky Way HVC population. If HI clouds exist in the NGC 2403 group, their masses are less than 2.2 x 10^6 M_sun. We also compared our results to structures that are expected based on recent cosmological models, and found none of the predicted clouds. If NGC 2403 is surrounded by a population of dark matter halos similar to those proposed for the Milky Way in recent models, our observations imply that their HI content is less than 1% of their total mass.Comment: Accepted by A

Neutral Hydrogen Clouds in the M81/M82 Group

We have observed a 3 degree x 3 degree area centered on the M81/M82 group of galaxies using the Robert C. Byrd Green Bank Telescope (GBT) in a search for analogs to the High Velocity Clouds (HVCs) of neutral hydrogen found around our galaxy. The velocity range from -605 to -85 km/s and 25 to 1970 km/s was searched for HI clouds. Over the inner 2 degrees x 2 degrees the 7-sigma detection threshold was 9.6 x 10^5 M_sun. We detect 5 previously unknown HI clouds associated with the group, as well as numerous associated filamentary HI structures, all lying in the range -105 < V_helio < +280 km/s. From the small angular distance of the clouds to group members, and the small velocity difference between group members and clouds, we conclude that the clouds are most likely relics of ongoing interactions between galaxies in the group.Comment: 17 pages, 5 figures, AJ accepte