A SEARCH FOR DIFFUSE NEUTRAL HYDROGEN AND H I CLOUDS IN THE NGC 2403 GROUP

We have observed the NGC 2403 group of galaxies using the Robert C. Byrd Green Bank Telescope 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 H I cloud with a size ≤10 kpc within 50 kpc of a group galaxy, our 7σ mass detection limit is 2.2 x 106 M sun for a cloud with a line width of 20 km s-1, over the velocity range from -890 to 1750 km s-1. At this sensitivity level, we detect three new H I 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-1, 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 H I clouds exist in the NGC 2403 group, their masses are less than 2.2 x 106 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 H I content is less than 1% of their total mass.