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.
