Distribution of Faint Atomic Gas in Hickson Compact Groups
  We present 21cm HI observations of four Hickson Compact Groups with evidence
for a substantial intragroup medium using the Robert C. Byrd Green Bank
Telescope (GBT). By mapping H I emission in a region of
25$^{\prime}\times$25$^{\prime}$ (140-650 kpc) surrounding each HCG, these
observations provide better estimates of HI masses. In particular, we detected
65% more \HI than that detected in the Karl G. Jansky Very Large Array (VLA)
imaging of HCG92. We also identify if the diffuse gas has the same spatial
distribution as the high-surface brightness (HSB) HI features detected in the
VLA maps of these groups by comparing the HI strengths between the observed and
modeled masses based on VLA maps. We found that the HI observed with the GBT to
have a similar spatial distribution as the HSB structures in HCGs 31 and 68.
Conversely, the observed HI distributions in HCGs44 and 92 were extended and
showed significant offsets from the modeled masses. Most of the faint gas in
HCG44 lies to the Northeast-Southwest region and in HCG 92 lies in the
Northwest region of their respective groups. The spatial and dynamical
similarities between the total (faint+HSB) and the HSB HI indicate that the
faint gas is of tidal origin. We found that the gas will survive ionization by
the cosmic UV background and the escaping ionizing photons from the star
forming regions and stay primarily neutral for at least 500 Myrs.
