DISTRIBUTION OF FAINT ATOMIC GAS IN HICKSON COMPACT GROUPS

We present 21 cm H (I) observations of four Hickson Compact Groups (HCGs) with evidence for a substantial intragroup medium using the Robert C. Byrd Green Bank Telescope (GBT).(5) By mapping H (I) emission in a region of 25' x 25' (140-650 kpc) surrounding each HCG, these observations provide better estimates of H (I) masses. In particular, we detected 65% more H (I) than that detected in the Karl G. Jansky Very Large Array (VLA) imaging of HCG 92. We also identify whether the diffuse gas has the same spatial distribution as the high. surface brightness (HSB) H (I) features detected in the VLA maps of these groups by comparing the H (I) strengths between the observed and modeled masses based on VLA maps. We found that the H (I) observed with the GBT has a similar spatial distribution to the HSB structures in HCG. 31 and HCG 68. Conversely, the observed H (I) distributions in HCG. 44 and HCG 92 were extended and showed significant offsets from the modeled masses. Most of the faint gas in HCG 44 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 H (I) 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 Myr.