Detection of Diffuse Neutral Intragroup Medium in Hickson Compact Groups

Abstract

We present new Green Bank Telescope (GBT) 21 cm neutral hydrogen (HI) observations of a complete distance limited sample of 22 Hickson Compact Groups (HCGs) with at least four true members. We detected an average HI mass of $8 \times 10^{9} M_{\odot}$ (median= $6\times 10^{9} M_{\odot}$), which is significantly larger than previous single-dish measurements. Consequently, the HI-deficiencies for these HCGs have been reduced, although not completely eliminated. Spectral comparison of the GBT data with complementary Very Large Array (VLA) data shows significant HI excess in the GBT spectra. The observed excess is primarily due to the high surface brightness sensitivity of the GBT detecting diffuse, low column density HI in these groups. The excess gas forms a faint diffused neutral medium which is an intermediate stage in the evolution of high-surface brightness HI tidal debris in the intra-group medium (IGM) before it is fully ionized. The excess gas mass fraction (ratio of excess HI to total HI) for our complete sample varies from 5% to 81% with an average of 36% (median=30%). The excess gas mass fraction is highest in slightly HI deficient groups where the tidal debris has had enough time to evolve. We also find the excess gas content increases with the evolutionary phase of the group described in Verdes-Montenegro et al. 2001. Theoretical calculations indicate that an HI cloud of radius >200 pc would survive in an IGM of 2 million Kelvin for more than the typical dynamical lifetime of a group. However, smaller clouds get evaporated and assimilated into the hot IGM in a much shorter timescale.