Detection of Diffuse Neutral Intragroup Medium in Hickson Compact Groups
  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.
