Where is the Neutral Atomic Gas in Hickson Groups?

Abstract

We have analyzed the total HI contents of 72 Hickson compact groups of galaxies (HCGs) and the detailed spatial distributions and kinematics of HI within a subset of 16 groups using the high angular resolution observations obtained with the VLA in order to investigate a possible evolutionary scenario for these densest systems in the present day galaxy hierarchy. For the more homogeneous subsample of 48 groups, we found a mean HI deficiency of DefHI = 40 (40% of the expected HI for the LBs and morphological types of the member galaxies). The individual galaxies show larger degrees of deficiency than the groups globally, DefHI = 0.62 (24% of the expected HI), due in most cases to efficient gas stripping from individual galaxies into the group environment visible in the VLA maps. The degree of deficiency is found to be similar to the central galaxies of Virgo and Coma cluster, and Coma I group, in spite of the significantly different characteristics of these environments. Phase transformation of the HI should explain the deficiency. The detection rate of HCGs at X-ray wavelengths is larger for HI deficient groups. In the evolutionary scenario we propose, the amount of detected HI would decrease further with evolution, by continuous tidal stripping and/or heating. The H2 content also tends to be lower than expected for the galaxies in HI deficient groups, this may suggest that the HI stripping by frequent tidal interaction breaks the balance between the disruption of molecular clouds by star formation and the replenishment from the ambient HI.