Where is the neutral atomic gas in Hickson groups?

We have analyzed the total HI contents of 72 Hickson compact groups of gala xies (HCGs) and the detailed spatial distributions and kinematics of HI wit hin a subset of 16 groups using the high angular resolution observations ob tained with the VLA in order to investigate a possible evolutionary scenari o for these densest systems in the present day galaxy hierarchy. For the mo re homogeneous subsample of 48 groups, we found a mean HI deficiency of De f(HI) = 0.40 +/- 0.07, which corresponds to 40% of the expected HI for the optical luminosities and morphological types of the member galaxies. The in dividual galaxies show larger degrees of deficiency than the groups globall y, Def(HI) = 0.62 +/- 0.09 (24% of the expected HI), due in most cases to e fficient 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 (number of galaxies, veloci ty dispersion) of these environments. It does not seem plausible that a sig nificant amount of extended HI has been missed by the observations. Hence p hase transformation of the atomic gas should explain the HI deficiency. The groups richer in early type galaxies or more compact with larger velocity dispersions show a weak tendency to be more HI deficient. The detection rat e of HCGs at X-ray wavelengths is larger for HI deficient groups, although the hot gas distribution and hence its origin is only known for a few cases . In the evolutionary scenario we propose, the amount of detected HI would decrease further with evolution, by continuous tidal stripping and/or heati ng. The H-2 content also tends to be lower than expected for the galaxies i n HI deficient groups, this may suggest that the HI stripping by frequent t idal interaction breaks the balance between the disruption of molecular clo uds by star formation and the replenishment from the ambient HI.