The definition of Hickson compact groups (HCG) by the number of members (N 4) within three magnitudes of the brightest galaxy (), compactness, and degree of isolation (Hickson 1982, 1993) could not decide on the membership of individual galaxies. When redshifts became available for almost all galaxies in HCGs (Hickson 1993) it became evident that most of these compact associations of galaxies were physical groups with only a few by chance projections of field galaxies in the foreground or background. A study of the environment of HCGs (Palumbo et al. 1995) revealed that most compact groups are isolated, only 16% of them seem to be the compact inner parts of larger groups. The mean fraction of spiral galaxies in HCG's is lower than in the field. These results contradict the view that most compact groups are the result of chance alignments. Hickson compact groups are among the densest aggregates in the universe. As such, these systems should be the ideal place to look for galaxies in interaction. Roughly 7% of HCG spirals show tidal tails, which is a signature of strong dynamical interaction. Another claim for interaction is drawn from radio properties of spiral galaxies in HCGs. The radio continuum from the nuclear regions is more than 10 times that from comparable regions in a comparison sample (Menon 1995). Massive inflows of gas towards the centres of galaxies would result in star formation activity at the center yielding supernovae and the subsequent radio emission. However the number of identified merger candidates is extremely low, and although star formation is enhanced with respect to isolated galaxies, the level is lower than in pairs (Moles et al. 1994, Sulentic and Rabaca 1994, Zepf et al. 1991).
There is an increasing amount of theoretical and observational work on Hickson groups (e.g. Mamon 1995). Many of the Hickson groups are detected in X-ray (, Ponman and Bertram 1993, Sulentic et al. 1995, Pildis et al. 1995, Ebeling et al. 1995). At least a third of the individual galaxies show morphological disturbances in optical light (Hickson 1990). In compact groups where tidal interactions are continuous and dynamically important, perturbations in abundance, morphology and kinematics, together with ejection of atomic gas to produce an intragroup medium are clearly important.
Single dish 21-cm HI observations of a sample of spiral rich HCGs using the Green Bank 300ft and the Arecibo telescopes (Williams&Rood 1987) yielded HCGs to be somewhat HI-deficient (by a factor of 2 on average) compared to loose groups. Subsequent interferometer observations of 9 HI-bright groups (Shostak et al. 1984, Williams and van Gorkom 1988, 1995, Williams et al . 1991, Verdes-Montenegro et al. 1997, in preparation) revealed a wide range of morphologies and kinematics and evidence for close interactions in many cases, including several examples of a single HI cloud containing the entire group (Williams & van Gorkom 1995). Apertur synthesis observations tend to detect additional dwarf members (e.g. Verdes-Montenegro et al. 1997 in preparation) of these groups not included in Hicksons definition ().
The aim of this project was to complete the HI survey of HCGs and look especially for HI in groups with a high content of S0 and E galaxies. The sample was limited by the telescope's declination limit () and to a radial velocity range up to 12000 due to sensitivity.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998