Hickson groups are among the densest galaxy systems in the Universe, with apparent surface density enhancements that range from 300 to 2000 (Sulentic 1987). They are defined by the number of members, (n 4), compactness and isolation degree (Hickson 1982, 1993). The original subsample has been enlarged in the southern sky by means of an automated search by Prandoni et al. (1994) and relaxing Hickson criteria (Tassi & Iovino, 1995). The small number of members in Hickson groups allows a detailed study of individual galaxies. In addition, and contrary to other compact groups such as those of Shakbazyan (Del Olmo, Moles & Perea, 1995), Hickson groups exhibit a wide range of morphologies and degrees of interaction. This variety is quite different from that of other high density regions such as the centers of rich clusters.
Their high surface density enhancements, together with low velocity dispersions ( = 200 km s-1, = 330 km s-1 Hickson et al. 1992), argue that the groups are physical but with short dynamical lifetimes ( yrs). The number of merger candidates, however, appears to be extremely low (Zepf et al. 1991; Moles et al. 1994) and, although star formation is enhanced with respect to isolated galaxies, it is lower than in strongly interacting pairs (Moles et al. 1994; Sulentic & Rabaca 1994). A diffuse medium surrounding entire groups has been detected as atomic gas (Williams & Van Gorkom 1995, and references therein), X-ray emission (Bahcall et al. 1984; Ponman & Bertran 1993; Ebeling et al. 1994; Saracco & Ciliegi 1995; Mulchaey et al. 1996; Pildis et al. 1995a; Sulentic et al. 1995), and diffuse optical light (Pildis 1995; Pildis et al. 1995b; Sulentic 1987, and references therein). The study of Hickson group environments shows that some of them are embedded in more extended physical systems (Ramella et al. 1994; Rood & Struble 1994), but with low densities and in general well isolated (Sulentic 1987; Rood & Williams 1989).
Different models have been proposed in order to explain the nature of Hickson groups. Mamon (1986, 1995) suggested that 50% of the groups are chance superposition of pairs and unrelated galaxies within loose groups. Hernquist et al. (1995) interpret them as end-on views of filaments of galaxies. In the context of the last model, and based on X-ray results by Mulchaey et al. (1996), Ostriker et al. (1995) suggest that low spiral fraction groups are real physical entities, but high spiral fraction groups are projected filaments. The application of their Q parameter to Hickson groups has been found however inappropriate by Pildis et al. (1996) because the groups are gas poor. Dell'Antonio et al. (1995) have also argued that the large axial ratios inferred by Ostriker et al. (1995) could be just a reflection of the difference in gas fraction between groups and the reference rich clusters. Diaferio et al. (1994) proposed that compact groups could continually form in rich groups, while Athanassoula et al. (1996) find that, if they have a sufficiently massive and not too centrally concentrated common dark halo, and/or appropriate initial kinematics, their lifetimes should be considerably longer than would be naively expected. Since no significant enhancement in the SFR is observed nor many candidates for mergers of late types galaxies, the effects of the environment on the gaseous and stellar component of Hickson groups could appear only at faint levels in which case detailed observations should be needed to reveal them. In order to bring new elements to this debate we will discuss here one Hickson compact group with a high spiral fraction, Hickson 96.
Our aim is to look for optical signatures of interactions by studying the dynamics of the interaction - both in terms of the overall appearance of the group and the local effects on member galaxies - and the possible perturbations of the star formation activity. We report here deep photometric data and spectroscopic observations for the four galaxies of the group. In Sect. 2 we describe the observations and data reduction, in Sect. 3 we show our results for the group as a whole and for the individual galaxies, and we discuss them and present our main conclusions in Sect. 4.
A Hubble constant is used throughout this paper.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998