1 Introduction

Hickson Compact Groups of galaxies are defined as aggregates of four or more galaxies with high galaxy densities similar to cluster cores (Hickson 1982) and low velocity dispersions (< $\sigma >\ = 200$ km s^-1, Hickson et al. 1992). The original catalog, composed of 100 groups, was produced without kinematical information, and was later revised by Hickson et al. (1992) on the basis of redshifts obtained for most of the members of the groups. When chance projections of pairs and triplets with discordant galaxies were removed, the catalog was reduced to 69 groups. Still, reality of some of the proposed groups are questioned. For example, detailed investigations suggest that HCG 18 is probably a single knotty galaxy based on the HI and optical data (Williams & Van Gorkom 1988; Plana et al. 2000).

HCG 54 was proposed by Hickson (1989) to be a small group composed of four dwarf galaxies, named a through d. The brightest galaxy has a "Sdm'' morphological type while the rest are classified as "Im''. Some authors have later claimed (e.g., Arkhipova et al. 1981) that HCG 54 is in fact a single galaxy composed of several bright HII regions. Vílchez & Iglesias-Páramo (1998) found multiple emission knots in H$\alpha$. HCG 54 occupies the lower end of the total luminosity, velocity dispersion and optical diameter distributions of the HCGs sample, with $\log(L_B) = 9.27$, $\log(\sigma_v) = 2.05$ and $R_{\rm 25} = 4$ kpc.

We were motivated to study HGC 54 in the context of our ongoing studies of HCGs (e.g. Verdes-Montenegro et al. 2001; Vílchez & Iglesias-Páramo 1998) in order to confirm or discard its compact group nature based on optical images and spectra, together with HI VLA data. A large HI tidal tail, clear signature of a tidal interaction, as well as the presence of multiple optical ripples or shells, added new motivations to this study. In particular, the presence of ripples/shells and the apparent Sdm morphology pose an interesting theoretical problem, since the formation of these features has been generally attributed to a bright elliptical accreting a small companion (see e.g. Hernquist & Quinn 1987). The observations are presented in Sect. 2, the results discussed in Sect. 3 and we present possible scenarios in Sect. 4. A distance of 19.6 Mpc ( $H_{\rm0} = 75~\rm km~s^{-1}~Mpc^{-1}$) is assumed throughout this paper for HCG 54.