Astron. Astrophys. 321, 409-423 (1997)

4. Discussion and conclusions

4.1. Individual members of Hickson 96

We have shown that the geometrical center of the H96a disk is shifted by relative to the photometric center, in the direction opposite to H96c and perpendicular to the observed bar. One of the two arms of H96a vanishes in its outer parts, and the same occurs for H96c, both in the area where their disks overlap. An offset of the H96c disk would be difficult to detect due to the small size of this galaxy. Galaxies whose centers of the disk are offset from the center of the bar are not a rare case, and the asymmetry is more important for the later Hubble types, being often associated to one-armed spiral structures (see review by Odewah, 1996). Their origin is not well understood. Impact with a companion can produce them, as shown by Athanassoula (1996) with the help of numerical simulations, but other mechanism should exist since isolated asymmetric galaxies have been found (e.g. Phookun et al. 1993). In the case of H96a and c, the interaction hypothesis seems plausible, since both galaxies are bi-symmetrical in their inner parts, while they loose their symmetry in the area where their disks are in contact. It is also there where the tails start.

H96b could, at a first glance, appear unaffected by the other members of the group. We have, however, shown that this elliptical galaxy has clear morphological and kinematical perturbations. Its surface brightness profile, while well represented by a r law in the outer parts, deviates from it in a sinusoidal way with the highest excess of light in the inner ( 4 kpc). For the same inner radii the ellipticity of the isophotes suffers an important increase relative to the outer parts. These results, together with the kinematical perturbations found for the same range of radii, clearly suggest the presence of a very elongated component in the center of H96b, kinematically decoupled relative to the overall velocity pattern of the galaxy.

These properties have been observed in other elliptical galaxies. Nieto et al. (1991) found that all ellipticals in his sample with decoupled cores (33%) show photometric structures in a similar range of radii. Forbes et al. (1995) suggest from their observations that kinematically-distinct cores show disk-like shapes, although the presence of dust and/or low inclinations of the disk can make the detection of those disks difficult (Forbes et al. 1995; Kormendy et al. 1994). This might be the case for H96b, for which we find indications of a small-scale elongated dust lane towards the center (Sect. 3.4.). In fact higher ellipticities of the central isophotes, as we have measured in H96b, have been attributed in the literature to evidences of disks (Franx & Illingworth, 1988; Forbes 1994; Forbes et al. 1995) finding in some cases bar-like structures (Surma & Bender 1995).

Several models have been proposed to explain the origin of these kind of features, and we examine here whether they may apply to H96b. Kormendy (1984) and more recently Balcells & Quinn (1990) explain decoupled cores by the capture of a small spherical/compact galaxy. Since H96b is found in a group with likely existence of dwarf galaxies, it seems reasonable that this giant elliptical had previously swallowed a smaller companion causing the observed kinematical perturbations, and the observed wave-like perturbation of its light profile. The recent model by Hau & Thomson (1994) explains these signatures through a retrograde interaction with a secondary galaxy without invoking a merger. A completely different approach is the streaming motion model by Statler (1991) which does not consider an external origin. In the case of H96b the merger/interaction models are also favoured by its broad and diffuse tidal-like structure, that has been predicted by simulations as an interaction signature (e.g. Barnes & Hernquist 1992 and references therein) and photometrically described by Schombert et al. (1990).

It is difficult to establish the existence of peculiarities in H96d due to its small size, that usually implies a barely defined shape. As shown in Sect. 3.5, it is a disk system, probably a Sm galaxy, showing knots related to the presence of bursts of star formation. This young population still keeping ionized gas is mainly distributed in the side closer to H96a, and might be therefore induced by this galaxy. In fact, the knotty aspect of H96d is quite similar to that of IC 5283, for which the existence of local episodes of star formation processes, induced by the interaction with the companion galaxy NGC 7469, has been claimed to explain its morphology (Márquez & Moles 1994).

4.2. Intragroup medium and dynamical state of the group

The morphological and kinematical perturbations of the galaxies in Hickson groups strengthen the argument that they are real entities, and help to define their dynamical state. Hickson (1990) estimates that one third of the galaxies in Hickson groups show morphological disturbances, and one third of all groups contain at least 3 morphologically disturbed galaxies. The ratio of perturbed galaxies is larger (2/3) on the basis of spectroscopic data (Rubin, Hunter & Ford 1991). Tikhonov (1990) suggests that 53% of the groups he observed show clear signs of interactions, but the majority being in pairs (64%). Deeper observations of compact groups however should change these ratios. Hickson 96 is an example of a group with an evident interacting pair of galaxies but, as we conclude from our observations, not only the pair but all of its members are suffering the effects of gravitational interaction.

Toomre & Toomre (1972) showed that tails, bridges or plumes are signs of gravitational interaction. In Hickson 96 two long tails emerge from the region between the spiral galaxies a and c, while a plume comes out from the elliptical b which in turn is connected with the dwarf disk galaxy d through a bridge of matter (Fig. 3). The existence of these features argues very strongly for the fact that Hickson 96 is a physical interacting group. Those features fit well with the extensive photometric study of tidal features in interacting galaxies performed by Schombert et al. (1990), which indicates that the internal velocity dispersion of the galaxy from where the material originates strongly influences the appearance of the tidal feature. Sharp features are more abundant in spiral systems, while broad, diffuse features are more likely associated to hot components, as is here the case.

A diffuse intragroup component has been detected at different wavelengths toward several Hickson groups (see Sect. 1). We have not detected diffuse light toward Hickson 96 in the B, V and R bands at 3 levels of 27.2 mag arcsec^-2, 27.1 mag arcsec^-2, and 26.9 mag arcsec^-2 respectively. Neither has diffuse X-ray emission associated to the group been reported, although it has been observed with the ROSAT Position Sensitive Proportional Counter (PSPC) with an exposure time of 3800s. This, however, cannot exclude the existence of such emission since simulations made by Diaferio et al. (1995) indicate that a time longer than 10⁴ s would be necessary in order to detect hot intragroup gas. X-ray emission has been only detected toward H96a (NGC 7674), probably originated from its Seyfert nucleus, similarly to X-ray detections of active galaxies in H4, H16, H44 and H91 (Ebeling et al. 1994, Saracco & Ciliegi 1995, Pildis et al. 1995a). Dynamical evolution would be however expected to occur, since the material being ejected from the individual galaxies as tidal features into the intragroup medium could build a diffuse light component in the future evolution of Hickson 96.

The observed harmonic radius amounts to kpc. This value, together with the velocity dispersion obtained in Sect. 3.1, implies a virial mass of = 1.7 10¹² and a crossing time of H . The group luminosity, obtained as the sum of the light of the individual galaxies extrapolated to infinite, is 7 10¹⁰ which correspond to the rather low value for the mass luminosity ratio of 20 / . The low velocity dispersion and crossing time contradicts in the case of Hickson 96 the argument that groups are chance superposition of pairs and non related galaxies within loose groups.

From the point of view of isolation Hickson 96 is a well isolated group since it is not part of any loose group or cluster and only two galaxies fainter than H96a and b can be found in a radius of 1 Mpc and with a difference in velocity smaller than 1500  km s^-1 (Sect. 3.1).

4.3. Concluding remarks

We present here evidence that Hickson 96 is a dynamical entity. Tidal features associated to all of its members, a small velocity dispersion (160 km s^-1), together with its high degree of isolation argue very strongly against alternatives for the origin of Hickson 96 such as a transient group in a loose one or chance projections. Therefore we conclude that the most probable alternative is that Hickson 96 is a real compact group, where the observed features can be accounted for by multiple tidal interactions in a dense environment such as that of compact groups.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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