2. HCG environments

Do the closest and brightest HCGs belong to known loose groups or clusters? I've searched for membership in larger systems of all HCGs with at least two member galaxies with similar redshifts (Av < 1000 km s-1) included in a galaxy redshift survey (the remaining HCG galaxies being out of the boundaries of the galaxy survey, or too faint to be included in it). There were five such HCGs. All of them were found to belong to loose groups, as shown in Table 1.

NOTES: N is the number of galaxies with accordant redshifts, WR87, S87, and RW89, are the studies of Williams & Rood (1987), Sulentic (1987), and Rood & Williams (1989), and GH, NBG, MKW, and RGH stand for the galaxy systems of Geller & Huchra (1983), Tully (1987), Morgan, Kayser & White (1975), and Ramella, Geller, & Huchra (1989), respectively. The number in parentheses is the number of galaxies in the embedding system. I also list as HCG 101 the compact group that I discovered in the Virgo cluster (Mamon 1989).

In addition to these, Tikhonov (in these proceedings) found six more HCGs within LGs. Sulentic (1987) would classify all of the HCGs listed in Table 1 as isolated, as the parent groups or cluster span far beyond his angular limits. But more surprising is the fact that Williams & Rood (1987) would have classified 5 out of 6 of these compact groups as isolated. On the other hand Rood & Williams (1989) would classify only one of the 6 compact groups as isolated, which tends to suggest that their isolation criterion is much better although not perfect. It would not be surprising that once deeper redshift surveys are established, the majority of HCGs would belong to greater 3D structures. Note however that while this saves the chance alignment hypothesis, it does not prove it since, if HCGs are bound dense systems, they would have to form within greater structures (see my review on dense groups in these proceedings).

3. HCG galaxy morphological types

Tikhonov (these proceedings) finds a significantly smaller fraction of spirals in more distant HCGs, thus suggesting that ellipticals are oversampled. I checked this using the very accurate morphologies based upon CCD frames given in Hickson, Kindl, & Auman (1989): HCG galaxies are 24±5% spiral at z ≥ 0.04, compared to 56±4% for z < 0.04, a ~ 60 result. It thus seems preferable to exclude the 24 HCGs with z> 0.04 when analysing morphologies. Once Rood & Williams divide their HCG sample into the isolated and non-isolated HCGs (see previous section), they find that the morphologies are not significantly different between the 67 isolated HCGs and their sparse environments. A significant difference is present for the 33 remaining HCGs and their neighborhoods, but only for this minority.

White (1990) has argued that morphological concordance is the result of similar conditions at galaxy formation, and linked to a correlation of galaxy morphological types with some yet unknown physical quantity. I find that the quartets in the CfA LG catalog (Geller & Huchra 1983) present no such significant morphological concordance: perhaps because the correlation of quantities is less strong in LGs.

4. HCG elongations

Hickson et al. (1984) also studied the distribution of HCG elongations using simple dynamical models of both groups and subgroups within groups, and found the two distributions statistically consistent. Thus, the issue of dense bound groups versus chance alignments was thus not resolved here, in contrast to what is stated by Hickson & Rood (1988).

5. Demographics

The logic of Barnes is based upon the assumption that on average each loose group sees one (and only one) bound dense group form within it in its lifetime. Now, my simulations of LGs (Mamon 1987) were stopped at a Hubble time, while the LG lifetimes, based upon their crossing times and the merger rates summarized in my review in these proceedings, was about double. Hence, Barnes would expect me to have found bound dense groups within half of my simulated LGs, whereas I had only found roughly 5%, or, in other words, the formation rate of dense groups within LGs is ten times too low to explain HCGs.

6. Galaxy interaction in HCGs

Now to the fundamental point. If HCGs are caused by chance alignments, then these are not simply well separated individual galaxies lying along the line of sight. A compact quartet, could be such a “1+1+1+1" system, but could also be an alignment of binaries, or a triplet aligned with a single galaxy. The galaxies that are physically associated can moreover be bound to one another or not, for example one could have a transient unbound triplet. The mix of these populations is of fundamental importance in assessing the nature of HCGs, and unfortunately has not been yet looked for in the explicit-physics simulations (where statistical results can be obtained). I thus allowed myself to guess the expected mix for quartets appearing as chance alignments, and these appear in Table 2, where I also used the results of my simulations (Mamon 1987).

Table 2: Approximate distribution of HCGs

NOTES: The underlined numbers in bold correspond to bound systems of galaxies.

From the numbers in Table 2, I expect 32% or 24% of strongly interacting galaxies, if the dark matter resides in individual halos or a common intergalactic background, respectively. The weak interactors account for an additional 19% or 44% of the galaxies for the two dark matter situations, respectively. So a chance alignment model of HCGs turns out to be fully consistent with galaxy interactions. As a cautionary note, in the one dense group simulation of Barnes (1989), mergers occur mainly at the beginning and the end of the simulation, and little interaction is seen during most of the life of the group.

Now the predicted existence of galaxy interactions in chance aligned HCGs should affect somewhat the mix of morphologies in these systems relative to their environments, and might thus explain the differences discovered by Rood & Williams (1989).

Future Prospects

In summary, while a minority of HCGs display strong signs of interaction, these are consistent with chance alignments, which moreover are well justified from statistical arguments. While the controversy surrounding the nature of compact groups will certainly not end soon, there are quite a few tests that ought to be tried out, which I list below.

The explicit-physics simulations that I've carried out (Mamon 1987), although inaccurate in reproducing the details of the galaxy interactions, are probably the best way to assess the amount of binaries and triplets in chance aligned quartets and quintets. The distribution of group elongations can also be properly assessed in this way. One can also test the distribution of mass-to-light ratios of the compact configurations occurring in projected simulated loose groups, and compare with the HCG sample.

More detailed simulations like the restricted 3-body simulations being undertaken by Borne & Levison (in preparation) are then needed to obtain statistical estimates of the strength and the duration of galaxy interactions in dense groups, and in loose groups as well. In the long run, one will of course strive for a statistical set of self-consistent dense group simulations, perhaps with 1000 particles per galaxy.

Observationally, the day will come when POSS plates will be automatically scanned and standard group as well as HCG algorithms can be applied to these. We will thus find out if compact groups are always situated in looser groups or in clusters, and the importance of the latter environment. We would also have better statistics to assess the difference in morphologies.

Barnes, J. 1989, Nature, 338, 123.

References

Geller, M.J., and Huchra, J.P. 1983, Ap. J. Suppl., 52, 61.

Hickson, P. 1982, Ap. J., 255, 382.

Hickson, P., Kindl, E., and Auman, J.R. 1989, Ap. J. Suppl., 70, 687.

Hickson, P., Kindl, E., and Huchra, J.P. 1988, Ap. J., 331, 64.

Hickson, P., Menon, T.K., Palumbo, G.G.C., and Persic, M. 1989, Ap. J., 341, 679.

Hickson, P., Ninkov, Z., Huchra, J.P., and Mamon, G.A. 1984, in Clusters and Groups of Galaxies, ed. F. Mardirossian, G. Giuricin, and M. Mezzetti (Dordrecht: Reidel), p.

367.

Hickson, P. and Rood, H.J. 1988, Ap. J. (Letters), 331, L69.

Mamon, G.A. 1986, Ap. J., 307, 426.

Mamon, G.A. 1987, Ap. J., 321, 622.
Mamon, G.A. 1989, Astr. Ap., 219, 98.

Menon, T.K., and Hickson, P. 1985, Ap. J., 296, 60.
Menon, T.K., and Hickson, P. 1990, in preparation.

Morgan, W.W., Kayser, S., and White, R.A. 1975, Ap. J., 199, 545.
Postman, M., and Geller, M.J. 1984, Ap. J., 281, 95.

Ramella, M., Geller, M.J., and Huchra, J.P. 1989, Ap. J., 344, 57.
Rood, H.J., and Williams, B.A. 1989, Ap. J., 339, 772.

Rose, J.A. 1977, Ap. J., 211, 311.

Rose, J.A. 1979, Ap. J., 231, 10.

Rubin, V.C., Hunter, D., and Ford, W.K. 1990, in preparation.
Sulentic, J.W. 1987, Ap. J., 322, 605.

Tully, R.B. 1987, Ap. J., 321, 280.

Walke, D.G., and Mamon, G.A. 1989, Astr. Ap., 295, 291.

White, S.D.M. 1990, in Dynamics and Interactions of Galaxies, ed. R. Wielen, in press. Williams, B.A., and Rood, H.J. 1987, Ap. J. Suppl., 63, 265.

Zepf, S.E., and Whitmore, B.C., 1990, in preparation.

DISCUSSION

Sulentic: Three comments: 1) The failure of simulations to produce dense groups is not a basis for rejecting that they are physical systems. 2) I am surprised by the lack of infrared emission from the dense groups it supports your view. I believe that Hickson et al. overestimate the FIR luminosities of group members (due to resolution of IRAS). 3) At least some dense groups do have luminous haloes. At least 3 are discordant groups--your model would increase the significance of these associations because these 3 groups would populate the reduced sample of physical groups.

Mamon: If F. Hammer were here, I guess he would respond to your last comment by arguing that the discordant member, if it is a background object, would be gravitationally lensed by the remaining group. This would amplify it enough to make it almost as bright as the other group members. Perhaps gravitational lensing of the background object could also be responsible for the diffuse light that you find in some of Hickson's compact groups.

Whitmore: 1. Vera Rubin has also looked at about a dozen ellipticals in compact groups, and finds that most of them have emission in Ha, and one case is a counterrotating system. 2. I recently reexamined the morphology-density relation using Dressler's data for 55 clusters. I found that the fundamental correlation is probably with the distance from the center of the cluster rather than local density (i.e., - high fractions of ellipticals are only found near the centers of clusters). This would explain why the morphology-density relation is offset for compact groups (i.e., not all ellipticals as would be predicted by high local density) without implying they are not physically related. The small number of galaxies in compact groups cannot produce the deep potential well found in clusters with hundreds of galaxies.

Hamon: I believe that loose groups cannot produce deep potential wells like those found in clusters. However, there is a morphology-density relation on loose groups (e.g., Postman & Geller) which follows that of clusters. So one must explain the offset of Hickson's compact groups relative to loose groups in the morphology-density relation.

Hickson: I think that you and I both agree that the probability of a sufficiently compact chance alignment of galaxies occuring in a typical loose group is extremely small. When you include the Virgo cluster you find a mean probability which is about an order of magnitude higher than when Virgo is excluded. This means that if compact groups are mostly chance alignments, they should almost all be in clusters as least as rich as Virgo. Yet very few of the compact groups in my catalog are anywhere near such clusters.

Mamon: One should therefore perform automated searches inside clusters (with N>50 accordant redshifts) such as the one I performed on Virgo, and one should perhaps expect to find a large number of new HCG's in such clusters.