Published in Astrophysical Journal, vol. 162, p.L149, 1970

Abstract. A classification of clusters of galaxies based on the relative contrast of the brightest member galaxy is described. Clusters containing a supergiant D galaxy define Type I; at the other extreme (Type III) are clusters which contain no members significantly brighter than the general bright population. A list of seventy-six clusters classified according to the present system is included.

Terminology. Compact cluster; medium compact cluster; open cluster. - The galaxy-cluster classification of Zwicky, Herzog, and Wild (1961).

Regular cluster, Irregular cluster. - The two-part classification of Abell (1965; 1970).

Com; Vir; UMa. - Standard clusters of galaxies in the classification of Morgan (1962).

D galaxy. - A galaxy which has, on ordinary photographs, an elliptical-like nucleus surrounded by an extensive envelope.

cD galaxies. - Outstandingly large, luminous D galaxies, as observed on ordinary photographs. The nuclei may be single, double, or multiple.

Table of Contents

INTRODUCTION

THE "BRIGHTEST GALAXY" MORPHOLOGY

THE OBSERVATIONS

DISCUSSION

THE SIGNIFICANCE OF TYPES I AND II

CONCLUSION

REFERENCES

1. INTRODUCTION

Perhaps the most useful classifications of clusters of galaxies at the present time are those of (1) Abell (1958); (2) Zwicky, Herzog, and Wild (1961); and (3) the recent two-part division of Abell (1965, 1970). Each has its special criterion: richness, in (1); concentration, in (2); and the separation in (3) into clusters of two differing average stellar populations. A more detailed expansion of (3) is implicit in a discussion by Morgan (1962), which suggests a classification in which the Coma, Virgo, and Ursa Major clusters can be considered as type-objects for a morphology paralleling that of open star clusters in the Galaxy; its use, however, is limited to the nearer clusters, because of the necessity of determining form types for the brighter individual galaxies in each cluster. The importance of the Abell two-part classification is great, since it can be applied to clusters over great ranges in distance.

2. THE "BRIGHTEST GALAXY" MORPHOLOGY

There is another property of clusters of galaxies which has some interest as a parameter for classification: the degree to which the brightest member stands out against the general cluster background. Such bright objects are clearly important astrophysically, as well as for problems of galactic structure.

We have therefore devised a five-part form classification which depends on the relative contrast of the brightest galaxy to others in each cluster. The notation is given in Table 1.

Types for seventy-six clusters are given in Table 2. Column (1) gives a running number; column (2), the Abell number, with asterisks used in the sense defined by Abell (1958), i.e., the cluster does not meet the requirements for inclusion in the statistical sample; columns (3) and (4), distance and richness classes from Abell; and column (5), the type as determined from paper prints and the glass second-negative copy of the National Geographic Society - Palomar Observatory Sky Survey at the Yerkes Observatory. The final two entries (Hya A and Cyg A) were classified from photographs reproduced in Matthews, Morgan, and Schmidt (1964) and the Notes to Table 1 of that paper.

Table 2 is divided into three sections. The first section gives clusters of distance classes less than 4, for which fairly good types could be derived from the second negatives of the Sky Survey plates. Five clusters with uncertain types (followed by a colon) have been included because of features of special interest. The second section lists the nine richest clusters in Abell's catalog, together with A2029 and A2670, which are good examples of class I (and which are illustrated in Matthews et al. 1964). The last section of the table consists of two entries, the clusters containing the radio galaxies Hya A and Cyg A.

From the first part of Table 2 we can derive two preliminary conclusions: (1) There is no close correspondence between cluster richness and form-type (for the nearest clusters) on the present system; clusters of Type I range in richness from 0 to 2; clusters of Type III also range between the same limits. (2) The phenomenon of two centrally located galaxies in a common envelope is encountered in a number of clusters of Types II and III.

From the nine richest clusters (second part of Table 2) we note that all classes from I to III are represented; from this independent material we again conclude that there is no close relationship between classified type and richness. There is also another interesting characteristic of the richest clusters: the majority are irregular in shape, and two (A545 and A665) are highly irregular, with density minima within their main bodies.

The two radio galaxies in the final section of Table 2 are of special interest: both have double nuclei; and one (Cyg A) has an emission spectrum that resembles typical Seyfert galaxies (see spectrum reproduced in Schmidt 1965). Some similarity in the spectrum of Cyg A to Seyfert galaxies had been noted earlier by Baade and Minkowski (1954).

5. THE SIGNIFICANCE OF TYPES I AND II

According to the criteria of Table 1, Type I is defined by a centrally located cD galaxy - that is, by an outstandingly large, luminous galaxy having an elliptical-like nucleus surrounded by an extensive envelope. The most striking characteristic of such galaxies is their extremely large linear dimensions. At Type II, the brightest member galaxies are smaller in linear dimensions than at Type I; however, they may be comparable in luminosity with some cD galaxies in Type I clusters. For example, in the Coma cluster (A1656) Abell has found (private communication) that the brightest member galaxy stands out in integrated luminosity from its cluster background by as large an amount as does the cD galaxy NGC 6166 from its Type I cluster (A2199).

6. CONCLUSION

In an abstract of the present classification (Bautz and Morgan 1970), an additional Type IV was described. This type has been omitted here, since its discriminant (spirals as brightest members of clusters of galaxies) differs in nature from that used for the other types (degree of contrast of brightest member of cluster).

We are greatly indebted to Dr. G. O. Abell for several discussions, for unpublished information, and for preprints of two of his papers. One of us (W. W. M.) wishes to acknowledge support by a grant from the National Science Foundation.

REFERENCES

1. Abell, G. O. 1958, Ap. J. Suppl., 3, 211.
2. Abell, G. O. 1965, Ann. Rev. Astr. and Ap., 3, 1.
3. Abell, G. O. 1970, in Stars and Stellar Systems, Vol. 9, Galaxies and the Universe, ed. A. Sandage and M. Sandage (Chicago: University of Chicago Press) (in press).
4. Baade, W., and Minkowski, R. 1954, Ap. J., 119, 215.
5. Bautz, L. P., and Morgan, W. W. 1970, Bull. A.A.S. (in press).
6. Dewhirst, D. W. 1959, Paris Symposium on Radio Astronomy, ed. R. N. Bracewell (Stanford, Calif.: Stanford University Press), p. 507.
7. Maltby, P., Matthews, T. A., and Moffet, A. T. 1963, Ap. J., 137, 153.
8. Matthews, T. A., Morgan, W. W., and Schmidt, M. 1964, Ap. J., 140, 35.
9. Morgan, W. W. 1962, Ap. J., 135, 1.
10. Schmidt, M. 1965, Ap. J., 141, 1.
11. Zwicky, F., Herzog, E., and Wild, P. 1961, Catalogue of Galaxies and of Clusters of Galaxies, Vol. 1 (Pasadena: California Institute of Technology), p. iii.