We now pass to a more general consideration of complexes of stars: clusters of galaxies can also be regarded as being made up of stars. Thus, in spite of the higher order of complexity reached on passing from individual galaxies to clusters of galaxies, we still have some justification for considering the problem as one of stellar classification.
We proceed to define a diagram which bears a certain resemblance to the HR diagram for star clusters. The co-ordinates of this diagram are the concentration class as defined above, as abscissa, and the apparent visual magnitude as ordinate; the plotted points are galaxies - in place of the individual stars of the HR diagrams.
Figure 5 illustrates this form-magnitude diagram for the brightest three magnitudes of the inner part of the Virgo Cluster; the galaxies plotted are listed in Table 3. The two-decimal magnitudes are from Holmberg (1958); the one-decimal values are corrected to Holmberg's system from the Shapley-Ames catalogue. The concentration classes were determined according to the definitions in Section II, above. The area of the cluster included is defined by an ellipse having a northern extremity of declination at +19° and a southern extremity at +5°. The columns of Table 3 give (1) the NGC number, (2) the photovisual magnitude according to Holmberg, (3) the concentration class, and (4) the form class on the Y system.
Figure 5. Form-magnitude diagram for brighter members of inner part of Virgo Cluster. The abscissae are degrees of central concentration of luminosity; the ordinates are the photovisual magnitudes of Holmberg. Note absence of bright galaxies of concentration class 1.
Figure 5 has the following characteristics: the brightest half-dozen members of the inner Virgo Cluster are all of the strongly concentrated type. On passing to the two fainter magnitudes, all concentration classes are represented except class 1; there is a preponderance among these fainter two magnitudes for the higher concentration classes. Approximately one magnitude fainter than the brightest members of the cluster there appear a number of spirals of especial interest. If their apparent diameters are defined in terms of their well-defined spiral arms, then the diameters are remarkably similar to each other.
The brightest six members of the inner Virgo Cluster are reproduced in Figure 6; they are all of the highly concentrated variety. The bright spirals just referred to are reproduced in Figure 7. The galaxies reproduced in Figures 6 and 7 were obtained by Mr. R. F. Garrison with the Yerkes 40-inch refractor combined with a Meinel reducing camera constructed under the direction of Dr. W. A. Hiltner. Kodak 103a-D plates were used, in conjunction with a Schott GG11 filter. The exposure times were 30 minutes. The writer is indebted to Dr. Hiltner and his Air Force Contract on galactic research for making this part of the program possible.
Figure 6. Photographs of the six brightest (photovisual) members of inner Virgo Cluster. Top: left, 4472; right, 4486. Center: left, 4649; right, 4406. Bottom: left, 4382; right, 4374. Plates by R. F. Garrison with Yerkes 40-inch refractor and Meinel Hiltner reducing camera. Kodak 103a-D plates with GG11 filter.
Figure 7. Photographs of six brightest
(photovisual) spirals in
inner part of Virgo Cluster. Top: left, 4321; right,
4501. Center: left, 4579; right, 4569. Bottom:
left, 4535; right, 4254. The brightest of these spirals
(4321) is about 0m.1 fainter than the
(4274) illustrated in Fig. 6.
in Figs. 6 and 7 are
to the same scale.
The brightest galaxies in the region of the Ursa Major Cloud are plotted similarly in Figure 8. Here the magnitudes are of considerably lower accuracy; they consist, in most cases, of the Shapley-Ames photographic magnitudes reduced to the visual magnitude system of Holmberg. The general appearance of Figure 8 differs markedly from that of Figure 5: the brightest galaxies in the Ursa Major Cloud do not belong to the highest concentration classes; furthermore, the most populous concentration class is 1 - the class that is not even represented in Figure 5. We then have the conclusion that the physical conditions of the brighter galaxies in the Ursa Major Cloud are systematically different from those in the Virgo Cluster.
Figure 8. Form-magnitude diagram for brightest members of Ursa Major Cloud. Abscissae are similar to Fig. 5; ordinates are Shapley-Ames magnitudes reduced to Holmberg's photovisual system. Note large number of galaxies in concentration class 1 and absence of bright members in concentration classes 6 and 7.
The Ursa Major Cloud, is of course, an ill-defined physical group of galaxies; its volume density is exceedingly low, and it is doubtful whether we know any other groups like it at the present time. This is probably due to the impossibility of recognizing such poor physical groupings at greater distances.
We do have, however, a valuable catalogue of rich clusters of galaxies, the recently published catalogue of George O. Abell (1958), which makes possible investigation of the physical nature of many other clusters in the manner outlined above for the Virgo Cluster. The inner parts of the twenty clusters listed by Abell in his nearest distance group were photographed by Garrison with the Yerkes 40-inch, in the same manner as that described for the Virgo galaxies. The photographic resolution of these plates is sufficiently high to permit schematic diagrams to be constructed in a manner similar to that for Figure 5.
This investigation, which derives out of work begun as a guest investigator at the Mount Wilson-Palomar Observatories in the fall of 1960, gives the following results: The nearest rich clusters of galaxies belong to one of two general types: (a) the Virgo type, in which the brightest members are of the highly concentrated type, with lower-concentrated spirals appearing from half a magnitude to a magnitude fainter than the brightest members, and (b) the Coma type, in which all galaxies in the brightest two magnitudes are of the highly concentrated class.
The following bright clusters of Abell's catalogue are of the Virgo type: Nos. 262, 347, and 2151 (Hercules Cluster). The majority of all the nearer rich clusters are of the Coma type; examples are Nos. 1656 (Coma Cluster), 194, and 779.
The division into Virgo type and Coma type is not a division according to cluster richness; Abell 2151 is of the Virgo type; it has a richness class of 2 - the same richness class assigned to the Coma Cluster itself. At the other extreme, a very poor clustering surrounding NGC 3158 - so poor that it is not eligible for inclusion in the Abell catalogue - is of the Coma type, as far as the kinds of galaxies are concerned.
There is, therefore, a property which is common to all the rich clusters in Abell's nearest distance group: the brightest members of each cluster of galaxies are of concentration classes 6 or 7; that is, in all rich clusters located in the nearest distance group of Abell's catalogue, the brightest component galaxies are of the highly concentrated type.
The rich clusters of galaxies observed have, therefore, a certain analogy in their stellar population with the kinds of stars observed in the brightest two magnitudes of "evolved" star clusters like NGC 6356 and 6553 - two of the strong-lined globular clusters in the region of the Galactic nucleus. The observational comparison between star clusters and clusters of galaxies is summarized in Figure 9. The upper plot gives a schematic representation of star clusters at the two extremes of integrated spectral type: the brightest two magnitudes of (1) the Orion Nebula cluster, from the observations of S. L. Sharpless (1952), and (2) the strong-lined globular cluster NGC 6356, from the observations of Sandage and Wallerstein (1960). Star clusters in the nuclei of O associations will have, in general, HR diagrams similar to that of the Orion Nebula cluster; other strong-lined globular clusters will probably have HR diagrams roughly similar to that of NGC 6356.
Figure 9. Schematic diagrams of population characteristics of brightest members of star clusters (above) and clusters of galaxies (below). The members of the brightest two magnitudes of the Orion Nebula Cluster (Sharpless) and the globular cluster NGC 6356 (Sandage and Wallerstein) represent the extremes in stellar population by spectral types.
The lower part of Figure 9 summarizes the results for clusters of galaxies. Most of the nearer rich clusters - and many of the poorer ones - resemble the Coma Cluster, in that all the brighter component galaxies are of the strongly concentrated form class. A number of nearby clusters of galaxies resemble the Virgo Cluster; the brightest component galaxies are of the concentrated form class; however, on passing to galaxies approximately one magnitude fainter than the bright limit, little-concentrated spirals are encountered. Within the distance limits included in this survey, there seem to be no physical groups of galaxies rich enough to be called clusters, which are analogous to the Orion Nebula star cluster and other early-type star clusters in O associations.
A similar diagram for clusters of galaxies is shown below; the upper envelopes for the Coma and Virgo Clusters and the Ursa Major Cloud are illustrated, Clusters of galaxies of the Coma type bear a certain analogy to the star cluster NGC 6356 in the absence of appreciable fractions of hot, early-type stars. However, among the nearer clusters of galaxies there seem to be no cases analogous to the star clusters of the Orion Nebula type.
The work outlined here was carried out under a grant on stellar classification by the National Science Foundation. It had its origins in work carried on at the Mount Wilson-Palomar Observatories under a contract with the Office of Naval Research. I wish to express my thanks to Director I. S. Bowen and the Research Committee of the Mount Wilson-Palomar Observatories for making possible my entry into this field.