.629B THE HERCULES CLUSTER OF NEBULAE* .130. G. R. Burbidge and E. Margaret Burbidge 9ApJ. Yerkes and McDonald Observatories Received March 26, 1959 195 ABSTRACT The northern of two clusters of nebulae in Hercules, first listed by Shapley in 1933, is an irregular group of about 75 bright nebulae and a larger number of faint ones, distributed over an area about Io X 40'. A set of plates of parts of this cluster, taken by Dr. Walter Baade with the 200-inch Hale reflector, is shown and described. More than three-quarters of the bright nebulae have been classified, and, of these, 69 per cent are spirals or irregulars and 31 per cent elliptical or SO. Radial velocities for 7 nebulae were obtained by Humason, and 10 have been obtained by us with the 82-inch reflector. The mean red shift is 10775 km/sec. From this sample, the total kinetic energy of the nebulae has been esti- mated. By measuring the distances between all pairs on a 48-inch Schmidt enlargement, the total poten- tial energy has been estimated. From these results it is concluded that, if the cluster is to be in a stationary state, the average galactic mass must be ^1012Mo. Three possibilities are discussed: that the masses are indeed as large as this, that there is a large amount of intergalactic matter, and that the cluster is expanding. The data for the Coma and Virgo clusters are also reviewed. It is concluded that both the Hercules and the Virgo clusters are probably expanding, but the situation is uncertain in the case of the Coma cluster. Some discussion of the implication of these results is given. I. INTRODUCTION The clustering tendency of nebulae is well established. Clusters are of all sizes, ranging from those containing very few nebulae up to the rich clusters with hundreds or thousands of members. Rich clusters may have a high degree of central concentration, or they may contain a large number of members spread over a considerable area of the sky with a less steep gradient of density within the cluster. Clearly, this clustering tendency of nebulae is of the greatest cosmologie significance, though only a few results have as yet been obtained which bear at all on the fundamental questions associated with the origin and evolution of those clusters. Of the rich compact clusters, a number have been studied in the last twenty years by the method of making counts, by Zwicky and his collaborators (1941, 1942a, b, 1951, 1957), Shane and Wirtanen (1954), and Omer (1952). These include the Coma, Hydra, Cancer, and Pegasus clusters. Zwicky has shown that the density distribution of the nebulae in some of the clusters is that of an isothermal gravitational gas sphere (with a cutoff). In the inner parts of such clusters the distribution of the nebulae is highly symmetrical. If these clusters are stationary, then it is possible to use the virial theorem to determine the relation between the velocity dispersion of the cluster members and the total mass of the cluster and hence the average masses of cluster members. We shall discuss results which have been obtained in this way in Section III. Suffice it to remark here that the values obtained by Zwicky are often unrealistic, since he has never accepted any of the revisions in the distance scale since Hubble’s (1936) value of the red-shift constant of 526 km/sec per 106 pc. Besides the rich compact clusters, a large number of irregular clusters are also known. The first list of these was given by Shapley (1933). The best known is the Virgo cluster because it is so near and so conspicuous to us. Apart from this cluster, none of the irregular systems have, to our knowledge, been studied in detail. The Hercules cluster, which was first brought to our attention by W. Baade, falls into this category. It has been listed by Abell (1958) (No. 2151 in his catalogue), and there is another irregular cluster, Abell No. 2152, 1?3 south of it. Abell has put No. 2151 in richness category * Contributions from the McDonald Observatory, No. 310. 629 © American Astronomical Society • Provided by the NASA Astrophysics Data System .629B 630 G. R. BURBIDGE AND E. MARGARET BURBIDGE .130. 2, which means that it contains between 80 and 129 members down to 2 mag. fainter 9ApJ. than the third brightest member. Shapley (1933) also listed a cluster at this position (No. 15) but gives its membership as 256 down to the plate limit of 17.4 apparent photo- 195 graphic magnitude. With the necessary revision in the magnitude scale, this limit must be fainter than 17.4, but we have not estimated by how much. We have made counts on an enlargement from the original Palomar 48-inch Schmidt plate (red-sensitive) which was obtained for the Sky Survey. This is reproduced in Figure 1. Down to the faintest object that could be seen, we counted 242 nebulae, of which most of the faint ones must be field nebulae. Down to an estimated limit about 3-3J mag. fainter than the brightest nebula, we counted 73. These must be the nebulae that contribute most of the mass. Thus these 73 nebulae are equivalent to the 800 bright nebulae used in discussions of the Coma cluster, in which the number of faint nebulae has been estimated as thousands. It is this number, 73, that will be used in Section III, where the kinetic and potential energies of the Hercules cluster are obtained. II. DESCRIPTION AND OBSERVATIONS The Hercules cluster1 is an extremely interesting one, as will be seen from the plates reproduced in Figures 1-5. Figures 2-5 are reproduced from plates taken by Walter Baade with the Hale 200-inch reflector. The system is unsymmetrical, and its north- south and east-west extents are about Io and 40', respectively. The first feature that one notices is the presence of four conspicuous double systems which appear in Figures 2, 3, and 5. The members of the doubles are among the brightest nebulae in the cluster, a fact which may have some bearing on the way in which the system formed. There is at least one other double in the cluster, a relatively faint pair of spirals near the northern boundary. This is within the area covered in Figure 1 but outside the region covered by the 200-inch plates. It is more clearly visible on a plate taken with the 82-inch telescope. The second notable feature is the high proportion of spirals in comparison to elliptical and SO nebulae. Before discussing these proportions, we shall describe the results which have previously been obtained for the Virgo cluster. Hubble (1926) discussed about 400 nebulae, mostly field nebulae, and found that 74 per cent were spirals. Of the 400, about 50 belong to the Virgo cluster, and, of those, about 57 per cent are spirals. Shapley and Ames (1930) and Shapley (1950)have given percentages by types in the Virgo cluster in different apparent magnitude ranges. The percentages of spirals are as follows: lO^O-lO1^, 22 per cent; llm0-llm9, 59per cent; 49 per cent; IS^O-ld1^, 48 per cent; 14m0-14m9, 32 per cent; 15m0- 15na9, 20 per cent. In each range a small proportion—at maximum, 9 per cent—is classi- fied as irregular. From the Humason, Mayall, and Sandage (1956) catalogue of red shifts, 73 nebulae from the Virgo cluster are listed, and, of these, 27, or 37 per cent, are classified as spirals. By contrast with this it is well known that in highly condensed, centrally concentrated clusters, such as the Coma and Corona Borealis clusters, the proportion of spiral and irregular nebulae is very small (cf. Hubble, quoted by Spitzer and Baade 1951). We have not been able to find any good figures based on counts by type in clusters such as these. However, in the Coma cluster, of the 23 members for which red shifts are given by Humason et al., only 1, or about 4 per cent, has been classified as a spiral. We have classified as many of the members of the Hercules cluster as possible, using both 200-inch plates taken by Baade and plates taken at the prime focus of the 82-inch McDonald telescope. The latter plates cover a larger area than the 200-inch plates but have lower resolution. Out of the 61 galaxies classified, 69 per cent are spiral or irregular and 31 per cent elliptical or SO. This remarkably high proportion of spirals, which is apparently significantly higher than the corresponding proportion in the Virgo cluster, 1 1950 co-ordinates of center: a = ô = T 17054'. © American Astronomical Society • Provided by the NASA Astrophysics Data System 9B 62 . .130. 9ApJ. 195 ■#* Fig. 1.—Hercules cluster of nebulae. Reproduced from© American original red-sensitive Astr onomical 48-inch Society Schmidt • ProvidedolatCj Courtesy by the National NASA GeographicAstrophysics Society-Palomar Data System Observatory Sky Survey. North is at the top, east at the left Fig. 2.—Part of Hercules cluster of nebulae. Plate obtained with 200-inch Hale reflector at Palomar Observatory by Dr. Walter Baade. Plate center (1950) : = ló^B, 5 = +17054'. Orientation Éià(Asi«iciimiTig^sêr!ôi»OHtiuidaSaH;ifièT;tl?aPtliL\^tdiJcÂ\iàklfâ.NthSi\ftV4tmïplws3tii.Data System Fig. 3.—Central part of Hercules cluster of nebulae. Plate obtained with 20(Mnch Hale reflector at Palomar Observatory by Dr. Walter Baade. Plate center (1950) : a = 16h2Ii19, ô = +17054/. Orienta0(^if^i^l^AlÍPoB¡ábdcal Society • Provided by the NASA Astrophysics Data System .629B .130.