.192Z . .143 . 66ApJ. 19 COMPACT GALAXIES AND COMPACT PARTS OF GALAXIES. II F. Zwicky Mount Wilson and Palomar Observatories, Carnegie Institution of Washington, California Institute of Technology Received July 2, 1965 ABSTRACT The range of surface brightness characteristics for ordinary and for compact galaxies is discussed. Long- and short-exposure photographs of NGC 4874 and its neighbors illustrate the respective pres- ence or absence of compact cores of galaxies which in appearance are stellar and comparable in luminosity to the isolated medium galaxies. Representatives of a subclass of compact galaxies are discussed whose spectra are continuous, with a variety of allowed and forbidden emission lines of H, He n, C m, C iv, [O il], [O m], [Ne in], [Ne v], and [S n] superposed. The symbolic velocities of recession of the objects chosen range from F« = 764 km/sec to 11470 km/sec; their indicative absolute photographic magni- tudes, from Mp — —11.8 to —20.9; their surface brightness <r, from 15.6 to 19.0 per square second of arc; and the indicative diameters of their compact cores, from 68 to 1400 pc. If the double system at R.A. 9h30m30s and deck +55°27/ (1950) can be assumed to be stationary, its indicative mass is 5D7 > 7.2 X 108 $D?o and its indicative relative mass-luminosity ratio 9Î > 13.6. Some characteristics of two compact galaxies, one with a continuous featureless spectrum and the other with a broad-banded emission spectrum are discussed. About one hundred compact galaxies and compact parts of galaxies have been in- vestigated so far with the 200-inch telescope, and it was found that they begin to populate all the empty regions which existed in the diagram of structural characteristics of galaxies versus spectral types as it was established for about 550 of the galaxies investigated by Humason, Mayall, and Sandage. I. ON SOME CHARACTERISTICS OE COMPACT GALAXIES During the past few years compact galaxies and galaxies with compact parts have been searched for and discovered in great numbers with the 48-inch Schmidt telescope. A list of 209 compact and possible posteruptive galaxies as characterized by knots and and pronounced jets was made available to the various meetings of Commission 28 of the International Astronomical Union at Hamburg, August 25-September 3, 1964. Two further lists containing several hundred additional similar objects have been made avail- able and are still available to all interested upon request to the author. The systems dis- cussed in §§ IV, V, and VII were contained in the original Hamburg list. During the past 3 years over a hundred direct photographs and spectra of the objects on the three lists have been obtained with the 200-inch telescope. Attention should be called to the fact that, in order to make possible this fast survey, low dispersion (400 Â/mm) and wide slits were used. These choices do not severely affect the accuracy of the determination of the average redshifts of the galaxies involved when data on many emission lines are available, as the analyses presented in §§ HI, IV, and V show. On the other hand, not too much weight must be assigned the listed widths of the emission lines until spectra at higher dispersion and using narrower slits become available. Most of the compact objects which have so far been investigated differ materially from the previously known stellar systems such as those listed in the NGC and the Shapley-Ames catalogues (1932). These ordinary galaxies are characterized by an aver- age surface brightness a photographic magnitudes per square second of arc which lies in the range 26.0 > <r > 21.0; objects of <7 > 26.0, however, cannot generally be detected by conventional photographic methods. The preceding statements also apply to most of the systems in the Catalogue of Galaxies and of Clusters of Galaxies by Zwicky, Herzog, and Wild (1961, 1963). The objects designated as compact, very compact, and extremely compact in this catalogue were assigned these attributes from the inspection of films obtained with the 18-inch Schmidt telescope on Palomar. Very few among them, how- ever, fall into the various categories of the compact galaxies which are being discussed 192 © American Astronomical Society • Provided by the NASA Astrophysics Data System COMPACT GALAXIES 193 in the present series of papers (Zwicky 1964a). Actually, as our knowledge of compact galaxies grows, it will become necessary to characterize the individual objects through the use of ever more significant parameters. Among these, the average surface bright- ness <r, combined with the spectral type and the width of the spectral lines, as well as the absolute luminosity and the absolute dimensions will be important. For purposes of a preliminary orientation we list the average diameters d and the average surface bright- ness g of a few sample galaxies. The compact galaxies Z\ and Z2, as well as the dwarf galaxy in Virgo, listed in Table 1 were described in Paper I of this series (Zwicky 1964a). Among the luminous compact galaxies or compact parts of galaxies, those will be of interest to us whose photographic, visual, or red surface brightness is smaller than <7 = 20 per square second of arc. Galaxies No. 32 and No. 64 in Cl 0124-0133 are the two among the eighty-five galaxies in the field of this cluster which have, respectively, the greatest and lowest surface brightness (Zwicky and Humason 1964). TABLE 1 Apparent Photographic Magnitudes mp and Surface Brightness 0- per Square Second of Arc for a Select Group of Galaxies and Compact Parts of Galaxies Epoch 1950 Object mp R.A. Decl. Galaxy 64 in Cl 0124-0133. 0h- 27“2 -01° 30' + 14.9 87". +24.6 Galaxy 32 in Cl 0124-0133. 0 21.4 -01 53 + 15.3 11.7 +20.4 Compact dwarf in Virgo. .. 12 28.0 +12 46 +14.5 12 + 19.6 Compact (Zi) galaxy 1 20.6 +34 19 + 13.9 6.7 + 17.8 Disk of compact (Z2) galaxy 1 01.2 +29 52 + 17.6 2.4 + 19.2 Nucleus of M31 0 40.0 +41 00 + 12.8 2.5 +14.5 3C 273 12 26.5 + 2 20 + 12.0 < 0.25 <+10.2* Sun -26.2 32' -10.2 * Assuming that the radius of 3C 273 is r < 0T25. II. ON SOME GALAXIES WITH COMPACT PARTS IN THE COMA CLUSTER For our understanding of the physico-chemical evolution of the Universe, knowledge of the amount and the distribution of mass is indispensable. The assumption that the mass of clusters of galaxies, for instance, can be derived from the integrated luminosity of these clusters has led to results which are in flagrant contradiction with the values of the masses deduced from the internal velocity dispersion (Zwicky 1933). This disagree- ment would disappear if the existence among the member galaxies of a cluster of a suffi- cient number of objects could be demonstrated, whose mass-luminosity ratios are much larger than those of ordinary galaxies. The conjecture that compact galaxies and com- pact parts of galaxies may play a role in this connection is indicated by the comparison of the images of various types of cluster galaxies as shown in Figure 1, a and b. Atten- tion is called in particular to the appearance on the long- and short-exposure photographs of the four galaxies listed in Table 2. It is seen that the brightest among the four galaxies, NGC 4874, with diminishing exposure time, fades away the fastest since it has no bright, compact central disk. Of the other three systems, Nos. 1-3, the outlying “suburban” populations disappear on shortening the exposures, but starlike images of their central disks remain long after the whole image of NGC 4874 has disappeared. Plates of the Coma Cluster obtained © American Astronomical Society • Provided by the NASA Astrophysics Data System 194 F. ZWICKY with the 48-inch Schmidt telescope with different exposure times show that about 10 per cent of the galaxies in the range 13.0 < mv < 15.7 possess compact central parts of stellar appearance. For purposes of a rudimentary morphological classification of the distribution of surface brightness in galaxies, and particularly in those of the E and So types, it is proposed to investigate the relative luminosities of the nuclei, of the central disks, and of the suburban formations of galaxies (spiral arms, secondary belts, halos, etc.). A similar proposal was made by V. A. Ambartsumian in one of his reports to the I.A.U. assembly at Hamburg in August, 1964. In Paper I of this series (Zwicky 1964a) data on three compact galaxies were pre- sented whose spectra show them to be essentially agglomerations of G-, F-, and A-type stars with or without low-excitation lines, such as X 3727.2 of [On] appearing in emis- sion. In the following we discuss compact galaxies whose spectra show many emission lines superposed on featureless continua in which, at the dispersion used, no absorption lines are in evidence. TABLE 2 Galaxies near the Center of the Coma Cluster Epoch (1950) Vs* No. NGC nip (km/sec) R.A. Decl. 4871 12h57m05s +28° 13 .'7 15.1 4873 12 57 08 +28 15.1 15.4 4872 12 57 09 +28 13.0 15.3 +6910 4874 12 57 11 +28 13.8 13.7 +7171 * Vs = heliocentric symbolic velocity of recession (Humason, Mayall, and Sandage 1956). III. THE COMPACT GALAXY AT R.A. 16h22m06s, DECL. +41°12' (l95(fi This galaxy, a photograph of which is reproduced in Figure 2, appears as a bright disk with a sharp outline.