Preparing this review was my just punishment for stating only two years ago - in another review (Weedman 1977) - that Seyfert galaxies are not strong X-ray sources. I said that because, as recently as three years ago, NGC 4151 was the only Seyfert galaxy known as an X-ray source. Now we have 36 Seyfert 1 galaxies, along with 12 other galaxies with strong emission-line nuclei, that are X-ray sources. And this is all without even having HEAO-2 data at our disposal yet. The study of active galactic nuclei with X-ray astronomy is progressing so rapidly that a reviewer feels almost hopeless. The best I can do is summarize what is known as of the summer of 1979 and give a simple overview of how X-ray and other properties relate.

Some excellent reviews of the X-ray properties of Seyfert and other emission-line galaxies already exist. I especially recommend that by Andrew Wilson (1979). He provides very complete references as of a year ago, but X-ray astronomy is progressing so rapidly that he then had only somewhat more than half the active nuclei now in Tables 1 and 2. It was the group working with the Ariel V SSI that made the initial comprehensive X-ray studies of Seyfert galaxies (Ward et al. 1977, Elvis et al. 1978). The UHURU results for Seyfert galaxies followed soon after and are summarized by Tananbaum et al. (1978); the HEAO-A-2 survey results are now in press (Marshall et al. 1979) I have tried to incorporate these and other recent results in Tables 1 and 2.

In this contribution I will present some results of a program aimed at determining the optical variability characteristics of Seyfert galaxy nuclei. It has been known for more than a decade that the continuous, non-stellar light of Seyfert nuclei can vary significantly on a time scale of months to years (see, e.g., Lyutyi, 1973, 1977; Penston et al., 1974; Penfold, 1979). In recent years there also have been various reports of variations in the intensity of the broad optical emission lines (Boksenberg and Netzer, 1977; Tohline and Osterbrock, 1976) following the pioneering work by Souffrin et al. (1973) and Cherepashchuk and Lyutyi (1973). However, very little is known on the details of the continuum variations and how they relate to the line variability. Such information could provide valuable constraints on the fashionable photo-ionization models for the broad line region (BLR) and the structure of the latter.

It has been just a few years since Type 1 Seyferts were established as a class of X-ray sources with luminosities in the range 1042 - 1045 ergs s-1 by Elvis et al. (1978) using data from the sky survey instrument on Ariel V, and by Tananbaum et al. (1978) using data from UHURU.

The average error-box sizes for X-ray sources identified with Type 1 Seyferts in the 2A catalog (Cooke et al. 1978) is ˜ 0.4 sq. degrees, and ˜ 1.0 sq. degrees for those in the 4U catalog (Forman et al. 1978). Improvement in these positions has been made over the past two years by the modulation collimators on board the satellites SAS-3 and HEAO-1. In particular, the HEAO-1 scanning modulation collimator has been used to position a total of 20 X-ray sources, confirming the identification in each case, with the possible exception of Mkn 279 (Dower et al. 1979, Griffiths et al. 1979a). Of the 37 X-ray sources which were discovered prior to the launch of the Einstein Observatory and which have been associated with Type 1 Seyferts, 21 have been positioned to ˜ 1 arc minute, representing an improvement by factors of ˜ 20 to 100 over the previous 2A and 4U error box sizes. Some examples of the error boxes and identifications confirmed with the HEAO-1 scanning modulation collimator are shown in figs. 1 and 2. In fig. 1 both NGC 7213 (Philips 1979) and MCG - 2 - 58 - 22 (Ward et al. 1978) were discovered to be Seyferts by optical spectroscopy of candidate objects in the error regions of the corresponding X-ray sources. NGC 7213 is a Seyfert nucleus in a galaxy of Type SO (Philips 1979). In fig. 2, NGC 931 was likewise discovered to be a Seyfert as a result of its X-ray emission (Ward and Wilson 1978).

Active galaxy-X-ray sources are well known to be variable on timescales of days to years (Ricketts, et al. 1977, Mushotzky, et al. 1979, Lawrence 1979). Here we present some new data from the Einstein Observatory which shows that these sources also vary in less than one day, on timescales of hours. Large luminosity changes in such short times promise to allow the investigation of the physics of such sources in several new ways. We shall give some examples of how this can be begun. This is only a preliminary report of this work. A full account will be given elsewhere.

The Einstein Observatory (Giacconi, et al. 1979) is capable of exploring this regime of variability because its imaging capability gives it two unique advantages. Firstly, the background rate in one beam area is negligible so that intensity determinations are limited only by Poisson counting statistics. Secondly, the background counts in the remainder of the field can be integrated to give a simultaneous monitor with the same instrument of cosmic ray and background X-ray events. Thus, any peculiarities in detector behaviour, telemetry or software can be traced and separated from real source variations. This is a significant advantage and gives us a great deal more confidence in our results. Many sources do not show variability. For example, in our data, Cen A is constant to ˜ 2% over 1 day.

The topic of this paper really falls somewhere between this session on active galaxies and the session on clusters. What I will report is really a cluster phenomenon but one which depends on the presence of a dominant, massive galaxy in the cluster. Specifically, we have detected several X-ray emission lines from the vicinity of M87 in the Virgo Cluster and NGC 1275 in Perseus. The lines are indicative of material which is cooler than the bulk of the intracluster gas. This material is most likely accreting onto the central galaxy with the accretion rate controlled by the rate of radiative cooling.

The observations I am reporting were made with the Focal Plane Crystal Spectrometer (FPCS)on the Einstein Observatory. The instrument is a Bragg crystal spectrometer which has a resolving power (E/ΔE) of 50 to 500 over the energy range of 0.2 to 3 keV. It operates much like an optical scanner in that it has a narrow passband which is swept over some restricted spectral range containing an emission line. Detailed descriptions are given elsewhere (Canizares et al. 1977, 1979, Giacconi et al. 1979).

The activity of galactic nuclei entails a continuous release of energy in the form of hot gas, violent motions, relativistic particles and ordered magnetic fields. The spectrum radiated spans most of the electromagnetic band, from radiowaves to γ-rays, and the power can reach or exceed 10441 - 1046 ergs sec-1. Various extensive surveys dealing with this subject have been published recently (see, e.g., Hazard and Mitton 1979; Physica Scripta 1978; Setti 1976; Wolfe 1979): we refer the reader to these publications for details. In the following we shall just outline the main ideas which are being discussed to explain the origin of these phenomena which pose one of the most important problems of modern astronomy.

Dr. Pacini has dealt with the innermost parsec; my job is to comment on the next 999,999. I have four comments.

1. A reminder of the importance of using the bulk kinetic energy of whatever beam or jet there is - forms of energy that exert pressure will get lost on the way out because of adiabatic expansion (Longair et al. 1973).

2. What is the asymmetry of jets trying to tell us?

Over the past several years a considerable body of evidence has accumulated, suggesting that extended radio sources are powered quasi-continuously from the nuclei of their parent galaxies. This view is supported by the recent discovery that several radio galaxies have narrow radio jets which connect their active nuclei with the large radio lobes and which often extend for several tens of kiloparsecs. Because of their presumed association with the energy transport outward from the active nuclei, radio jets are at present being intensively studied with high-resolution radio techniques.

The closest galaxy known to have a radio jet is the giant elliptical M87 (e.g., Wilkinson 1974), and in this case there is a well-known optical counterpart (e.g., Curtis 1918; de Vaucouleurs, Angione and Fraser 1968), This optical jet is highly polarized (Baade 1956), implying that at least part of the emission is non-thermal. This and the good agreement between the optical and radio structure suggests that these features are closely related.

The launch of the Einstein Observatory has added a new and exciting dimension to the study of active galaxies. Not only have a large number of optical and radio active galaxies been detected, but many new examples of high energy activity have been found. The ease with which a large number of quasars may now be studied in the X-ray regime out to a redshift of at least four promises to improve our understanding of the nature of these tremendous powerhouses and the evolution of the universe.

The Columbia Astrophysics Laboratory (CAL) is carrying out an extensive program to study active galaxies with the imaging proportional counter (IPC) on board the Einstein Observatory (Giacconi et al. 1979). These observations have already yielded a large number of positive detections including four Seyferts, five N galaxies, seven BL Lacs, and 17 quasars. Upper limits were obtained for eight additional quasars. Six new Seyfert I and/or quasars have been identified from X-ray observations (Chanan 1979). Preliminary results from the first six months of the CAL survey of active galaxies will be presented below. A few representative objects of interest will be discussed briefly. Simple statistical tests will be applied to determine whether X-ray properties can be used to understand the differences and similarities between the various classes of active galaxies. Particular emphasis will be placed on the quasars in our sample. Our results for the quasar survey will be compared with those discussed by Tananbaum et al. (1979). Finally, the implications of the discovery of a large number of quasars will be briefly discussed. (Cosmological parameters of qo = 0 and Ho = 50 km (s Mpc)-1 are used throughout.)

We briefly describe the soft X-ray image of M87 obtained using the imaging proportional counter aboard the Einstein Observatory. These data provide further strong evidence for the existence of a massive halo of dark matter surrounding M87 and allow a much more precise determination of its mass. Two pointing positions of the satellite were analyzed; one centered on M87, the other 63’ south and 25’ east of M87. The field of view of the imaging proportional counter is 60’ × 60’, and it attains a two dimensional spatial resolution of ˜ 1.5’ in a spectral range spanning 0.1 to 4.5 keV.

As previous reports had suggested (Gorenstein et al. 1977; Fabricant et al. 1978), the present observations show M87 to be a strong, very extended, thermal X-ray source with a temperature near 2 keV, surrounded by weaker and still more extended emission from hotter gas associated with the Virgo cluster as a whole (Davison, 1978; Lawrence, 1978). We find M87 to have a total 0.5-4.5 keV X-ray luminosity of about 2 × 1043 ergs/sec, and an extrapolated 2-6 keV luminosity of approximately 1 × 1043 ergs/sec. The total mass of gas inferred from the X-ray measurement exceeds 1012 solar masses.

We would like to present early results from the EINSTEIN X-ray Observatory on three radio galaxies: Centaurus A, NGC 315 = DW0055+30, and Cygnus A = 3C405. We hope to demonstrate that imaging X-ray astronomy can provide important insights into the physics and environment of radio galaxies and their extended radio components.

NGC 5128, the parent galaxy of the double-double radio source Centaurus A, is the nearest radio galaxy, providing the best testing ground for high resolution X-ray studies. The X-ray morphology has proved to be rich and varied. We detect four distinct components to the X-ray emission: (1) the strong, compact nucleus detected by earlier satellites; (2) extended emission around the nucleus; (3) emission from the inner radio lobes; and (4) a unique X-ray jet between the nucleus and the NE radio lobe. A detailed presentation of these observations can be found in Schreier et al. (1979).

Clusters and groups of galaxies contain the majority of galaxies in the universe. The rich clusters, while less numerous than the many poor groups, are the densest and largest systems known, and can be easily recognized and studied even at relatively large distances. Their study is important for understanding the formation and evolution of clusters and galaxies, and for a determination of the large-scale structure in the universe.

In the Westerbork Survey of Rich Clusters of galaxies (WSRC), seven nearby and rich clusters of galaxies have been observed with the Westerbork Synthesis Radio Telescope. The results obtained at different frequencies are published in several papers of a series, and Table 1 serves as a general reference to these publications. In these papers, both discussions on individual radio sources and the presentation of cluster radio luminosity functions (RLF) are given. Here we summarize some of the results of the WSRC which, from a statistical point of view, impose some constraints on the rate of the radio activity of galaxies and on the influence of the galaxy environments on their activity. The rate of radio activity of a sample of galaxies in a cluster can be described by the integral RLF of the cluster, which represents the fraction of galaxies that emit in the radio domain above a certain power. When it is presented as a function of the optical luminosities of the cluster galaxies, it is called the bivariate radio luminosity function (BRLF). The BRLFs are most suitable for representing in an unbiased way the rate of activity of a sample of galaxies, since they are presented in absolute parameters and are normalized to the optical luminosity function (or distribution) of the galaxy samples. This is important since, both inside and outside clusters, it has been shown that the RLF depends strongly on the optical luminosities.

The question which we have addressed is “what is the clustering of galaxies about extragalactic radio sources and how does it vary with the properties of the sources such as radio luminosity, optical spectral properties, radio morphology, etc”. Only a small fraction of strong radio sources belong to rich clusters and most of the work on weaker associations is of a somewhat subjective nature. Our approach has been to work with complete, statistically defined samples of bright radio sources and to cross-correlate the positions of the sources with the Lick counts of galaxies. In this approach, we never define exactly what we mean by a cluster as group of galaxies - clustering is simply defined as the amplitude of a correlation function. This approach is only possible because the Lick counts of galaxies have been studied in detail by Seldner et al. (1977) who calibrated the Lick counts over the whole of the sky.

In this contribution I shall review briefly some of the recent research being carried out at the Harvard-Smithsonian Center for Astrophysics in the study of X-ray emission from clusters of galaxies. This work is being done by several of us at CFA, and I particularly wish to thank Drs. Christine Jones, William Forman, and J. Patrick Henry for permission to discuss their results. The data have been obtained from the Einstein X-ray Observatory (HEAO-2) using the imaging instruments, and in particular the Imaging Proportional Counter (IPC). This gives X-ray images with about l½ arc minute resolution over a field of view of ½° × ½° and moderate energy resolution over a band from 0.5 to 3.0 keV. (For further details see Giacconi et al. 1979).

Now that the predominantly thermal nature of the X-ray emission from clusters of galaxies is well established (see e.g. Mushotzky et al. 1978, Smith et al. 1979), it is appropriate to ask what further information can be gleaned from X-ray spectroscopic observations. While the above publications made use of data from the OSO-8 satellite, two newer experiments have been flown by our group on the HEAO-1 and HEAO-2 (Einstein) satellites. The first of these, the A-2 experiment, was comprised of gas proportional counters like our OSO-8 detectors, while the second, the Solid State Spectrometer (SSS), is a cooled Si detector with narrow field of view and heightened energy resolution.

The EINSTEIN X-ray Observatory has been used to study the X-ray emission from the center of the Perseus cluster, including the active galaxy NGC 1275. Both a point source and extended (˜ 6’) source are observed from NGC 1275. The central 40’ × 40’ region of the Perseus cluster around NGC 1275 displays an interesting temperature and surface brightness distribution. Simple hydrostatic isothermal sphere models do not well describe the cluster emission. The surface brightness of the high resolution image of NGC 1275 can be fit with a constant-pressure but centrally-cooling (i.e., temperature increases with radius) gas which suggests a radiative cooling accretion flow onto NGC 1275.

With the advent of imaging in X-ray astronomy, we can, for the first time, study extended sources with the same detailed spatial and spectral resolution employed by radio and optical astronomers. As in other fields of astronomy, the study of the X-ray emission from clusters of galaxies is beginning to feel the tremendous impact of this advance. The accompanying contributions by Murray and by Grindlay show that the division of clusters into classes based solely on X-ray morphology holds clear implications for the origin of the intracluster medium and the evolution of the cluster as a whole. Comparisons with optical and radio data, along with detailed X-ray studies of nearby objects and the extension of cluster observations to redshifts greater than 0.5, will help connect these ideas to a sound theoretical base. In this report on the cluster observations performed by the Columbia Astrophysics Laboratory with the Einstein Observatory, we restrict ourselves to three brief comments concerning the following areas: (1) The correlation of X-ray brightness with cluster morphology, (2) the detection of a spiral-rich cluster at z ˜ 0.4, and (3) detailed observations of Coma, the nearest rich cluster.

A brief review is given of cosmological tests based on quasars, of source evolution and of the X-ray background. The cosmological tests are still inconclusive because of the limited material available and the possibility of serious selection effects. Quasars and other extra-galactic sources account for most or all of the background in the 1-10 kev range and very possibly also at higher X- and γ-ray energies.

The origin of the diffuse background has been an important question in X-ray astronomy starting from the earliest measurements (Giacconi, et al. 1962). When it was recognized that the X-ray background above 2 keV was isotropic and hence extragalactic (c.f. Schwartz 1970, 1979), it became evident that understanding its origin would have cosmological implications (c.f. Hoyle 1963, Rees 1973). Two general explanations representing opposite points of view have received the most attention. One is that the background is composed of faint unresolved objects which are of the same classes as, but perhaps in earlier phases than, the objects which can be detected directly and identified. If this explanation is correct, then the luminosities and/or proper densities of the objects must be larger at earlier times in the universe, and the magnitude and graininess of the background can be used to place important constraints upon the evolution of the objects. The other is that the background is truly diffuse. The most probable explanation of this type is a hot plasma that pervades the intergalactic medium. If it accounted for only 10% of the background, the mass that could be attributed to such a plasma would represent more matter in the universe than has been detected so far by all other means. Such a large mass would be important dynamically, and in determining the deceleration parameter, although it would not “close” the universe. Furthermore, the density of such a plasma would place constraints upon determining the epoch of galaxy formation.