The Molonglo Observatory Synthesis Telescope has been used for a deep survey of 25 southern fields of about 1° diameter. All these fields have been observed on several occasions in 120 × 12-hour sessions during 1983–85. Radio sources have thus been detected down to a few millijansky at 843 MHz in 20 deg2 of sky from these observations and in a few selected fields, it has been possible to reach a detection limit of 1 mJy. The first results on the source counts and optical identifications are presented. By supplementing these data with some existing observations of stronger radio sources, we have been able to obtain source counts at 843 MHz for the entire range of flux densities above 1 mJy.

During the last two years we have completed an extensive project of direct CCD imaging with the Palomar 200 inch Four-shooter on several milliJansky and microJansky radio samples, now totaling 200 sources. The purpose is to study the nature and redshift distribution of the radio galaxies that cause the upturn in the milliJansky source counts (WMOKK) and to search for candidates of primeval radio galaxies.

This review is mainly concerned with the discussion of the statistical properties of X-ray and optically selected samples of quasars and Sy 1 nuclei and their intercomparison. The inconsistencies which have been found are clearly exemplified by the discrepancy between the observed and the predicted X-ray source counts. It is shown that a satisfactory understanding of these problems has not yet been reached. The last section deals with the long debated contribution of AGNs to the extragalactic X-ray background (XRB), which is still uncertain within a factor of 2, although a lower bound of 30% at the (now convenient) energy of 2 keV can be set.

We present a progress report on a major extension of the Einstein Observatory Medium Sensitivity Survey (MSS). The basic properties of the extragalactic sources identified with Active Galactic Nuclei (AGN) and clusters of galaxies are discussed. Results from previous work are briefly summarized.

We briefly discuss the problem of estimating the slope of the number-counts relations for the specific case of imaging X-ray surveys. Results have been obtained from extensive simulations of Einstein Observatory imaging X-ray data. We conclude that the bias which affects the X-ray number-counts slope determination is much smaller than that which affects the radio number-counts slope.

We have performed a survey of ∼570 square degrees of high galactic latitude sky using a large number of EXOSAT X-ray images. 130 serendipitous sources were detected and ∼60% of them have been identified with catalogued objects. The large majority of the remaining sources have faint optical counterparts on POSS or ESO plates and are expected to be extragalactic. The comparison of our results with those of the Einstein Medium Sensitivity Survey and an analysis of the correlation between the spatial distribution of EXOSAT serendipitous sources and the amount of galactic Hydrogen column density indicate that the average spectral index of extragalactic X-ray sources is steeper than the canonical AGN slope.

A correlation of the form , with γ = 0.83, is found between 2 keV monochromatic X-ray luminosity and 1.65 μm monochromatic infrared luminosity for a sample of optically selected radio-quiet QSOs. This is equivalent to an average 1.65 μm to 2 keV spectral index of αir/x = 1.28+0.05 log(lir/1031 ergs s−1 Hz−1). The ratio of X-ray to infrared luminosity decreases with increasing infrared luminosity in a manner similar to the relationship between X-ray and optical luminosity. The standard deviations of the distributions of data about the best-fit functions are comparable for the X-ray versus infrared and X-ray versus optical fits. Thus, contrary to a previous claim, the infrared luminosity is not better than the optical luminosity at predicting the X-ray emission from QSOs.

As the sky in the microwave band is dominated by a cosmic background, so too is the X-ray sky. In this presentation the observational situation regarding the extragalactic X-ray background is reviewed, emphasizing data obtained from HEAO-1 and the Einstein Observatory (HEAO-2). Spectral characteristics and spatial variations are described and discussed within the context of what is currently known about individual extragalactic sources. It is concluded that the bulk of the cosmic X-ray background is yet to be understood. Possibilities range from genuinely diffuse emission to an “as yet” unknown large population of unresolved discrete sources. The role of cosmological effects is examined and could be important.

Counts of optically selected quasars as a function of magnitude and redshift show the effects of strong evolution. If quasars have relatively short life times, then the observed numbers at a given redshift are mostly determined by their birth rate and mean luminosity over their lifetime. In this case the evolution of the luminosity function can be described by density evolution, where the rate of evolution may depend on luminosity and other properties. On the other hand, if all quasars were formed at large redshift and have been decaying in luminosity since that time, then the evolution of the luminosity function is best described in terms of luminosity evolution. We discuss some of the consequences of luminosity evolution for the mass of quasars and for the X-ray background.

We explore the observational aspects of the redshift cutoff of quasars. The situation is complicated by the unavoidable bias in slitless surveys against weak-line objects. Since quasar emission lines show a wide range of equivalent widths, a spectral survey will be characterized by a distribution of limiting continuum magnitudes rather than by a single value. The decline in the space density of quasars at large redshift may depend on luminosity, and may also have structure, such as a steep drop, but not a total cutoff, in density at a redshift near 3.

The distribution of quasars has become one of the most interesting problems in observational cosmology. This is due mainly to the development of theory of the formation of large scale structure in the universe. In recent years, several scenarios of clustering have been proposed. In the adiabatic case, the clustering process is from larger scales to smaller ones, i.e., the first systems to form out would be on the scale of superclusters, then these systems fragment to form smaller scale systems such as galaxies. In the isothermal case, the clustering is from smaller scales to larger ones, namely, galaxies condense out at first and larger scale systems, such as clusters and superclusters, then form later via hierachical build-up processes. In the universe contain two components, the scenario of clustering might be different from both standard adiabatic and isothermal cases(1). According to this new scenario, there should be two kinds of small scale objects, one is formed due to fragment of larger scale systems, another is formed before large scale systems form.

The analysis of all published slitless quasar surveys suggests that the space density of quasars increases quickly with redshift up to z ∼ 2.45, then drops abruptly by about a factor of five, increases again up to z ∼ 3.8 at least to drop once more below the present day limit of sensitivity.

We are in the process of compiling a large catalogue of faint (B < 20.9 mag), UVX selected QSOs with complete spectroscopic identification using the fibre optic (FOCAP) system at the Anglo-Australian Telescope. From the 220 QSOs thus far identified we find that QSO evolution is most simply parameterised by a uniform increase in luminosity towards higher redshifts. We also find evidence for strong QSO clustering at scales < 10h−1 Mpc.

Hereafter the term “quasar” is applied to both optically selected (and/or spectroscopically confirmed) UVX objects, and to radio-detected objects of that kind. A recent attempt to model number-counts of radio-selected QSOs (Condon, private communication) has revealed that the counts cannot be modelled by simply translating the general radio luminosity function (RLF) leaving the cosmological evolution unchanged. While it may be true that the radio sources in QSOs evolve in the same way as do all radio sources (i.e. translation function), the radio sources are probably not always in QSOs. In particular, it may be that increasing the luminosity (especially the core luminosity) of a radio source increases the probability that it is in a QSO. If that is true, a radio detection rate of optical QSOs should be strongly dependent on the optical (core) luminosity, i.e. on the absolute magnitude.

The space distribution of quasars discovered in our CFHT blue grens survey is discussed in detail. Redshifts for about 200 of the quasar candidates show that the sample is relatively complete for 0.2 < z < 3.4 and m < 20.5. Two-thirds of the quasars have z < 1.8 and only 5% have z > 2.5, indicating that high redshift quasars are rare. The surface density of quasars brighter than m = 20.5 is 30 deg−2. Seven quasars with z = 1.165 ± 0.007 discovered in one of the fields have typical separations of ≈20 Mpc and may belong to a very large structure. Statistical tests on our data indicate that clustering among quasars is not common, however. The luminosity dependent density evolution models proposed by Schmidt and Green (1983) combined with a redshift cutoff at high redshift are consistent with our data and that of Schmidt and Green (1983), Marshall et al. (1984), Koo, Kron and Cudworth (1986), and Schmidt, Schneider and Gunn (1986). The model indicates that there was a broad maximum in the comoving density of quasars near z = 1.7. The results will be reported in detail in the March 1987 issue of The Astrophysical Journal.

We present the current status of a survey of faint, optically selected quasars covering 0.7 square degrees. Candidates have been selected through multicolour search, grism spectroscopy, and, to some extent, variability analysis. Slit spectroscopy for almost all the high priority candidates brighter than J=20.84 yielded 22 confirmed quasars. In the magnitude range J=21–22 our results are in very good agreement with other existing surveys. The extension of the survey on an adjacent field is in progress.

We describe how the application of techniques which utilise all the information contained in multi-colour surveys of stellar-like objects can be employed to detect quasars over an extended redshift range. The method is particularly effective for the identification of quasars with redshifts z>2.2 where the application of the ultra-violet excess criterion breaks down. Spectroscopy of a small sample of objects from the first survey field has resulted in the detection of two new faint quasars with redshifts z=3.42 and z=4.01 (ref. 1).

The IR-optical-UV continua of quasars are often represented by two components: (i) a flat spectrum component dominating in the optical-UV (the “Big Bump”) and sometimes attributed to thermal radiation from an accretion disk with temperatures of about 20000 to 40000 K - we will call it the “disk” component - and (ii) a near IR component characterized by a steep rise, α ∼ 1 for λ > 1 μm, often thought to be a synchrotron spectrum - an extrapolation of the cm or mm wavelength radio spectrum - although some have preferred an explanation in terms of thermal re-radiation of the ionizing continuum by hot dust (e.g., Hyland and Allen 1982, Neugebauer et al. 1979).

We report briefly on our optical spectroscopy of QSOs from the radio survey in progress at the University of Texas Radio Astronomy Observatory (UTRAO). The radio survey, under the direction of Dr J. N. Douglas, covers declinations north of −36 degrees, in strips that are 9 degrees wide in declination. Preliminary results for the strip centered at +18 degrees have been published (Douglas et al. 1980) and other strips are currently being prepared for publication. The complete catalogue will list 1 arcsec positions and some structure information for about 75,000 sources stronger than about 250 mJy at 365 MHz. Optical identifications are made from the Palomar Observatory Sky Survey plates using an interactive laser measuring machine (Ghigo 1977), and the accuracy of the radio source positions allows the identifications to be made regardless of the colours of the objects. Optical spectroscopy of a sample of about 300 QSO candidates brighter than 19 mag, drawn from the +36, +18 and −12 degree strips, is being carried out at McDonald Observatory using an Intensified Dissector Scanner at the cassegrain focus of the 2.7m reflector. Two image-tube and dissector chains are available, together covering the range 3200–8500 A; a typical spectrum takes 1 hour to obtain, with wavelength resolution of 10 A over a total range of 3000 A.

We describe our complete sample of flat-spectrum radio sources with fluxes >0.5 Jy selected from the Parkes 2700 MHz catalogue. The sample covers all right ascensions and declinations from +10° to −45°, but excluding the galactic plane (b < 10°), and contains some 400 sources.

After taking account of the selection effects in the identification of emission lines and choosing the sample within a narrow range of absolute magnitude, we can investigate the evolutionary function of quasars from their redshift distribution. From data given by slitless surveys with limiting apparent magnitude 19.5, we find that the evolutionary function takes form of ρ = ρo(1+z)6.5±1. The analysis has also showed that the observational redshift distribution of quasars is compatible with cosmological principle.