There is strong evidence that quasars are powered by the gravitational energy released by in fall of matter into a compact supermassive object which is most likely a black hole with mass M>106 to 109MO. The question then arises as to how such massive black holes came to be formed. What sort of objects were the progenitors? While looking for an answer we have to bear in mind another bit of intriguing evidence that even high Z quasars show lines of heavier elements (Oxygen, magnesium, etc) with solar abundances, strongly suggesting that matter had already undergone nuclear processing at even earlier epochs. Again even the oldest stars in our galaxy show evidence of metal content. All this suggests that atleast part of the metal content could have been generated by pregalactic supermassive stars, sometimes referred to as Pop.III objects. Such stars with masses ranging from 102–106 M⊙ could have formed in clusters consisting of about a few hundred or thousand of these objects at Z ⋍ 10. Now objects >106 M⊙ would have central temperatures not high enough for nuclear reactions the reason being a well known instability of general relativistic origin which is reached when GM/R⋍9c2/16 (M⊙/M)½; MO ⋍M⊙ giving rise to a maximum central temperature Tc reached at the instability, i.e. just before collapse. This is given by Tc = 2×1013/(M/MO) giving Tmax = 107K (necessary for CNO reactions) for M ⋍ 106M⊙ and for M> 106 M⊙, Tc is too low. It is believed that 10−5 of the primordial material might have been processed in Pop III objects which would have undergone nuclear reactions and evolved over some fraction of a Salpeter time, cσT/4πGmp ⋍ 108 yrs; (σT = Thompson cross section, mp is proton mass)., producing heavier elements in the process. It turns out that objects of mass M <300 M⊙ can explode and scatter the heavy elements. For instance a 116 M⊙ oxgen core of a 200MO object can completely disrupt and the evolution of such objects can explain anomalies like the O/Fe enrichment in metal poor stars and the G dwarf problem. Again there can be substantial mass loss from these superstars (a typical empirical relation like M ⋍ (tD/tKH)¼. L/GM/R (where tD is the dynamical time, and tKH is the Kelvin-Helmholtz scale, L is the luminosity) predicting · ≈ 10−3MO/yr) again leading to enrichment of the medium. Stars >300 MO do not explode but would collapse. The relaxation time scale for a cluster of these objects is again 5×108 to 109 yrs, so that they would all collapse to form a central black hole of M ∼ 108 −109 MO, the surrounding earlier ejected matter enriched in heavier elements being now accreted onto it. There would have been atleast a few times 108 of these black holes formed. Assuming near Eddington luminosity the rate of accretion given by ·(t) ⋍ 8πGM(t)/kc⋍16πG2M2(t)X ρ(t)V/c4, ρ(t) being a function of Z) would put a constraint on when Pop III stars formed and accreted enough matter to trigger quasar activity as quasars do not seem to be present before z = 4. This gives the result that they could not have formed before z ⋍8–10. Another possibility of forming a central black hole is by the collapse of a dense star cluster (consisting mainly of neutron stars or white dwarfs), the central relaxation time is tR∼VC3 / mc21n(0.4 Nc) ∼109yr. One obtains the result that for a core of compact stars to evolve to a relativistic state in a Hubble time it should have a minimal velocity dispersion given by Vc (min)⋍ 103(mc) (1-A/B) (7-3A/B)km/s, A, B are constants. This corresponds to clusters roughly having 107–109 compact stars and the gravitational collapse of such a cluster would then lead to black holes in the required mass range. An observed example is the central 3.5 pc core of NGC 4151 which has a mean stellar density of 2×108 M pc−3 corresponding to V ⋍ 103 km/s. As the central black hole grows the relative tidal force decreases and quasar activity can stop when the hole can no longer break up stars. (Tidal break up is also producing and scattering heavy elements.) For a central black hole accreting white dwarfs M <106MO and for it to break up neutron stars M <2 · 103MO. The break up of a neutron star by such a hole would give a burst of energy ∼1052ergs/sec which has so far not been recorded in any quasar or AGN, the upper limit to the luminosity being ∼1049 ergs/s. This may be indirect evidence that the mass of the central black hole is well above 104 MO in all cases. It is a pleasure to thank Professor Martin Rees for valuable discussions.

Stimulated Raman scattering (SRS) processes offer an attractive and efficient method for producing both essentially the entire non-thermal continuum as well as fast electrons in active galactic nuclei (AGN). In this picture, electrons are accelerated by Langmuir waves which are generated by Raman forward scattering (RFS); these electrons then rapidly radiate their energy by means of Raman back scattering (RBS) off of spatially periodic magnetic fields. Such periodic fields can be produced by magnetic modulational instabilities of the Langmuir field The emission is envisaged to arise from an expanding region, with the highest frequency radiation originating from the smallest volumes at the core of the AGN. Time variability is dominated by density fluctuations in these magnetohydrodynamic flows.

The flat radio spectra of core dominated AGNs are often attributed to a superposition of optically thick synchrotron components. This is known as the cosmic conspirac A radio index pr ∼0 is not a natural consequence of this model, and an alternative model proposed here interprets the source components as regions in which relativistic electrons stream along a magnetic field. Stream collimation is determined by plasma instabilities, and components are assumed to be optically thin, with cyclotron turnover at low frequencies. An appealing feature of this model is that radio photon index depends only upon electron energy distribution, which may be obtained from the photon index above the turnover frequency. An energy power-law with index s≥3 is typical for these sources, and implies a flat radio spectrum with pr = (4/3)-p ∼0.

In this review, the currently published, complete, spectroscopically identified samples of quasars are assembled to produce a composite luminosity function, independent of evolutionary assumptions. Two interpretations of the change with cosmic time provide reasonable fits to the data. Luminosity evolution implies a fixed population of host objects, with nuclear luminosity that fades with advancing cosmic time; some dependence of the timescale on intrinsic luminosity is required. Density evolution traces objects of comparable luminosity to find the change in space density, without a requirement of long lifetime. The change in co-moving volume density depends on luminosity; newer data suggest that somewhat stronger evolution is required at the low luminosity end than the models of Schmidt and Green allowed. Caution is advised in drawing direct physical conclusions about the evolution of individual quasars from mathematical representations of ensemble properties.

Systematic biases that are redshift dependent can influence the optical discovery of quasars and the evolution laws derived from counts of quasars. New data and their interpretation for quasars brighter than MB = −24 in the Palomar Bright Quasar Survey (BQS) (Schmidt and Green, 1983) are consistent with no evolution. A comparison of BQS quasars with the brightest quasars from the CTIO Schmidt Telescope Survey (Osmer and Smith, 1980) shows that if qo is near zero, the co-moving density of bright quasars in a Friedmann cosmology is about 15 times higher for the CTIO survey quasars (mean z ≈ 2.8) than for the BQS quasars (mean z ≈ 1.8). In this case spectral evolution is also required since the CTIO quasars have stronger CIV λ1548 lines than the BQS quasars of similar luminosity. Alternatively, if qO is taken to be near 1, the CTIO survey quasars would then have lower luminosity than the BQS quasars and these data would be consistent with no evolution. Strong CIV λ1548 lines for the CTIO quasars would then fit the general correlation between absolute quasar luminosity and emission line strength (Wampler, Gaskell, Burke and Baldwin, 1984).

The topics of clustering, isotropy, and redshift cutoff are in reality just different aspects of the problem of the three-dimensional distribution of quasars, assuming, of course that the redshifts are cosmological and therefore an indication of radial distance. The distribution in redshift has additional interest because of the substantial lookback times involved, up to four fifths of the age of the universe. The radial variation of quasar density between redshift zero and two, and the attendant questions of density and luminosity evolution, are discussed elsewhere in this symposium by Green and shall not be treated here. Rather we shall concern ourselves with the behavior at redshifts larger than two and the specific question of a steep decline of quasar density at redshifts near three. For simplicity we may characterize the problem as one of studying either the formation of quasars as we normally see them in a cosmologically short time or of the properties of the universe and its optical depth, should intergalactic absorption contribute significantly to blocking our view at redshift three. Of course other hypotheses are also possible.

Evidence is now accumulating that the most powerful extragalactic radio sources display a cutoff in comoving density at relatively low redshift. For both radio galaxies and quasars, the luminosity function falls by a factor ≥ 3 between z = 2 and 4. We describe the data which lead to this conclusion, and discuss the prospects for studying the behaviour of low-luminosity active galaxies at high redshift. The strong evolution of highly luminous radio-quiet quasars at z > 2 may be an artifact of gravitational lensing.

Although the majority of astronomers interpret the redshifts of quasars according to Hubble's law, there are isolated cases as well as detailed studies indicating that the redshifts of quasars and even some galaxies may have noncosmological components. The evidence discussed here will be on two counts: (i) the periodicities in redshift distributions, (ii) the associations between discrepant redshifts objects. In addition, theoretical constraints on physical models based on the cosmological hypothesis will be briefly mentioned.

This paper summarizes various uses which have been made of QSO pairs. Statistical studies using pairs show that the sky distribution of QSOs of arbitrary redshifts is random, and that QSOs of similar redshifts are clustered on a linear scale similar to that of galaxies today. Close pairs are used to set limits on the masses of QSOs. And absorption line studies of QSO pairs support the extrinsic hypothesis for the origin of the narrow absorption lines and the cosmological interpretation of the redshift, they demonstrate the presence of absorbing matter around QSOs, and they set limits on the sizes of the absorbers.

CCD images of quasars having redshifts between 0.3 and 0.65 are analyzed to study the association of galaxies with quasars. Average luminosity functions (LF) of the excess galaxies associated with the radio-loud quasars are determined. It is found that for the sub-sample with z>0.55, there is a significant brightening of the characteristic magnitude M*, if qo is assumed to be 0. Comparing computed quasar-galaxy spatial-covariance amplitudes, we can conclude, at the 0.025 significance level, that the spatial-covariance amplitudes of the sub-sample with z>0.55 are greater than those of the lower redshift quasars. This indicates that there has been a strong evolution of preferred sites for bright radio-loud quasars, implying some number-density evolution of quasars has taken place, and that some rich clusters at z∼0.6, in comparison with the local rich clusters, have significantly different physical conditions.

Both the soft (Maccacaro et al., 1982; Gioia et al., 1984; Giacconi et al., 1979; Griffiths et al., 1983) and the hard (Piccinotti et al., 1982) X-ray surveys provide crucial information on the evolution of AGNs in the X-ray band. Before combining the two data sets, however, the consistency of the two flux scales must be checked. To this end we have compared the soft (SX) and the hard (HX) X-ray luminosities of all powerful (log(SX)>43.5) AGNs for which both IPC and hard X-ray fluxes are available in the literature (lower luminosity objects frequently exhibit large intrinsic, luminosity dependent, absorbing columns which cannot be accurately accounted for). We find <log(HX)/(SX)> = 0.17±0.06, to be compared with the expected value <log(HX)/(SX)>≃0 for a typical X-ray energy index αX≃0.7. Thus, an average correction factor of ≃1.5 needs to be applied to the IPC fluxes to bring them into the hard X-ray flux scale.

If the BL Lac Objects are active nuclei with a beamed component that is dominant when directed at us, their observed luminosity function must comprise a flat faint branch: N(L)dL ∝ L1+1/pdL with p=4.5 (Urry and Shafer 1984). If this is flatter than the LF NP(L) of the parent objects at equal observed L, then we expect the counts of BL Lacs to flatten out in turn at fluxes quite higher than the counts of the parents, even when both populations evolve strongly and uniformly with comparable timescales (Cavaliere, Giallongo and Vagnetti 1985).

Comprehensive testing of alternative cosmologies on the basis of complete samples of discrete sources is described. Application is made to the comparison of the Friedman and chronometric cosmologies (denoted C1 and C2) using quasar samples. C2 predicts the results of analyses predicated on C1 that represent evolution in the C1 framework, but is itself nonevolutionary, nonparametric, and consistent with observation.

It is very important to study the large-scale structure by means of the space distribution of quasars. Using this method, one may search for the more distant superclusters and explore the super-large-scale structure, i.e., the existence of super-superclusters. The answer to the problem would be of great interest. It is related to the question about the transition of the clustering of galaxies on about 100 Mpc to the uniformity of the universe. Recently Oort et al and de Ruiter et al suggested that the quasars are located in superclusters. So we soppose that analysing the space distribution of quasars might give us some information about the super-large-scale structure of the universe. But up to the present the study of the clustering of quasars has not obtained universally accepted conclusions; in fact, some of them, including grouping and clustering (Arp; Chu and Zhu), no clustering (Chu and Zhu; Osmer; Webster), clustering for z < 2 and no clustering for z > 2 (Fang et al) and stringing (Deng et al), are contradictory.

The peaks and dips in the quasar redshift distribution seem to be incompatible with the cosmological principle. This lays the cosmological redshift hypothesis under suspicion and censure, and has been considered by some investigators as a manifestation of the intrinsic nature of quasar's redshift. So it is worthwhile studying whether the redshift distribution of quasars could be explained in the framework of cosmological redshift. As we know, this distribution is affected not only by the possible physical origin of redshift but also by the selection effects in the observations (Zhou, Deng, Zhou 1983). From this we have the redshift distribution function f(z) = P(z)R(z), where P(z) is the real distribution function which depends on the evolutionary properties of quasars and the space-time structure of the Universe, and R(z) is the factor caused by the selection effect in the line identification.

Some years ago, Bolton, Peterson, Wills and Wills (1976, BPWW) reported a statistically-significant excess of QSOs within 2' arc of flat spectrum radio-selected QSOs (specifically, they found 5 such objects while only 1 was expected by chance). None of the QSO pairs had the same redshifts so the result could be regarded as evidence for non-cosmological redshifts, non-uniform QSO distributions, or a statistical fluctuation. Because of the potential importance of BPWW's result, we later used the same technique to examine an independent sample of about 150 QSOs with flat radio spectra and found results that are consistent with the known surface density of QSOs. If we assume a surface density of 3.3 QSOs per square degree brighter than B = 19 (e.g. Marshall et al. 1983), the 150 fields each of 2' arc radius should contain 1.7 random QSOs above this limit and we found 2 (e.g. Wills 1978). A third object is very close to B = 19, and another field contains an object that is extremely blue, with B < 19, on the Palomar Sky Survey, but below our plate limit (i.e. B > 21); we have so far been unable to obtain a spectrum for it. One of the 3 confirmed QSOs is 119″ arc from the radio-emitting one, so rather small changes in the magnitude limit, search radius and choice of epoch at which the magnitudes are measured can result in there being anywhere between 1 and 4 secondary QSOs (or 1–3 if the variable object is not a QSO), compared with 2 expected by chance, and perhaps 10 that would be predicted by BPWW's results.

The region of the Virgo cluster is the subject of an intensive quasar search, since quasars mill provide valuable probes of absorption due to galaxy haloes or the intracluster medium. It is also interesting to test the quasar-galaxy association in such a region of the nearest major cluster of galaxies. More than 200 quasar candidates were found in the Virgo cluster region using both slitless technique and machine search, 32 of them have been confirmed as quasars.

Several U.K. Schmidt objective prism plates are being searched for quasar clustering. The spectra are measured by the automated plate measuring facility (APM) and emission line objects selected for which redshifts can be estimated. Most redshifts tested (based on Lyman-α) are accurate to 2%. Initial results from the South Galactic Pole (SGP) field show little evidence of clustering on scales between 60 and 500 Mpc (Ho=50, qo=0, at z=2.3).

The Sculptor Region has been searched for quasar candidates: three IIIa-J objective prism plates from the UK Schmidt-telescope centred around the NGC55 and NGC300 galaxies have been used. 76 quasars have been found doubling the number of quasars previously known in this field. A test using Monte-Carlo simulations to reshuffle redshifts of the quasar sample shows that a simple eye examination of the simulated distributions produces as many quasar pairs or associations than in the original field. A further preliminary quantitative study using the autocorrelation function shows that quasar clustering is absent on any scale from 10 to 3500 Mpc (Ho=100).