We present details of a new sample of radio-loud quasars drawn from 0.013 sr of the 7C Redshift Survey. This sample is small (21 quasars) but complete in that every object with an unresolved nucleus and/or broad emission lines with S151MHz > 0.5 Jy has been discovered. The dependence of the quasar fraction with redshift and radio luminosity is investigated, providing new evidence supporting the...

The black hole (BH) mass in the centre of AGN has been estimated for a sample of radio-selected flat-spectrum quasars to investigate the relationship between BH mass and radio properties of quasars. We have used the virial assumption with measurements of the Hβ FWHM and luminosity to estimate the central BH mass. In contrast to previous studies we find no correlation between BH mass and radio p...

With a recently constructed composite quasar spectrum and the χ minimization technique, we demonstrated a general method to estimate the photometric redshifts of a large sample of quasars by deriving the theoretical colorredshift relations and comparing the theoretical colors with the observed ones. We estimated the photometric redshifts from the 5-band SDSS photometric data of 18678 quasars in...

There is controversy about the measurement of statistical associations between bright quasars and faint, presumably foreground galaxies. We look at the distribution of galaxies around an unbiased sample of 63 bright, moderate redshift quasars using a new statistic based on the separation of the quasar and its nearest neighbour galaxy. We nd a signiicant excess of close neighbours at separations...

During analysis of the relation of quasars to galaxies in the 2dF deep field a concentration of quasars was noted. Most striking was the closeness in redshift of 14 quasars about the mean redshift z = 2.149 with a range of ±.018. The cluster in spite of its high redshift subtends an area of diameter more than 2.3 degrees on the sky. At conventional redshift distance its diameter would be 181 me...

We investigate whether models based on the assumption that jets in quasars are powered by rotating black holes can explain the observed radio dichotomy of quasars. We show that in terms of the “spin paradigm” models, radio-loud quasars could be objects in which the black hole’s rotation rate corresponds to an equilibrium between spin-up by accretion and spin-down by the Blandford-Znajek mechani...

We report on our XMM observations of the high redshift quasars BR 2237–0607 (z=4.558) and BR 0351–1034 (z=4.351). We also report on XMM observations of 19 other z> 4 objects available in the public archive of which 14 were detected. We find that the optical to X-ray spectral index αox is correlated with the luminosity density at 2500Å, but does not show a correlation with redshift, consistent w...

We study the observational signatures of a potential population of low-luminosity quasars at high redshifts in a ΛCDM cosmology. We derive the evolution of the quasar luminosity function at fainter luminosities and higher redshifts than currently detected, based on three assumptions: (1) the formation of dark–matter halos follows the Press–Schechter theory, (2) the ratio of central black hole m...

We investigate the environments of quasars using a semi-analytic model which includes both galaxy and quasar formations based on a hierarchical clustering scenario. We assume that a supermassive black hole is formed and fueled by the accretion of cold gas during a major merger of galaxies. Our model can reproduce not only the general form of the galaxy luminosity functions in the local Universe...

The origin of the dichotomy of radio loudness among quasars can be explained using recent findings that the mass of the central supermassive black hole (SMBH) in extended radio-loud quasars is systematically a few times that of their counterparts in radio-quiet quasars. This sensitive dependence of radio jet ejection upon SMBH mass probably arises from the blockage of jets by the presence of su...