Radio-quiet quasar environments at 0.5 ≤ z ≤ 0.8

We quantify the galaxy environments around a sample of 0.5≤ z ≤0.8 radio-quiet quasars using the amplitude of the spatial galaxy–quasar correlation function, B _gq. The quasars exist in a wide variety of environments; some sources are located in clusters as rich as Abell class 1–2 clusters, whereas others exist in environments comparable to the field. We find that, on average, the quasars prefer poorer clusters of ≈Abell class 0, which suggests that quasars are biased tracers of mass compared with galaxies. The mean B _gq for the sample is found to be indistinguishable from the mean amplitude for a sample of radio-loud quasars matched in redshift and optical luminosity. These observations are consistent with recent studies of the hosts of radio-quiet quasars at low to intermediate redshifts, and suggest that the mechanism for the production of powerful radio jets in radio-loud quasars is controlled by processes deep within the active galactic nucleus itself, and is unrelated to the nature of the hosts or their environments.     

The environments of intermediate-redshift QSOs: 0.3 < z < 0.7

An angular correlation of low significance (2 σ ) is observed between 0.3< z <0.5 QSOs and V 23 galaxies. Overall, the cross-correlation function between 82 intermediate-redshift (0.3< z <0.7) X-ray selected QSOs and V ≲24 galaxies is investigated, but no signal is detected for the z >0.5 QSOs. After converting to an excess of galaxies physically associated with the QSO, this lack of strong correlation is shown to be consistent with the clustering of normal galaxies at the same moderate redshifts. Combined with previous observations, these results imply that the environments of radio-quiet QSOs do not undergo significant evolution with respect to the galaxy population over a wide range of redshifts (0< z <1.5). This is in marked contrast to the rapid increase in the richness of the environments associated with radio-loud QSOs over the same redshift range.     

Infrared photometry of z ∼ 1 3C quasars

We present JHKL ' photometry of a complete sample of steep-spectrum radio-loud quasars from the revised 3CR catalogue in the redshift range 0.65 z <1.20. After correcting for contributions from emission lines and the host galaxies, we investigate their spectral energy distributions (SEDs) around 1 μm. About 75 per cent of the quasars are tightly grouped in the plane of optical spectral index, α _opt, versus near-infrared spectral index, α _IR, with the median value of α _opt close to the canonical value, and the median α _IR slightly flatter. We conclude that the fraction of moderately obscured, red quasars decreases with increasing radio power, in accordance with the 'receding torus' model which can also explain the relatively flat median near-infrared spectra of the 3CR quasars. Two of the red quasars have inverted infrared spectral indices, and we suggest that their unusual SEDs might result from a combination of dust-scattered and transmitted quasar light.     

First results from the Isaac Newton Telescope Wide Angle Survey: the quasar survey

We report the first results of an observational programme designed to determine the luminosity density of high-redshift quasars     quasars) using deep multicolour CCD data. We report the discovery and spectra of three     high-redshift     quasars, including one with     . At     , this is the fourth highest redshift quasar currently published. Using these preliminary results we derive an estimate of the         quasar space density in the redshift range     of     . When completed, the survey will provide a firm constraint on the contribution to the ionizing UV background in the redshift range     from quasars by determining the faint-end slope of the quasar luminosity function. The survey uses imaging data taken with the 2.5-m Isaac Newton Telescope as part of the Public Isaac Newton Group Wide Field Survey (WFS). This initial sample of objects is taken from two fields of effective area ∼12.5 deg² from the final ∼100 deg².     

Compact radio sources variable at 151 MHz – II. High-resolution observations of an unbiased sample

High‐resolution observations, made with the Very Large Array (VLA) at 330 MHz, 1.4 GHz and 8.4 GHz and with the Ryle Telescope at 15 GHz, are presented of a sample of 23 sources which are variable at 151 MHz, concluding the observations of an unbiased sample of 40 such sources. The 8.4 GHz emission of almost all of the sources is dominated by structure on a scale ≲0.1 arcsec – the spectra of these compact components are such that they will also dominate the emission at 151 MHz; the number of sources for which this is not the case is consistent with the number of spurious variables expected to be found in the sample. About two-thirds of the sources have the self-absorbed, or flat, spectra expected from their size. The majority (∼75 per cent) of the sources belong to the compact steep-spectrum (CSS) class, but are generally more compact than other CSS sources; the CSS variables also appear to exhibit more spectral ageing than typical CSS sources, which may indicate a difference in the nature of the sources.     

The LBDS Hercules sample of mJy radio sources at 1.4 GHz – I. Multicolour photometry

The results are presented of an extensive programme of optical and infrared imaging of radio sources in a complete subsample of the Leiden–Berkeley Deep Survey. The LBDS Hercules sample consists of 72 sources observed at 1.4 GHz, with flux densities S _1.41.0 mJy, in a 1.2 deg² region of Hercules. This sample is almost completely identified in the g , r , i and K bands, with some additional data available at J and H . The magnitude distributions peak at r ≃22 mag, K ≃16 mag and extend down to r ≃26 mag, K ≃21 mag. The K -band magnitude distributions for the radio galaxies and quasars are compared with those of other radio surveys. At S _1.4 GHz≲1 Jy, the K -band distribution does not change significantly with radio flux density. The sources span a broad range of colours, with several being extremely red ( r − K ≳6). Though small, this is the most optically complete sample of mJy radio sources available at 1.4 GHz, and is ideally suited for studying the evolution of the radio luminosity function out to high redshifts.     

Compact radio sources variable at 151 MHz – I. Catalogue of sources

The Cambridge Low-Frequency Synthesis Telescope has been used to produce a representative sample of low-frequency variable sources. 20 fields, each covering an area of approximately 9°×9° cosec  δ , have been observed at 151 MHz at between 2 and 10 epochs over the period from 1984 to 1996. At each epoch, maps were made with rms noise levels of typically 10–15 mJy beam⁻¹. From a total of ∼6000 sources detected on these maps, 207 are found for which the flux density variations between at least two epochs appear significant at greater than the 3 σ level. A numerical model is used to assess the true significance of the variability, given the analysis method adopted. This shows that for about half of the sources which appear to vary by >3 σ the variability is genuine. For the other half it is caused by random statistical fluctuations; most of the spurious variables vary by ≲3.5 σ between a single pair of epochs. A catalogue of the variable sources is presented, which includes an estimate of the probability that a given source is a genuine variable. Fractional flux density variations of between 5 and 100 per cent (typically 15–25 per cent) have been detected on a range of time-scales from 1 to 12 years.     

The ASCA spectrum of the z = 4.72 blazar GB 1428+4217

The X-ray-luminous quasar GB 1428+4217 at redshift 4.72 has been observed with ASCA . The observed 0.5–10 keV flux is 3.2 Å– 10⁻¹² erg cm⁻² s⁻¹. We report here on the intrinsic 4 − 57 keV X-ray spectrum, which is very flat (photon index 1.29). We find no evidence for flux variability within the ASCA data set or between it and ROSAT data. We show that the overall spectral energy distribution of GB 1428+4217 is similar to that of lower redshift MeV blazars, and present models that fit the available data. The Doppler beaming factor is likely to be at least 8. We speculate on the number density of such high-redshift blazars, which must contain rapidly formed massive black holes.     

A 98 per cent spectroscopically complete sample of the most powerful equatorial radio sources at 408 MHz

A new sample of very powerful radio galaxies is defined from the Molonglo Reference Catalogue, according to the criteria S _408 MHz>5 Jy, −30°≤ δ ≤10° and | b |≥10°. The sample is selected to have similar properties to the northern 3CR revised sample, and to be visible to a combination of existing northern telescopes such as the Very Large Array radio interferometer and large southern hemisphere telescope facilities. The sample contains 178 sources, of which spectroscopic redshifts are available in the literature for 128. For the remaining 50 sources, new radio imaging, optical imaging and spectroscopic observations are presented to identify the host galaxies and determine their redshifts. With these new observations the total sample is 100 per cent optically identified and redshifts are available for 174 (98 per cent) of the sources. The sample consists of one starburst galaxy, one Seyfert galaxy, 127 radio galaxies and 49 quasars. Basic properties of the sample, such as the distributions of the quasar and radio-galaxy populations in redshift and their locations on the radio power versus linear size ( P − D ) diagram, show no significant differences from the revised 3CR sample. The equatorial location and the high spectroscopic completeness of this sample make it a valuable resource for detailed studies of the nature and environments of these important objects with the new generation of southern hemisphere telescopes.