Extragalactic sources with one-sided radio structure

A list has been compiled of 49 extragalactic sources, most of them identified with quasars, that appear to have a one-sided (D2 type) radio structure characterized by a single outer component displaced from a compact central (nuclear) component coincident with the optical object. The observed properties of a subsample of 28 D2 quasars that have an overall angular size larger than 5 arcsec are briefly discussed and compared with those of normal (Dl type) double quasars. It is found that the central components in most D2 sources account for more than half the total flux density at high frequencies in contrast to the Dl quasars which generally have less than 20 per cent of their total flux density in a central component. This makes it very unlikely that D2 sources are just those Dls in which there is a large intrinsic difference in the flux densities or separations of the two outer components. The observed properties of D2 sources are easier to understand in the relativistic beaming interpretation in which their axes are inclined at smaller angles with the line of sight compared to Dl sources

A VLA 20 and 90 centimetre radio survey of distant A-bell clusters with central cD galaxies

A radio survey, using the Very Large Array at 20 and 90 cm λ has been carried out in the direction of 46 distant Abell clusters (0.1 ≲ z ≲ 0.3) dominated by a cD galaxy (clusters classified to be Bautz-Morgan I type). A radio source coincident with the cD galaxy was detected in 16 clusters. We find that the radio luminosity function of the cD galaxies at 20cm λ, and below the luminosity P1.4GHz ≲ 1024.5 W Hz-1, is similar to that of brightest ellipticals in less clustered environments. Above this luminosity, the cDs seem to have a higher probability of becoming radio sources. The effect of optical brightness on radio emission is shown to be the same for the two classes. No significantly large population of very-steep-spectrum sources with spectral index α >1.2 (flux density ∝ frequency-α) was found to be associated with cD galaxies. A significant negative correlation is found between the radio luminosity of the cD galaxy and the cooling-time of the intra cluster medium near the galaxy. We also present evidence that the probability of radio emission from first-ranked galaxies is dependent upon their location relative to the geometrical centres of clusters and thus related to the morphological class and the evolutionary state of the clusters. We argue that both these effects are primarily caused by the dynamical evolution of these distant clusters of galaxies

The redshift dependence of spectral index in powerful radio galaxies

We present and discuss in this paper the rest frame radio spectra (1-25 GHz) of a sample of fourteen radio galaxies at z > 2 from the newly defined MRC/1Jy complete sample of 558 radio sources. These galaxies are among the most powerful radio sources known and range in luminosity from 1028-1028.8 watt Hz-1 at 1 GHz. We find that the median rest frame spectral index of this sample of galaxies at z > 2 is significantly steeper than that of a matched luminosity sample of 3CRR galaxies which are at a much lower redshift (0.85 < z < 1.7). This indicates that spectral index correlates primarily with redshift, at least in the luminosity range considered here. The difference between the distributions of rest frame spectral curvatures for the two samples does not appear to be statistically significant. We suggest a new explanation for the steeper spectra of radio galaxies at high redshift involving steeper electron energy spectra at injection. Electron energy spectra are expected to steepen in a first-order Fermi acceleration process, at both non-relativistic and relativistic shock fronts, as the upstream fluid velocity decreases. This may well be the case at high redshifts: the hotter and denser circum-galactic medium at high redshifts could result in slower speeds for the hotspot and the jet material behind it. The smaller sizes of radio sources at higher redshifts provide support to this scenario

Ooty occultations of 76 radio sources

Positional and structural information derived from lunar occultations observed at 327 MHz is presented for 76 radio sources, most of them of flux density less than 2×10-26Wm-2Hz-1. Only 16 sources are optically identified. Two of the stronger sources in the list, viz. PKS 1417-19 and MSH 16-205 have a head-tail type of radio structure. PKS 1417-19 appears to be associated with a small chain of faint galaxies. It is interesting that a close group of five radio sources, each with flux density ≥0.5 f.u. appears to be physically associated with the cluster Zw 0210.1 + 1857. One of the five sources coincides with a 19-mag BSO, while none of the other four is identified with an optical object on the basis of positional agreement

The Molonglo reference catalog 1 Jy radio source survey. III. Identification of a complete quasar sample

We present a new complete sample of 111 radio quasars (including six BL Lac objects) selected from the Molonglo Reference Catalog (MRC) at 408 MHz. The sample, which we call the Molonglo Quasar Sample (MQS), forms part of a complete survey of 557 MRC radio sources with S408 ≥ 0.95 Jy in the declination range -30 ° < δ < -20 ° , b > 20 ° but excluding the R.A. range 14h03m-20h20m. Quasar classifications are based on high-resolution radio images, deep optical identifications, and follow-up spectroscopy of sources in the strip. The relatively low radio frequency of the finding survey and the complete optical identification of quasars to faint magnitudes ensure that the MQS is relatively free from orientation biases that affect most other samples of radio-loud quasars. The MQS is therefore particularly well suited to investigating the effects of radio axis orientation on quasar properties. This paper describes in detail the formation of the MQS and presents basic radio and optical data, including VLA images of extended radio sources in the sample and a complete set of optical finding charts

The Molonglo Reference Catalog 1-Jy radio source survey IV. Optical spectroscopy of a complete quasar sample

Optical spectroscopic data are presented here for quasars from the Molonglo Quasar Sample (MQS), which forms part of a complete survey of 1-Jy radio sources from the Molonglo Reference Catalogue. The combination of low-frequency selection and complete identifications means that the MQS is relatively free from the orientation biases which affect most other quasar samples. To date, the sample includes 105 quasars and 6 BL Lac objects, 106 of which have now been confirmed spectroscopically. This paper presents a homogenous set of low-resolution optical spectra for 79 MQS quasars, the majority of which have been obtained at the Anglo-Australian Telescope. Full observational details are given and redshifts, continuum and emission-line data tabulated for all confirmed quasars.Comment: 40 pages, ApJS in pres

The Molonglo reference catalog/1 Jansky radio source survey. I. Radio galaxy identifications

This is the first in a series of papers discussing the properties of a complete set of radio sources selected near S408 = 1 Jy. We present optical or infrared identifications for a sample of 452 radio galaxies. The sources were selected from the 408 MHz Molonglo Reference Catalog, restricted only by flux density and position on the sky, as follows: S408 ≥ = 0.95 Jy and -30° < decl. (1950) < -20°, and 9h20m < R.A.(1950) < 14h4m or 20h20m < R.A.(1950) < 6h14m. This complete sample, the MRC/1 Jy survey, contains 558 radio galaxies or quasars, of which six are occulted by bright stars or galaxies and two are multiple confused sources. Of the remaining 550 sources, 527, or 96%, are identified to an r magnitude of 25, and eight additional sources are identified to K = 19. Of the 17 unocculted objects that remain unidentified, 15 are sources for which we have either poor radio maps or inadequate optical/IR images. This paper presents finding charts and astrometric positions for the 452 sources that are not identified as either quasars or BL Lacertae objects. Magnitudes in the r passband accurate to typically 0.1 mag are given for 353 of the radio galaxy identifications. Redshifts for 268 of the galaxies are also listed; these have been derived from 450 spectroscopic observations. The radio observations, quasar and BL Lac identifications, spectroscopy, and near- IR images will be presented in subsequent papers in this series

The alignment of distant radio sources

The relative orientation of the axes of extragalactic extended radio sources was first investigated by Willson, who noted that in the 3CR sample there was a tendency for the axes to lie parallel to each other for sources separated by ≤10° on the sky. Using two larger samples, Sanders has recently concluded that this tendency to align is present at a high confidence level (>3 σ) only for distant (redshift, z>0.4) sources and could arise from gravitational lensing by superclusters, if most of the matter in a closed universe is in thin filaments. Because of the important cosmological implications, we have now attempted to verify the effect using two more independent and larger source samples and find no significant tendency for axis alignment. In view of this negative result, we also investigated a sample formed by adding well-observed radio quasars to 3CR sources (similar to one of the two samples used by Sanders) and found that, even in this sample, the effect shows up only at a marginally significant level (<2 σ)