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Quasars found in low frequency radio surveys are often assumed to have their jet axes randomly oriented in the sky because relativistically beamed radio cores rarely contribute significantly to the total flux density at metrewaves. But an orientation bias can still arise from the optical magnitude limit of the sample (Kapahi & Shastri 1987) if radio beaming is accompanied by an enhancement of the optical continuum as well (due to beaming or other effects). Such a bias can explain the finding (de Ruiter et al. 1986) that the typical value of R (ratio of core to extended flux density at 5 GHz) for the magnitude limited Bologna sample is about 5 times larger than for 3CR quasars, eventhough both samples are from low frequency surveys. It is thus consistent with the unified scheme for quasars (Orr & Browne 1982; Kapahi & Saikia 1982), which in fact requires an aspect dependence of the optical continuum (eg. Browne & Wright 1985; Browne & Murphy 1987).
Redshift distributions of almost complete samples of steep as well as flat-spectrum radio quasars are investigated. It is found that flat-spectrum (core-dominated) quasars contain a higher fraction of high redshifts (z ≳ 1.4) compared to steep-spectrum ones. The difference can be understood in the ‘unified scheme’ in which the two types of quasars differ only in the orientation of their jet axes with respect to the observer.
The angular size - redshift (θ - z) relation can in principle be used to discriminate between world models because the angular size subtended by a rigid rod is quite a sensitive function of cosmology, specially at z ≳ 0.5. The test is simpler to apply to objects for which a metric diameter is measured than to objects with isophotal diameters (Sandage 1961). It was first suggested by Hoyle (1958) at the Paris symposium on Radio Astronomy, that the separation between the two lobes of extragalactic radio sources such as Cyg-A, could be used for performing such a test. In an Einstein-de Sitter Universe sources like Cyg-A cannot have angular sizes ≲ 15 arcsec (the minimum occuring at z = 1.25) whereas in the Steady State Universe their sizes should asymptotically approach a value near 4 arcsec at large redshifts. It was not until the early seventies that the test was actually applied to samples of radio quasars with redshifts of upto ∼ 2 (Legg 1970; Miley 1971; Wardle & Miley 1974). The angular sizes were found to show a large scatter due to a wide distribution of physical sizes and the projection effects associated with the essentially linear radio structures. The upper envelope to the θ -values (which would be expected to show much less scatter) nevertheless appeared to fall off monotonically with increasing z, more or less like the Euclidean relation θ ∝ z−1. The θ - z test thus appeared to be incompatible with the predictions of uniform world models in which the linear sizes of quasars are independent of epoch.
We present some interesting results from a radio study of 17 radio galaxies at z >2, selected from a complete sample of Molonglo sources with S408 > 0.95 Jy for which optical identifications and spectroscopic redshifts are being obtained in a major observational programme (Kapahi et al. 1995).
Observations and results : The 17 galaxies were mapped using the VLA at 1.4, 4.8 and 8.3 GHz with the highest resolution of ≃ 0.3″ at 8.3 GHz. The integrated spectra are generally quite steep and convex in many sources, steepening to α ≥ 1.2 (S ∝ v–α) at the highest frequency. Most of the sources have a double structure with sizes ranging up to 22″. Unresolved radio cores, coincident with the optical galaxies, are detected at the mJy or sub-mJy level in most cases. Several sources also show one-sided jetlike features. Surprisingly, most of the cores appear to have steep spectra between 5 and 8.3 GHz!
The MRC/1Jy sample of 559 radio sources with S408 MHz ≥ 0.95 Jy (McCarthy et al. 1996; Kapahi et al. in preparation) is a factor of 5 to 6 times deeper than the 3CRR sample; it is therefore, well suited for disentangling the redshift (z) and luminosity (P) dependence of several properties of extragalaxtic radio sources. Here we present results on the spectral index — redshift correlation for radio galaxies, based on a comparison of the well documented radio spectra (in the rest frame frequency range of about 1 to 16 GHz) of the following two matched-luminosity samples, (a) 14 high redshift radio galaxies (HRRG) from MRC with 2.0 < z < 3.2 and linear size i > 10 kpc, and (b) 21 intermediate z radio galaxies (IRRG) from 3CRR with 0.85 < z < 1.7 and l > 10 kpc. Both samples have P1.4 GHz in the range 1028 and 1028.8 WHz−1.
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