Origin of the viewing-angle dependence of the optical continuum emission in quasars

The orientation-dependence of the optical continuum emission in radio-loud quasars is investigated using a new, complete sample of low-frequency-selected quasars, the Molonglo Quasar Sample (MQS). The optical continuum is found to be highly anisotropic, brightening continuously from lobe- to core-dominated quasars by 3-5 mag. It is argued that aspect-dependent extinction, rather than relativistic boosting as has been previously proposed, provides the simplest explanation consistent with the data. The reddening hypothesis is supported by both the steeper optical slopes and the larger Balmer decrements found in lobe-dominated quasars, as well as the stronger anisotropy seen at blue wavelengths. The dust responsible is shown to be physically associated with the quasar, lying mostly at radii between the broad and narrow-line regions in a clumpy distribution. Such a geometry is reminiscent of a torus. However, substantial numbers of dust clouds must lie within the torus opening angle, contributing to an increasing average optical depth with increasing viewing angle away from the jet axis.Comment: 14 pages, 21 figures, MNRAS accepte

Deep optical imaging of the field of PC1643+4631A&B, II: Estimating the colours and redshifts of faint galaxies

In an investigation of the cause of the cosmic microwave background decrement in the field of the z = 3.8 quasar pair PC1643+4631, we have carried out a study to photometrically estimate the redshifts of galaxies in deep multi-colour optical images of the field taken with the WHT. To examine the possibility that a massive cluster of galaxies lies in the field, we have attempted to recover simulated galaxies with intrinsic colours matching those of the model galaxies used in the photometric redshift estimation. We find that when such model galaxies are added to our images, there is considerable scatter of the recovered galaxy redshifts away from the model value; this scatter is larger than that expected from photometric errors and is the result of confusion, simply due to ground-based seeing, between objects in the field. We have also compared the likely efficiency of the photometric redshift technique against the colour criteria used to select z>3 galaxies via the strong colour signature of the Lyman-limit break. We find that these techniques may significantly underestimate the true surface density of z>3, due to confusion between the high-redshift galaxies and other objects near the line of sight. We argue that the actual surface density of z=3 galaxies may be as much as 6 times greater than that estimated by previous ground-based studies, and note that this conclusion is consistent with the surface density of high-redshift objects found in the HDF. Finally, we conclude that all ground-based deep field surveys are inevitably affected by confusion, and note that reducing the effective seeing in ground-based images will be of paramount importance in observing the distant universe.Comment: 18 pages, 60 figures, submitted to MNRAS, 2 large figure avaliable at ftp://ftp.mrao.cam.ac.uk:/pub/PC1643/paper2.figure50.eps and ftp://ftp.mrao.cam.ac.uk:/pub/PC1643/paper2.figure51.ep

Deep optical imaging of the field of PC1643+4631A&B, I: Spatial distributions and the counts of faint galaxies

We present deep optical images of the PC1643+4631 field obtained at the WHT. This field contains two quasars at redshifts z=3.79 & 3.83 and a cosmic microwave background (CMB) decrement detected with the Ryle Telescope. The images are in U,G,V,R and I filters, and are complete to 25th magnitude in R and G and to 25.5 in U. The isophotal galaxy counts are consistent with the results of Metcalde et al. (1996), Hogg et al. (1997), and others. We find an excess of robust high-redshift Ly-break galaxy candidates with 25.0<R<25.5 compared with the mean number found in the fields studied by Steidel et al. -we expect 7 but find 16 - but we do not find that the galaxies are concentrated in the direction of the CMB decrement. However, we are still not sure of the distance to the system causing the CMB decrement. We have also used our images to compare the commonly used object-finding algorithms of FOCAS and SExtractor: we find FOCAS the more efficient at detecting faint objects and the better at dealing with composite objects, whereas SExtractor's morphological classification is more reliable, especially for faint objects near the resolution limit. More generally, we have also compared the flux lost using isophotal apertures on a real image with that on a noise-only image: recovery of artificial galaxies from the noise-only image significantly overestimates the flux lost from the galaxies, and we find that the corrections made using this technique suffer a systematic error of some 0.4 magnitudes.Comment: 17 pages, 40 figures, submitted to MNRAS, 1 large figure avaliable at ftp://ftp.mrao.cam.ac.uk:/pub/PC1643/paper1.figure18.p

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 Radio-Optical Correlation in Steep-Spectrum Quasars

Using complete samples of steep-spectrum quasars, we present evidence for a correlation between radio and optical luminosity which is not caused by selection effects, nor caused by an orientation dependence (such as relativistic beaming), nor a byproduct of cosmic evolution. We argue that this rules out models of jet formation in which there are no parameters in common with the production of the optical continuum. This is arguably the most direct evidence to date for a close link between accretion onto a black hole and the fuelling of relativistic jets. The correlation also provides a natural explanation for the presence of aligned optical/radio structures in only the most radio luminous high-redshift galaxies.Comment: MNRAS in press. Uses BoxedEPS (included