A B S T R A C TUsing 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 by-product 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 on to 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.

We present low-resolution, composite optical spectra for 60 quasars drawn from the 408 MHz-selected Molonglo Quasar Sample. Individual spectra have been co-added according to the ratio of radio core-to-lobe flux density R in order to show global changes as a function of quasar orientation. Compact steep-spectrum (CSS) quasars have been combined separately. With increasing implied viewing angle to the radio-jet axis, we find that (1) the optical continuum steepens, (2) the 3000 Å broad emission feature decreases in relative strength, and (3) the narrow-line equivalent widths, broad-line widths, and Balmer decrements increase. The composites show that reddening is considerable in most lobe-dominated quasars ( A V 1 2-4). Moreover, the extinction appears to be aspect-dependent, which suggests an intrinsic origin for the dust. The CSS composite spectrum is particularly striking, showing evidence for significant reddening, a steep power-law continuum, relatively strong lowionization narrow-line emission, and no discernible 3000 Å bump.

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We assess the global properties of associated C ivλλ1548,1550 absorption lines measured in the spectra of radio-loud quasars drawn from a near-complete, low-frequency selected sample. The observations span restframe C iv in two redshift ranges-0.7 < z < 1.0 and 1.5 < z < 3.0-which were targetted in the UV with HST/STIS and in the optical with ground-based telescopes, respectively. First, we corroborate trends for C iv associated absorption to be found preferentially in steep-spectrum and lobedominated quasars, implying the absorbing material tends to lie away from the radio-jet axis. Furthermore, we find a clear anticorrelation between C iv absorption strength and the projected linear size of steep-spectrum quasars, indicative of an evolutionary sequence. We also find that heavily-absorbed quasars are systematically redder, implying dust is mixed in with the C iv-absorbing gas. No redshift dependence was found in any of the trends considered. These new results show that radio sources are triggered in galaxies which are exceptionally rich in gas and dust, which then dissipates on a timescale comparable with, but less than, that of the radio source. This observational sequence, together with the lack of redshift-dependence, points to a direct causal link between the event which triggered the radio source and the build-up of absorbing gas and dust, whose make-up is tantalisingly similar to the products of a post-merger starburst. Thus, these new results provide direct evidence for the clearing of absorbing material around quasars with time, as well as the probable association of starburst activity with the onset of the radio activity in AGN.