Associated absorption in radio quasars. I. C IV absorption and the growth of radio sources

We assess the global properties of associated C IV 1548,1550 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 lobe-dominated 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.