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

We assess the global properties of associated C IV lambdalambda1548, 1550 absorption lines measured in the spectra of radio-loud quasars drawn from a near-complete, low-frequency selected sample. The observations span rest-frame C IV in two redshift ranges, 0.7 < z < 1.0 and 1.5 < z < 3.0, which were targeted in the UV with the Hubble Space Telescope Space Telescope Imaging Spectrograph and in the optical with ground-based telescopes. First, we corroborate trends for C IV associated absorption to be found preferentially in steep-spectrum and lobe-dominated quasars, implying that 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 that 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 that are exceptionally rich in gas and dust, which then dissipates on a timescale comparable to 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 that triggered the radio source and the build-up of absorbing gas and dust, whose make-up is tantalizingly similar to the products of a postmerger 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 active galactic nuclei.