The quasar-frame velocity distribution of narrow C iv absorbers

Abstract

We report on a survey for narrow (full widths at half-minimum <600 km s−1) C iv absorption lines in a sample of bright quasars at redshifts 1.8 ≤z < 2.25 in the Sloan Digital Sky Survey. Our main goal is to understand the relationship of narrow C iv absorbers to quasar outflows and, more generally, to quasar environments. We determine velocity zero-points using the broad Mg ii emission line, and then measure the absorbers' quasar-frame velocity distribution. We examine the distribution of lines arising in quasar outflows by subtracting model fits to the contributions from cosmologically intervening absorbers and absorption due to the quasar host galaxy or cluster environment. We find that a substantial number (≥43 ± 6 per cent) of absorbers with Wλ15480 > 0.3Å in the velocity range +750 ≲v≲+ 12 000 km s−1 are intrinsic to the active galactic nucleus outflow. This ‘outflow fraction’ peaks near v=+2000 km s−1 with a value of foutflow≃ 0.81 ± 0.13. At velocities below v≈+ 2000 km s−1, the incidence of outflowing systems drops, possibly due to geometric effects or to the over-ionization of gas that is nearer the accretion disc. Furthermore, we find that outflow absorbers are on average broader and stronger than cosmologically intervening systems. Finally, we find that ∼14 per cent of the quasars in our sample exhibit narrow, outflowing C iv absorption with Wλ15480 > 0.3Å, slightly larger than that for broad absorption line systems.