The Quasar-frame Velocity Distribution of Narrow CIV Absorbers

TL;DR

This study analyzes narrow CIV absorption lines in quasars to understand their relation to quasar outflows, revealing a significant fraction of absorbers are intrinsic to the outflows and characterizing their velocity distribution.

Contribution

It provides the first detailed measurement of the velocity distribution of narrow CIV absorbers and their association with quasar outflows, using SDSS data and a novel velocity zero-point calibration.

Findings

Over 43% of narrow CIV absorbers are intrinsic to quasar outflows.

Outflow absorbers peak at about 2000 km/s in velocity.

Approximately 14% of quasars show narrow outflowing CIV absorption.

Abstract

We report on a survey for narrow (FWHM < 600 km/s) CIV absorption lines in a sample of bright quasars at redshifts $1.8 \le z < 2.25$ in the Sloan Digital Sky Survey. Our main goal is to understand the relationship of narrow CIV absorbers to quasar outflows and, more generally, to quasar environments. We determine velocity zero-points using the broad MgII 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 a substantial number ($\ge 43\pm6$ per cent) of absorbers with REW $> 0.3$ \AA in the velocity range +750 km/s $\la v \la $ +12000 km/s are intrinsic to the AGN outflow. This `outflow fraction' peaks near $v=+2000$ km/s with a value of $f_{outflow} \simeq 0.81 \pm 0.13$. At velocities below $v \approx +2000$ km/s the incidence of outflowing systems drops, possibly due to geometric effects or to the over-ionization of gas that is nearer the accretion disk. Furthermore, we find that outflow-absorbers are on average broader and stronger than cosmologically-intervening systems. Finally, we find that $\sim 14$ per cent of the quasars in our sample exhibit narrow, outflowing CIV absorption with REW $> 0.3$\AA, slightly larger than that for broad absorption line systems.