Intrinsic Narrow Absorption Lines in Keck HIRES Spectra of a Sample of Six Quasars

Abstract

In the course of a large survey of intervening Mg II absorbers in the spectra of quasars, the C IV emission lines of six of the target objects (three radio-loud and three radio-quiet) were observed serendipitously. In four of these six quasars, we detected "associated" narrow absorption lines with velocities within 5000 km s-1 of the quasar emission-line redshift (three of these four quasars are radio quiet while the other is radio loud). As a result of the original target selection, the small sample of six quasars is unbiased toward finding associated absorption lines. In three of these four cases, the absorption-line optical-depth ratios deviate from the prediction based on atomic physics, suggesting that the background photon source(s) are only partially covered by the absorbing medium and, by extension, that the absorption lines are intrinsic to the quasar. We have used the method of Barlow &Sargent to determine the effective coverage fraction of background source(s) and we have extended it to constrain the coverage fraction of the continuum and broad emission-line sources separately. We have also applied this refined method to the narrow intrinsic absorption lines in three additional quasars for which the necessary data were available from the literature. We find that in two objects from our sample, the continuum source must be partially covered regardless of the covering factor of the emission-line source. We discuss these results in the context of the properties of absorption lines observed in different types of active galaxies and related outflow phenomena. We cannot distinguish between possible mechanisms for the origin of the partial coverage signature, although we do consider possible observational tests. Finally, we speculate on how the gas responsible for the narrow lines may be related to the accretion disk wind that may be responsible for the broad absorption lines observed in some quasars.