Occurrence and Global Properties of Narrow C IV λ1549 Å Absorption Lines in Moderate-Redshift Quasars

A statistical study is presented of (1) the frequency of narrow C IV λ1549 absorption lines in 1.5<=z<~3.6 radio-quiet and radio-loud quasars and (2) the UV and radio properties of the absorbed quasars. The quasar sample is unbiased with respect to absorption properties, and the radio-quiet and radio-loud subsamples are well matched in redshift and luminosity. A similarly high incidence (>~50%) of narrow C IV absorbers is detected for the radio-quiet and radio-loud quasars, and a constant ~25% of all the quasars, irrespective of radio type, display associated C IV absorbers stronger than EW_rest>=0.5 Å. Both radio-quiet and radio-loud quasars with narrow absorption lines have systematically redder continua, especially strongly absorbed objects. There is evidence of inclination-dependent dust reddening and absorption for the radio quasars. An additional key result is that the most strongly absorbed radio quasars have the largest radio source extent. This result is in stark contrast to a recent study of the low-frequency-selected Molonglo survey, in which a connection between the strength of the narrow absorbers and the (young) age of the radio source has been proposed. The possible origin of these discrepant results is discussed and may be related to the higher source luminosity for the quasars studied here.

Part of the observations reported here were obtained at the Multiple Mirror Telescope Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.