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J. Wise, M. Eracleous, J. C. Charlton, R. Ganguly (Penn State)
We present the results of a search for variability in the equivalent widths of narrow, associated (\delta v < 5,000 km/s) absorption lines found in the UV spectra of nearby (z<1.5) quasars. The goal of this search was to use variability as a means of identifying absorption lines arising in gas that is truly intrinsic to the quasar central engine. We have compared HST/FOS spectra of quasars observed in the QSO Absorption Line Key Project with recent spectra obtained as part of our own snapshot survey of the same objects with STIS. The typical intervals between observations are 5--10 years. We primarily focused on the C IV absorption lines, although we also studied other lines available in the same spectra (e.g., N V and Ly\alpha). Our main result is that in more than half of the quasars, the associated absorption line equivalent width varied by 20 percent or more (up to a factor of 2). Such variations are considerably higher than the sensitivity limit of our search, as judged by comparing the equivalent widths of Galactic, interstellar lines observed at the two different epochs. We conclude that at least half of the associated absorption lines observed in quasar spectra must be intrinsic, which is consistent with the previous estimates based on variability, partial coverage analysis, or statistical arguments. Moreover, we are now able to pick out specific intrinsic absorption-line systems to be followed up with high-dispersion spectroscopy in order to determine the geometry, location, and physical conditions of the absorber. This work was supported by NASA through grant HST-GO-08681.01-A.