Variable UV Absorption in the Spectrum of MRC 2251-178

Abstract

We present an ultraviolet spectrum of MRC 2251-178 taken with HST/STIS-G230L. The observation is part of a snap shot program of QSOs to systematically search for intrinsic absorption lines through variability. The sample consists of all QSOs observed with HST/FOS which showed associated narrow absorption lines. The FOS spectrum of MRC 2251-178, taken in 1996, showed an associated C IV doublet with an C IV \lambda1548 equivalent width 1.09+/-0.09 A (Monier et al. 2001). It is not detected in the STIS spectrum taken four years later, down to a 3-sigma threshold of 0.19 A. In addition to its other accolades, these observations make MRC 2251-178 the very first QSO at low redshift in which the associated absorption is shown to be truly intrinsic. We discuss the implications of this and suggest courses for future study.