Variable Ultraviolet Absorption in the Spectrum of MR 2251-178

Abstract

We present an ultraviolet spectrum of MR 2251-178 taken with Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) with the G230L grating. The observation is part of a snapshot program of quasars (QSOs) that systematically searches for intrinsic absorption lines through variability. The sample consists of all QSOs observed with the HST/Faint Object Spectrograph (FOS) that showed associated narrow absorption lines. The FOS spectrum of MR 2251-178, taken in 1996, showed an associated C IV doublet with a λ1548 equivalent width of 1.09+/-0.09 Å. It is not detected in the STIS spectrum taken 4 years later, down to a 3 σ threshold of 0.19 Å. In addition to its other accolades, these observations make MR 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. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.