QSO Absorption Lines from QSO s

We present the results of a search for metal absorption lines in the spectraof background QSOs whose sightlines pass close to foreground QSOs. We detectMgII(2796,2803) absorption in Sloan Digital Sky Survey (SDSS) spectra of fourz>1.5 QSOs whose lines of sight pass within 26-98 kpc of lower redshift(z~0.5-1.5) QSOs. The 100% [4/4 pairs] detection of MgII in the background QSOsis clearly at odds with the incidence of associated (z_abs ~ z_em) systems --absorbers which exist towards only a few percent of QSOs. Although the qualityof our foreground QSO spectra is not as high as the SDSS data, absorption seentowards one of the background QSOs clearly does not show up at the samestrength in the spectrum of the corresponding foreground QSO. This implies thatthe absorbing gas is distributed inhomogeneously around the QSO, presumably asa direct consequence of the anisotropic emission from the central AGN. Wediscuss possible origins for the MgII lines, including: absorption by gas fromthe foreground QSO host galaxy; companion galaxies fuelling the QSO throughgravitational interactions; and tidal debris left by galaxy mergers orinteractions which initiated the QSO activity. No single explanation isentirely satisfactory, and we may well be seeing a mixture of phenomena.Comment: Accepted for publication in ApJ Letters. 5 page