Discovery of the Galaxy Proximity Effect and Implications for Measurements of the Ionizing Background Radiation at Low Redshifts

We present an analysis of galaxy and QSO absorption line pairs toward 24 QSOsat redshifts between z~0.2 and 1 in an effort to establish the relationshipbetween galaxies and absorption lines in physical proximity to QSOs. Wedemonstrate the existence of a galaxy proximity effect, in that galaxies in thevicinities of QSOs do not show the same incidence and extent of gaseousenvelopes as galaxies far from QSOs. We show that the galaxy proximity effectexists to galaxy-QSO velocity separations of ~ 3000 km/s, much larger than thesize of a typical cluster (~ 1000 km/s), i.e. it is more comparable to thescale of the sphere of influence of QSO ionizing radiation rather than thescale of galaxy-QSO clustering. This indicates that the QSO ionizing radiationrather than some dynamical effect from the cluster environment is responsiblefor the galaxy proximity effect. We combine previous findings that (1) many ormost Lya absorption lines arise in extended galaxy envelopes, and (2) galaxiescluster around QSOs to show that the magnitude of the Lya forest proximityeffect is underestimated. Consequently, determinations of the UV ionizingbackground intensity using the proximity effect are likely overestimated. Weuse the galaxy-QSO cross-correlation function measured from our data toestimate the magnitude of this overestimate and find that it could be as highas a factor of 20 at z<1. This can have strong implications for models of theorigin and evolution of the ionizing background, and may indicate that QSOsproduce sufficient ionizing flux at all redshifts to account for the entirebackground radiation field.Comment: 26 pages, 3 figures, to appear in ApJ, October 20, 200