The Line‐of‐Sight Proximity Effect and the Mass of Quasar Host Halos

We show that the Lyman-alpha optical depth statistics in the proximityregions of quasar spectra depend on the mass of the dark matter halos hostingthe quasars. This is owing to both the overdensity around the quasars and theassociated infall of gas toward them. For a fiducial quasar host halo mass of(3.0+/-1.6) h^-1 x 10^12 Msun, as inferred by Croom et al. from clustering inthe 2dF QSO Redshift Survey, we show that estimates of the ionizing background(Gamma^bkg) from proximity effect measurements could be biased high by a factorof ~2.5 at z=3 owing to neglecting these effects alone. The clustering ofgalaxies and other active galactic nuclei around the proximity effect quasarsenhances the local background, but is not expected to skew measurements by morethan a few percent. Assuming the measurements of Gamma^bkg based on the meanflux decrement in the Ly-alpha forest to be free of bias, we demonstrate howthe proximity effect analysis can be inverted to measure the mass of the darkmatter halos hosting quasars. In ideal conditions, such a measurement could bemade with a precision comparable to the best clustering constraints to datefrom a modest sample of only about 100 spectra. We discuss observationaldifficulties, including continuum flux estimation, quasar systematic redshiftdetermination, and quasar variability, which make accurate proximity effectmeasurements challenging in practice. These are also likely to contribute tothe discrepancies between existing proximity effect and flux decrementmeasurements of Gamma^bkg.Comment: 25 pages, including 14 figures, accepted by Ap