Constraining quasar host halo masses with the strength of nearby Lyα forest absorption

Using cosmological hydrodynamic simulations, we measure the mean transmitted flux in the Ly alpha forest for quasar sightlines that pass near a foreground quasar. We find that the trend of absorption with pixel quasar separation distance can be fitted using a simple power-law form including the usual correlation function parameters r(0) and gamma, so that [F(r)] = exp{-tau(eff)[1+(r/r(0))(-gamma)]}. From the simulations, we find the relation between r0 and quasar host mass, and formulate this as a way to estimate quasar host dark matter halo masses, quantifying uncertainties due to cosmological and IGM parameters, and redshift errors. With this method, we examine data for similar to 9000 quasars from the Sloan Digital Sky Survey (SDSS) Data Release 5, assuming that the effect of ionizing radiation from quasars (the so- called transverse proximity effect) is unimportant (no evidence for it is seen in the data). We find that the best- fitting host halo mass for SDSS quasars with mean redshift z = 3 and absolute G- band magnitude -27.5 is log M/M-circle dot = 12.68(-0.67)(+0.81). We also use the Lyman- Break Galaxy (LBG) and Ly alpha forest data of Adelberger et al. in a similar fashion to constrain the halo mass of LBGs to be log(10) M/M-circle dot = 11.41(-0.59)(+0.54), a factor of similar to 20 lower than the bright quasars. In addition, we study the redshift distortions of the Ly alpha forest around quasars, using the simulations. We use the quadrupole to monopole ratio of the quasar Ly alpha forest correlation function as a measure of the squashing effect. We find its dependence on halo mass difficult to measure, but find that it may be useful for constraining cosmic geometry.