Constraining the Redshiftz ∼ 6 Quasar Luminosity Function Using Gravitational Lensing

Abstract

Recent discoveries by the Sloan Digital Sky Survey (SDSS) of four bright z ~ 6 quasars could constrain the mechanism by which the supermassive black holes powering these sources are assembled. Here we compute the probability that the fluxes of the quasars are strongly amplified by gravitational lensing and therefore the likelihood that the black hole masses are overestimated when they are inferred assuming Eddington luminosities. The poorly constrained shape of the intrinsic quasar luminosity function (LF) at redshift ~6 results in a large range of possible lensing probabilities. If the LF is either steep or extends to faint magnitudes, the probability for amplification by a factor μ ≳ 10 (and with only one image detectable by SDSS) can reach essentially 100%. We show that future observations, in particular, of either the current four quasars at the high angular resolution provided by the Hubble Space Telescope or an increased sample of ~20 z ~ 6 quasars at the current angular resolution, should either discover several gravitational lenses or else provide interesting new constraints on the shape of the z ~ 6 quasar LF.