A DIRECT MEASUREMENT OF THE INTERGALACTIC MEDIUM OPACITY TO H I IONIZING PHOTONS

We present a new method to directly measure the opacity from H I Lyman limit (LL) absorption kappa(LL) along quasar sight lines by the intergalactic medium (IGM). The approach analyzes the average (stacked) spectrum of an ensemble of quasars at a common redshift to infer the mean free path lambda(912)(mfp) to ionizing radiation. We apply this technique to 1800 quasars at z = 3.50-4.34 drawn from the Sloan Digital Sky Survey (SDSS), giving the most precise measurements on kappa(LL) at any redshift. From z = 3.6 to 4.3, the opacity increases steadily as expected and is well parameterized by lambda(912)(mfp) = lambda(0) - b(lambda)(z - 3.6) with lambda(0) = (48.4 +/- 2.1)h(72)(-1) Mpc and b(lambda) = (38.0 +/- 5.3)h(72)(-1) Mpc (proper distance). The relatively high lambda(912)(mfp) values indicate that the incidence of systems which dominate kappa(LL) evolves less strongly at z > 3 than that of the Ly alpha forest. We infer a mean free path three times higher than some previous estimates, a result which has important implications for the photoionization rate derived from the emissivity of star-forming galaxies and quasars. Finally, our analysis reveals a previously unreported, systematic bias in the SDSS quasar sample related to the survey's color targeting criteria. This bias potentially affects all z similar to 3 IGM studies using the SDSS database.