A Direct Measurement of the IGM Opacity to HI Ionizing Photons
  We present a new method to directly measure the opacity from HI Lyman limit
(LL) absorption k_LL along quasar sightlines 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 (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
k_LL at any redshift. From z=3.6 to 4.3, the opacity increases steadily as
expected and is well parameterized by MFP = (48.4 +/- 2.1) - (38.0 +/-
5.3)*(z-3.6) h^-1 Mpc (proper distance). The relatively high MFP values
indicate that the incidence of systems which dominate k_LL evolves less
strongly at z>3 than that of the Lya forest. We infer a mean free path three
times higher than some previous estimates, a result which has important
implications for the photo-ionization 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~3 IGM studies
using the SDSS database.
