A Direct Measurement of the IGM Opacity to HI Ionizing Photons

TL;DR

This paper introduces a new method to directly measure the IGM's opacity to HI ionizing photons by analyzing stacked quasar spectra, providing the most precise measurements of mean free path at high redshift and revealing biases in SDSS data.

Contribution

A novel technique for measuring IGM opacity using stacked quasar spectra, yielding more accurate mean free path estimates at z~3-4 and identifying biases in SDSS quasar selection.

Findings

Mean free path increases from 48.4 to 38.0 h^-1 Mpc between z=3.6 and 4.3.

Mean free path is three times higher than some previous estimates.

Identified a systematic bias in SDSS quasar color targeting criteria.

Abstract

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.