A Direct Measurement of the Intergalactic Medium Opacity to H I Ionizing Photons
Abstract
We present a new method to directly measure the opacity from H I Lyman limit (LL) absorption κLL 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 λ912 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 κLL at any redshift. From z = 3.6 to 4.3, the opacity increases steadily as expected and is well parameterized by λ912 mfp = λ0 - b λ(z - 3.6) with λ0 = (48.4 ± 2.1) h -1 72 Mpc and b λ = (38.0 ± 5.3) h -1 72 Mpc (proper distance). The relatively high λ912 mfp values indicate that the incidence of systems which dominate κLL evolves less strongly at z > 3 than that of the Lyα 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 ~ 3 IGM studies using the SDSS database.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2009
- DOI:
- 10.1088/0004-637X/705/2/L113
- arXiv:
- arXiv:0910.0009
- Bibcode:
- 2009ApJ...705L.113P
- Keywords:
-
- intergalactic medium;
- large-scale structure of universe;
- quasars: absorption lines;
- Astrophysics - Cosmology and Nongalactic Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 7 pages, 4 figures