The Column Density Distribution and Continuum Opacity of the Intergalactic and Circumgalactic Medium at Redshift <z>=2.4

Creators: Rudie, Gwen C.; Steidel, Charles C.; Shapley, Alice E.; Pettini, Max

Abstract

We present new high-precision measurements of the opacity of the intergalactic and circumgalactic medium (IGM, CGM) at =2.4. Using Voigt profile fits to the full Lyman alpha and Lyman beta forests in 15 high-resolution high-S/N spectra of hyperluminous QSOs, we make the first statistically robust measurement of the frequency of absorbers with HI column densities 14≾log(N_(HI))≾ 17.2. We also present the first measurements of the frequency distribution of HI absorbers in the volume surrounding high-z galaxies (the CGM, 300 pkpc), finding that the incidence of absorbers in the CGM is much higher than in the IGM. In agreement with Rudie et al. (2012), we find that there are fractionally more high-N_(HI) absorbers than low-N_(HI) absorbers in the CGM compared to the IGM, leading to a shallower power law fit to the CGM frequency distribution. We use these new measurements to calculate the total opacity of the IGM and CGM to hydrogen-ionizing photons, finding significantly higher opacity than most previous studies, especially from absorbers with log(N_(HI))< 17.2. Reproducing the opacity measured in our data as well as the incidence of absorbers with log(N_(HI)/cm^(-2)) > 17.2 requires a broken power law parameterization of the frequency distribution with a break near log(N^(HI))≈10^(15)cm^(-2). We compute new estimates of the mean free path λ_(mfp) to hydrogen-ionizing photons at Z_(em)=2.4, finding λ_(mfp) = 147 + 15 Mpc when considering only IGM opacity. If instead, we consider photons emanating from a high-z star-forming galaxy and account for the local excess opacity due to the surrounding CGM of the galaxy itself, the mean free path is reduced to λ_(mfp) = 121 + 15 Mpc. These λ_(mfp) measurements are smaller than recent estimates and should inform future studies of the metagalactic UV background and of ionizing sources at z≈2-3.

Additional Information

© 2013 American Astronomical Society. Received 2012 August 24; accepted 2013 April 22; published 2013 May 15. Based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration, and was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to thank Claude-André Faucher-Giguère for his careful reading of the draft and pertinent comments. We also thank Olivera Rakic for her contributions to the reduction of the QSO data set and for her helpful advice. The authors wish to acknowledge Ryan Cooke who contributed the fits to the damped profiles in our QSO spectra. Our thanks to Bob Carswell for his assistance with VPFIT. We wish to acknowledge the staff of the W.M. Keck Observatory whose efforts insure the telescopes and instruments perform reliably. Further, we extend our gratitude to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. This work has been supported by the US National Science Foundation through grants AST-0606912 and AST-0908805. CCS acknowledges additional support from the John D. and Catherine T. MacArthur Foundation and the Peter and Patricia Gruber Foundation. This research has made use of the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with the National Aeronautics and Space Administration.

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Submitted - 1304.6719v1.pdf

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