skip to main content

Title: Keck spectroscopy of gravitationally lensed z ≅ 4 galaxies: Improved constraints on the escape fraction of ionizing photons

The fraction of ionizing photons that escape from young star-forming galaxies is one of the largest uncertainties in determining the role of galaxies in cosmic reionization. Yet traditional techniques for measuring this fraction are inapplicable at the redshifts of interest due to foreground screening by the Lyα forest. In an earlier study, we demonstrated a reduction in the equivalent width of low-ionization absorption lines in composite spectra of Lyman break galaxies at z ≅ 4 compared to similar measures at z ≅ 3. This might imply a lower covering fraction of neutral gas and hence an increase with redshift in the escape fraction of ionizing photons. However, our spectral resolution was inadequate to differentiate between several alternative explanations, including changes with redshift in the outflow kinematics. Here we present higher quality spectra of three gravitationally lensed Lyman break galaxies at z ≅ 4 with a spectral resolution sufficient to break this degeneracy of interpretation. We present a method for deriving the covering fraction of low-ionization gas as a function of outflow velocity and compare the results with similar quality data taken for galaxies at lower redshift. We find an interesting but tentative trend of lower covering fractions of low-ionization gasmore » for galaxies with strong Lyα emission. In combination with the demographic trends of Lyα emission with redshift from our earlier work, our results provide new evidence for a reduction in the average H I covering fraction, and hence an increase in the escape fraction of ionizing radiation from Lyman break galaxies, with redshift.« less
Authors:
 [1] ; ;  [2] ;  [3]
  1. Department of Physics, University of California, Santa Barbara, CA 93106 (United States)
  2. Department of Astrophysics, California Institute of Technology, MS 249-17, Pasadena, CA 91125 (United States)
  3. Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ 85721 (United States)
Publication Date:
OSTI Identifier:
22348522
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 779; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; COMPARATIVE EVALUATIONS; EMISSION; GALAXIES; GRAVITATIONAL LENSES; LIMITING VALUES; PHOTONS; RED SHIFT; REDUCTION; RESOLUTION; SCREENING; SPECTRA; SPECTROSCOPY; STARS; VELOCITY