The Effects of Stellar Population and Gas Covering Fraction on the Emergent Ly α Emission of High-redshift Galaxies

Naveen A. Reddy, Michael W. Topping, Alice E. Shapley, Charles C. Steidel, Ryan L. Sanders, Xinnan Du, Alison L. Coil, Bahram Mobasher, Sedona H. Price, Irene Shivaei

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27 Scopus citations


We perform joint modeling of the composite rest-frame far-UV and optical spectra of redshift 1.85 ≤ z ≤ 3.49 star-forming galaxies to deduce key properties of the massive stars, ionized interstellar medium (ISM), and neutral ISM, with the aim of investigating the principal factors affecting the production and escape of Lyα photons. Our sample consists of 136 galaxies with deep Keck/LRIS and MOSFIRE spectra covering, respectively, Lyβ through C iii] λλ1907, 1909 and [O ii], [Ne iii], Hβ, [O iii], Hα, [N ii], and [S ii]. Spectral and photoionization modeling indicates that the galaxies are uniformly consistent with stellar population synthesis models that include the effects of stellar binarity. Over the dynamic range of our sample, there is little variation in stellar and nebular abundance with Lyα equivalent width, W λ (Lyα), and only a marginal anticorrelation between age and W λ (Lyα). The inferred range of ionizing spectral shapes is insufficient to solely account for the variation in W λ (Lyα); rather, the covering fraction of optically thick H i appears to be the principal factor modulating the escape of Lyα, with most of the Lyα photons in down-the-barrel observations of galaxies escaping through low column density or ionized channels in the ISM. Our analysis shows that a high star-formation-rate surface density, ςSFR, particularly when coupled with a low galaxy potential (i.e., low stellar mass), can aid in reducing the covering fraction and ease the escape of Lyα photons. We conclude with a discussion of the implications of our results for the escape of ionizing radiation at high redshift.

Original languageEnglish
Article number31
JournalAstrophysical Journal
Issue number1
StatePublished - Feb 1 2022

Bibliographical note

Funding Information:
We acknowledge support from NSF AAG grants AST1312780, 1312547, 1312764, and 1313171; grant AR13907 from the Space Telescope Science Institute; and grant NNX16AF54G from the NASA ADAP program. This work made use of v2.2.1 of the Binary Population and Spectral Synthesis (BPASS) models, as described in Eldridge et al. () and Stanway & Eldridge (), and v17.02 of the Cloudy radiative transfer code (Ferland et al. ). We wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, most of the observations presented herein would not have been possible.

Funding Information:
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 NASA and was made possible by the generous financial support of the W. M. Keck Foundation.

Publisher Copyright:
© 2022. The Author(s). Published by the American Astronomical Society.

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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