Accurately predicting the escape fraction of ionizing photons using rest-frame ultraviolet absorption lines

¹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
e-mail: John.Chisholm@unige.ch
² Johan Bernoulli Institute, University of Groningen, PO Box 407, 9700 AK Groningen, The Netherlands
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ KVI-Center for Advanced Radiation Technology (KVI-CART), University of Groningen, Zernikelaan 25, Groningen 9747 AA, The Netherlands
⁵ CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France
⁶ Observational Cosmology Lab, NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771, USA
⁷ MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Ave., Cambridge, MA 02139, USA
⁸ Department of Astronomy, University of Michigan, 500 Church St., Ann Arbor, MI 48109, USA
⁹ Department of Astronomy & Astrophysics, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637, USA
¹⁰ Kavli Institute for Cosmological Physics, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637, USA
¹¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway

Received: 2 February 2018
Accepted: 20 March 2018

Abstract

The fraction of ionizing photons that escape high-redshift galaxies sensitively determines whether galaxies reionized the early Universe. However, this escape fraction cannot be measured from high-redshift galaxies because the opacity of the intergalactic medium is large at high redshifts. Without methods to measure the escape fraction of high-redshift galaxies indirectly, it is unlikely that we will know what reionized the Universe. Here, we analyze the far-ultraviolet (UV) H I (Lyman series) and low-ionization metal absorption lines of nine low-redshift, confirmed Lyman continuum emitting galaxies. We use the H I covering fractions, column densities, and dust attenuations measured in a companion paper to predict the escape fraction of ionizing photons. We find good agreement between the predicted and observed Lyman continuum escape fractions (within 1.4σ) using both the H I and ISM absorption lines. The ionizing photons escape through holes in the H I, but we show that dust attenuation reduces the fraction of photons that escape galaxies. This means that the average high-redshift galaxy likely emits more ionizing photons than low-redshift galaxies. Two other indirect methods accurately predict the escape fractions: the Lyα escape fraction and the optical [O III]/[O II] flux ratio. We use these indirect methods to predict the escape fraction of a sample of 21 galaxies with rest-frame UV spectra but without Lyman continuum observations. Many of these galaxies have low escape fractions (f_esc ≤ 1%), but 11 have escape fractions >1%. Future studies will use these methods to measure the escape fractions of high-redshift galaxies, enabling upcoming telescopes to determine whether star-forming galaxies reionized the early Universe.