Volume 616, August 2018
|Number of page(s)||12|
|Section||Cosmology (including clusters of galaxies)|
|Published online||08 August 2018|
Accurately predicting the escape fraction of ionizing photons using rest-frame ultraviolet absorption lines
Observatoire de Genève, Université de Genève,
51 Ch. des Maillettes,
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
Accepted: 20 March 2018
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 (fesc ≤ 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.
Key words: dark ages, reionization, first stars / galaxies: irregular / galaxies: ISM / galaxies: starburst
© ESO 2018
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