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arXiv · 1803.03655

Accurately predicting the escape fraction of ionizing photons using restframe ultraviolet absorption lines

J. Chisholm·S. Gazagnes·D. Schaerer·A. Verhamme·J. R. Rigby·M. Bayliss·K. Sharon·M. Gladders·H. Dahle
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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 indirectly measure the escape fraction of high-redshift galaxies, 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 $1.4\sigma$) 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 Ly$\alpha$ 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 ($f_{\rm esc} \le 1$\%), but 11 have escape fractions $>1$\%. The methods presented here will measure the escape fractions of high-redshift galaxies, enabling future telescopes to determine whether star-forming galaxies reionized the early universe.

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J. Chisholm, S. Gazagnes, D. Schaerer, A. Verhamme, J. R. Rigby, M. Bayliss, K. Sharon, M. Gladders, H. Dahle. 2018-03-09. Accurately predicting the escape fraction of ionizing photons using restframe ultraviolet absorption lines. https://doi.org/10.1051/0004-6361%2F201832758

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