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

TL;DR

This study develops and validates indirect methods to accurately predict the escape fraction of ionizing photons from galaxies, crucial for understanding cosmic reionization, by analyzing UV absorption lines and other indicators in low-redshift galaxies.

Contribution

The paper introduces new indirect techniques, including UV absorption lines, Lyα escape fraction, and [O III]/[O II] ratios, to estimate ionizing photon escape fractions in galaxies.

Findings

Predicted escape fractions closely match observed values within 1.4σ.

Dust attenuation significantly reduces ionizing photon escape.

Many high-redshift galaxies likely emit more ionizing photons than low-redshift counterparts.

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.