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

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 1.4σ) 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%. 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.

The Lyman Continuum Escape Survey. II. Ionizing Radiation as a Function of the [O iii]/[O ii] Line Ratio

“…Verhamme et al 2015) and/or the depth of interstellar absorption lines in the rest-frame UV wavelength (e.g. Heckman et al 2011;Chisholm et al 2018) might provide further evidence of the geometrical hypothesis. Interestingly, deep composite UV spectra of LAEs are reported to present a tantalizing trend that LAEs on average show shallow interstellar absorption lines, i.e.…”

Intense C iv and He ii Emission in z ∼ 0 Galaxies: Probing High-energy Ionizing Photons∗

“…Specifically, the large ratios of high-to low-ionization emission that have been observed from all galaxies emitting LyC, have been previously used to argue for fully-ionized interstellar media (i.e., are density bounded) or optically-thin tunnels by which ionizing photons can escape (e.g., Jaskot & Oey 2013;Nakajima & Ouchi 2014;Rivera-Thorsen et al 2015;Gazagnes et al 2018). This idea is further supported by the low covering fractions of low-ionization absorption lines observed in many high-ionization galaxies (e.g., Heckman et al 2011;Alexandroff et al 2015;Henry et al 2015;Erb et al 2016;Chisholm et al 2018).…”

“…The deep profiles indicate that the gas in these galaxies are likely optically thick to low-ionization emission, with Si covering fractions of roughly 90% and 70% for J104457 and J141851, respectively. Using the empirical relationship from Chisholm et al (2018), these covering fractions correlate to escape fractions of 10% for ionizing photons with energies near 13.6 eV.…”

Evaluating the QSO contribution to the 21-cm signal from the Cosmic Dawn

“…LMACHs are given a higher value of gγ (7.1) than HMACHs (1.7), which reflects the likely presence of larger, more efficient Pop III stars (Stacy et al 2016) and/or higher escape fractions (e.g. Xu et al 2016;Chisholm et al 2018). These stellar spectra produce minimal X-rays, so do not contribute significantly to the heating of the neutral IGM.…”