The far-ultraviolet continuum slope as a Lyman Continuum escape estimator at high redshift

Most of the hydrogen in the intergalactic medium (IGM) was rapidly ionized at high redshifts. While observations have established that reionization occurred, observational constraints on the high-redshift ionizing emissivity remain elusive. Here, we present a new analysis of the Low-redshift Lyman Continuum Survey (LzLCS) and literature observations, a combined sample of 89 star-forming galaxies at redshifts near 0.3 with Hubble Space Telescope observations of their ionizing continua (or Lyman Continuum, LyC). We find a strong (6 sigma significant) inverse correlation between the continuum slope at 1550 angstrom (defined as F-lambda proportional to lambda(beta obs1550)) and both the LyC escape fraction (f(esc, LyC)) and f(esc, LyC) times the ionizing photon production efficiency (xi(ion)). On average, galaxies with redder continuum slopes have smaller f(esc, LyC) than galaxies with bluer slopes mainly due to higher dust attenuation. More than 5 per cent (20 per cent) of the LyC emission escapes galaxies with beta(1550)(obs) <-2.1 (-2.6). We find strong correlations between beta(1550)(obs) and the [O III]/[O II] flux ratio (at 7.5 sigma significance), galaxy stellar mass (at 5.9 sigma), the gas-phase metallicity (at 4.6 sigma), and the observed far-ultraviolet absolute magnitude (at 3.4 sigma). Using previous observations of beta(1550)(obs) at high redshift, we estimate the evolution of f(esc, LyC) with both redshift and galaxy magnitude. The LzLCS observations suggest that fainter and lower mass galaxies dominate the ionizing photon budget at higher redshift, possibly due to their rapidly evolving metal and dust content. Finally, we use our correlation between beta(1550)(obs) and f(esc, LyC) x xi(ion) to predict the ionizing emissivity of galaxies during the epoch of reionization. Our estimated emissivities match IGM observations, and suggest that star-forming galaxies emit sufficient LyC photons into the IGM to exceed recombinations near redshifts of 7-8.

Automated summary

This study analyzes the Low-redshift Lyman Continuum Survey (LzLCS) and literature observations to uncover the correlation between the continuum slope at 1550 angstrom and the escape fraction of LyC photons. The results show a significant inverse correlation, with redder slopes associated with smaller escape fractions due to dust attenuation. The study also finds strong correlations between the continuum slope and other galaxy properties. Using these findings, the ionizing emissivity of galaxies during the epoch of reionization is predicted.