Predicting Lyman-continuum emission of galaxies using their physical and Lyman-alpha emission properties

Abstract

Aims. The primary difficulty in understanding the sources and processes that powered cosmic reionization is that it is not possible to directly probe the ionizing Lyman-continuum (LyC) radiation at that epoch as those photons have been absorbed by the intervening neutral hydrogen. It is therefore imperative to build a model to accurately predict LyC emission using other properties of galaxies in the reionization era. Methods. In recent years, studies have shown that the LyC emission from galaxies may be correlated to their Lyman-alpha (Lyα) emission. In this paper we study this correlation by analyzing thousands of simulated galaxies at high redshift in the SPHINX cosmological simulation. We post-process these galaxies with the Lyα radiative transfer code RASCAS and analyze the Lyα – LyC connection. Results. We find that the Lyα and LyC luminosities are strongly correlated with each other, although with dispersion. There is a positive correlation between the escape fractions of Lyα and LyC radiations in the brightest Lyman-alpha emitters (LAEs; escaping Lyα luminosity LescLyα > 1041 erg s−1), similar to that reported by recent observational studies. However, when we also include fainter LAEs, the correlation disappears, which suggests that the observed relation may be driven by selection effects. We also find that the brighter LAEs are dominant contributors to reionization, with LescLyα > 1040 erg s−1 galaxies accounting for > 90% of the total amount of LyC radiation escaping into the intergalactic medium in the simulation. Finally, we build predictive models using multivariate linear regression, where we use the physical and Lyα properties of simulated reionization era galaxies to predict their LyC emission. We build a set of models using different sets of galaxy properties as input parameters and predict their intrinsic and escaping LyC luminosity with a high degree of accuracy (the adjusted R2 of these predictions in our fiducial model are 0.89 and 0.85, respectively, where R2 is a measure of how much of the response variance is explained by the model). We find that the most important galaxy properties for predicting the escaping LyC luminosity of a galaxy are its LescLyα, gas mass, gas metallicity, and star formation rate. Conclusions. These results and the predictive models can be useful for predicting the LyC emission from galaxies using their physical and Lyα properties and can thus help us identify the sources of reionization.