Diversity of the Lyman continuum escape fractions of high-$z$ galaxies and its origins

TL;DR

This study investigates the diverse escape fractions of Lyman continuum photons in high-redshift galaxies through simplified models, revealing how galaxy structure and clumpiness influence ionizing photon escape and contributing to understanding cosmic reionization.

Contribution

It provides a detailed analysis of how galaxy morphology and clumpiness affect LyC escape fractions using numerical ray-tracing simulations, clarifying the origins of observed diversity.

Findings

Escape fraction varies up to 3 orders of magnitude with galaxy structure.

Halo mass and disc scale height influence the trend of escape fraction.

Clumpiness increases escape fraction when the total clump mass dominates gas mass.

Abstract

The Lyman continuum (LyC) escape fraction is a key quantity to determine the contribution of galaxies to cosmic reionization. It has been known that the escape fractions estimated by observations and numerical simulations show a large diversity. However, the origins of the diversity are still uncertain. In this work, to understand what quantities of galaxies are responsible for controlling the escape fraction, we numerically evaluate the escape fraction by performing ray-tracing calculation with simplified disc galaxy models. With a smooth disc model, we explore the dependence of the escape fraction on the disposition of ionizing sources, and find that the escape fraction varies up to $\sim 3$ orders of magnitude. It is also found that the halo mass dependence of disc scale height determines whether the escape fraction increases or decreases with halo mass. With a clumpy disc model, it turns out that the escape fraction increases as the clump mass fraction increases because the density in the inter-clump region decreases. In addition, we find that clumpiness regulates the escape fraction via two ways when the total clump mass dominates the total gas mass; the escape fraction is controlled by the covering factor of clumps if the clumps are dense sufficient to block LyC photons, otherwise the clumpiness works to reduce the escape fraction by increasing the total number of recombination events in a galaxy.