Mapping the Escape Fraction of Ionizing Photons Using Resolved Stars: A Much Higher Escape Fraction for NGC 4214

TL;DR

This paper introduces a novel method using resolved stellar populations to measure the escape fraction of ionizing photons in NGC 4214, revealing a much higher escape fraction than previously reported, with implications for cosmic reionization.

Contribution

The study presents a new approach to measure the escape fraction of ionizing photons using resolved stars and spectral energy distribution modeling, providing the first non-zero escape fraction for a UV-faint galaxy.

Findings

Spatial escape fraction varies from 0-40%.

Global escape fraction is approximately 25%.

Higher escape fraction (~59%) if no internal dust is assumed.

Abstract

We demonstrate a new method for measuring the escape fraction of ionizing photons using Hubble Space Telescope imaging of resolved stars in NGC 4214, a local analog of high-redshift starburst galaxies that are thought to be responsible for cosmic reionization. Specifically, we forward model the UV through near-IR spectral energy distributions of $\sim$83,000 resolved stars to infer their individual ionizing flux outputs. We constrain the local escape fraction by comparing the number of ionizing photons produced by stars to the number that are either absorbed by dust or consumed by ionizing the surrounding neutral hydrogen in individual star-forming regions. We find substantial spatial variation in the escape fraction (0-40%). Integrating over the entire galaxy yields a global escape fraction of 25% (+16%/-15%). This value is much higher than previous escape fractions of zero reported for this galaxy. We discuss sources of this apparent tension, and demonstrate that the viewing angle and the 3D ISM geometric effects are the cause. If we assume the NGC 4214 has no internal dust, like many high-redshift galaxies, we find an escape fraction of 59% (an upper-limit for NGC 4214). This is the first non-zero escape fraction measurement for UV-faint (M$_{\rm FUV}$ = -15.9) galaxies at any redshift, and supports the idea that starburst UV-faint dwarf galaxies can provide a sufficient amount of ionizing photons to the intergalactic medium.