Stellar associations powering HII regions $\unicode{x2013}$ II. Escape fraction of ionizing photons

TL;DR

This study measures the escape fraction of ionizing photons from HII regions in 19 nearby galaxies, finding a high average escape fraction of around 82%, which impacts our understanding of stellar feedback and galaxy ionization.

Contribution

It provides the first detailed measurement of ionizing photon escape fractions from individual stellar associations in nearby galaxies using combined HST and MUSE data.

Findings

Average photon escape fraction is approximately 82%.

Up to 20% of regions have ionizing sources outside HII boundaries.

Galaxy-wide escape fractions are slightly lower but consistent with individual regions.

Abstract

Newly formed stars have a profound impact on their environment by depositing energy and momentum into the surrounding gas. However, only a fraction of the stellar feedback is retained in the cloud and observational constraints are needed to further our understanding of this process. In a sample of 19 nearby galaxies, we match HII regions from PHANGS$\unicode{x2013}$MUSE to their ionizing stellar source from PHANGS$\unicode{x2013}$HST and measure the percentage of ionizing radiation that is leaking into the surrounding diffuse ionized gas (DIG). Based on a catalogue, where each HII region is powered by a single young and massive stellar association, we measure a photon escape fraction of $f_\mathrm{esc}=82^{+12}_{-24}$ per cent. Comparable results are obtained when different procedures are used to match the ionized gas to its source. All samples we study contain a substantial fraction of objects (up to 20 per cent), where the stellar source is not sufficient to produce the H$\alpha$ flux observed from the nebula. Many of them are probably related to uncertain age estimates, but we also find numerous regions, where a significant fraction of the ionizing photon budget is contributed by stars that reside outside the boundaries of the HII region. This motivates the use of an alternative galaxy-wide approach, in which we include all HII regions and stellar sources, not just the ones that show a clear overlap. When summing up the ionization budget over entire galaxies, we measure slightly lower, but consistent values.