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

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$α$ 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.

Figures (12)

• Figure 1: Comparison between different population synthesis models. We use a sample with random ages and masses and compute the ionizing photon flux $Q(\mathrm{H}^0)$ with BC03, SB99, and BPASS. The grey points are the full sample and the red points show a young and massive subsample ($\mathrm{age}\leq8M$ and mass $>e4\Msun$). The Spearman correlation coefficient $\rho$ indicates good agreement between the different models across the robust subsamples, and all of the correlations are statistically significant with $p\text{-values}<0.05$.
• Figure 2: Examples for the overlap between H ii regions and their ionizing sources. Each cutout shows three-colour composite images, based on the five available HST bands, overlaid with the $\mathrm{H}\,\alpha$ line emission of MUSE in red. a) an H ii region with a fully contained association; b) an H ii region with a fully and a partially contained association and two compact clusters; c) an H ii region with two fully contained associations and a compact cluster; d) multiple H ii regions that form a single H ii region complex with multiple associations and clusters; e) an association that overlaps with two H ii regions.
• Figure 3: We compare the predicted ionizing photon flux $Q(\mathrm{H}^0)$ to the observed values from the H ii region $Q_{\mathrm{H}\,\alpha}$. The full samples are shown in grey and the robust subsamples in red. The contours indicate the distribution of the robust sample and the diagonal lines correspond to different escape fractions.
• Figure 4: Histograms of the observed escape fractions for the different samples. All regions with negative escape fractions are grouped together in the shaded left bar. The full sample is shown in gray and the robust subsample and its median in red.
• Figure 5: Contribution of external ionizing sources. For H ii regions with negative escape fractions, the average contribution from nearby associations, not overlapping with the nebula, is far greater than for those with positive escape fractions.