Studying the ISM at $\sim$10 pc scale in NGC 7793 with MUSE -- II. Constraints on the oxygen abundance and ionising radiation escape

TL;DR

This study uses high-resolution MUSE data to analyze the ionised interstellar medium in NGC 7793, estimating oxygen abundance, ionising photon escape fraction, and ionisation sources at ~10 pc scale, revealing detailed ISM structure and star cluster properties.

Contribution

First detailed spatially resolved analysis of oxygen abundance and ionising photon escape in NGC 7793 at 10 pc scale using MUSE data, complemented by HST and ALMA observations.

Findings

Median oxygen abundance $12 + \, \log(O/H) \sim 8.37$ with 0.25 dex scatter.

Ionising photon escape fraction $f_{esc} \approx 0.67$ in HII regions.

Young, less massive star clusters emit more ionising photons and are more likely to be in HII regions.

Abstract

We study the ionised ISM in NGC 7793 with MUSE/AO, at a spatial resolution of $\sim$ 10 pc. The data are complemented with young star clusters (YSCs), O stars and GMCs observed with HST and ALMA. Using a strong-line method, we find a median $12 + \log(O/H) \sim 8.37$ with a scatter of 0.25 dex, in agreement with previous estimates. The abundance map is rich in substructures, surrounding clusters and massive stars, although clear degeneracies with photoionisation are present. We determine the observed total amount of ionising photons, $Q(H^0)$, from the extinction corrected H$\alpha$ luminosity, and compare it to the expected $Q(H^0)$ obtained by summing the contributions of YSCs and massive stars, to obtain an escape fraction ($f_{esc}$). Overall, we find $f_{esc, HII} = 0.67_{-0.12}^{+0.08}$ for the population of HII regions. We also conclude that the sources of ionisation observed within the FoV are more than sufficient to explain the amount of diffuse ionised gas observed in this region of the galaxy. In general, we find that YSCs located in HII regions have a higher probability to be younger, less massive, and to emit a higher number of ionising photons than clusters in the rest of the field. Finally, we study the optical depth of the regions traced by [SII]/[OIII], finding no systematic trend between the resulting ionisation structure and $f_{esc}$. [abridged]