The nature of diffuse ionised gas in star-forming galaxies

TL;DR

This study uses high-resolution simulations with radiative transfer to analyze the diffuse ionised gas in star-forming galaxies, revealing its ionisation sources, properties, and how it can serve as a test for stellar feedback models.

Contribution

It introduces a detailed simulation framework that self-consistently models DIG ionisation, line emission, and compares results with observations, highlighting the role of stellar age and radiation hardening.

Findings

DIG is mainly ionised by 5-25 Myr stars.

Line ratio trends relate to star formation and radiation hardening.

Simulations match observed line ratios and correlations.

Abstract

We present an analysis of the diffuse ionised gas (DIG) in a high-resolution simulation of an isolated Milky Way-like galaxy, incorporating on-the-fly radiative transfer and non-equilibrium thermochemistry. We utilise the Monte-Carlo radiative transfer code COLT to self-consistently obtain ionisation states and line emission in post-processing. We find a clear bimodal distribution in the electron densities of ionised gas ($n_{\rm e}$), allowing us to define a threshold of $n_{\rm e}=10\,\mathrm{cm}^{-3}$ to differentiate DIG from HII regions. The DIG is primarily ionised by stars aged 5-25 Myr, which become exposed directly to low-density gas after HII regions have been cleared. Leakage from recently formed stars ($<5$ Myr) is only moderately important for DIG ionisation. We forward model local observations and validate our simulated DIG against observed line ratios in [SII]/H$\alpha$, [NII]/H$\alpha$, [OI]/H$\alpha$, and [OIII]/H$\beta$ against $\Sigma_{\rm H\alpha}$. The mock observations not only reproduce observed correlations, but also demonstrate that such trends are related to an increasing temperature and hardening ionising radiation field with decreasing $n_{\rm e}$. The hardening of radiation within the DIG is caused by the gradual transition of the dominant ionising source with decreasing $n_{\rm e}$ from 0 Myr to 25 Myr stars, which have progressively harder intrinsic ionising spectra primarily due to the extended Wolf-Rayet phase caused by binary interactions. Consequently, the DIG line ratio trends can be attributed to ongoing star formation, rather than secondary ionisation sources, and therefore present a potent test for stellar feedback and stellar population models.