A tale of two DIGs: The relative role of HII regions and low-mass hot evolved stars in powering the diffuse ionised gas (DIG) in PHANGS-MUSE galaxies

TL;DR

This study uses high-resolution integral field spectroscopy to investigate the sources of diffuse ionised gas in nearby spiral galaxies, finding that both leaking radiation from HII regions and hot low-mass evolved stars significantly contribute to the DIG's ionisation and emission properties.

Contribution

It presents a combined model showing how ionising radiation from HII regions and old stars jointly explain the physical conditions and emission line ratios of the DIG in galaxies.

Findings

Leaking radiation from HII regions accounts for the distribution of Hα in the DIG.

Hot low-mass evolved stars contribute to low-ionisation line ratios and [OIII] emission.

The DIG results from a superposition of young star ionisation and diffuse radiation from old stars.

Abstract

We use integral field spectroscopy from the PHANGS-MUSE survey, which resolves the ionised interstellar medium at ${\sim}50$ pc resolution in 19 nearby spiral galaxies, to study the origin of the diffuse ionised gas (DIG). We examine the physical conditions of the diffuse gas by first removing morphologically-defined HII regions and then binning the low-surface-brightness areas to achieve significant detections of the key nebular lines. A simple model for the leakage and propagation of ionising radiation from HII regions is able to reproduce the observed distribution of H$\alpha$ in the DIG. Leaking radiation from HII regions also explains the observed decrease in line ratios of low-ionisation species ([SII]/H$\alpha$, [NII]/H$\alpha$ and [OI]/H$\alpha$) with increasing H$\alpha$ surface brightness ($\Sigma_{H\alpha}$). Emission from hot low-mass evolved stars, however, is required to explain: (1) the enhanced low-ionisation line ratios observed in the central regions of some galaxies; (2) the observed trends of a flat or decreasing [OIII]/H$\beta$ with $\Sigma_{H\alpha}$; and (3) the offset of some DIG regions from the locus of HII regions in the Baldwin-Phillips-Terlevich (BPT) diagram, extending into the area of low-ionisation (nuclear) emission-line regions (LI[N]ERs). Hot low-mass evolved stars make a small contribution to the energy budget of the DIG (2% of the galaxy-integrated H$\alpha$ emission), but their harder spectra make them fundamental contributors to [OIII] emission. The DIG might result from a superposition of two components, an energetically dominant contribution from young stars and a more diffuse background of harder ionising photons from old stars. This unified framework bridges observations of the Milky Way DIG with LI(N)ER-like emission observed in nearby galaxy bulges.