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arXiv · 2111.14876

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

Francesco Belfiore·Francesco Santoro·Brent Groves·Eva Schinnerer·Kathryn Kreckel·Simon C. O. Glover·Ralf S. Klessen·Eric Emsellem·Guillermo A. Blanc·Enrico Congiu·Ashley T. Barnes·Médéric Boquien·Mélanie Chevance·Daniel A. Dale·J. M. Diederik Kruijssen·Adam K. Leroy·Hsi-An Pan·Ismael Pessa·Andreas Schruba·Thomas G. Williams
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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.

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Francesco Belfiore, Francesco Santoro, Brent Groves, Eva Schinnerer, Kathryn Kreckel, Simon C. O. Glover, Ralf S. Klessen, Eric Emsellem, Guillermo A. Blanc, Enrico Congiu, Ashley T. Barnes, Médéric Boquien, Mélanie Chevance, Daniel A. Dale, J. M. Diederik Kruijssen, Adam K. Leroy, Hsi-An Pan, Ismael Pessa, Andreas Schruba, Thomas G. Williams. 2021-11-29. 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. https://doi.org/10.1051/0004-6361%2F202141859

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