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. (arXiv:2111.14876v3 [astro-ph.GA] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Belfiore_F/0/1/0/all/0/1">Francesco Belfiore</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Santoro_F/0/1/0/all/0/1">Francesco Santoro</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Groves_B/0/1/0/all/0/1">Brent Groves</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schinnerer_E/0/1/0/all/0/1">Eva Schinnerer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kreckel_K/0/1/0/all/0/1">Kathryn Kreckel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Glover_S/0/1/0/all/0/1">Simon C. O. Glover</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Klessen_R/0/1/0/all/0/1">Ralf S. Klessen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Emsellem_E/0/1/0/all/0/1">Eric Emsellem</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Blanc_G/0/1/0/all/0/1">Guillermo A. Blanc</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Congiu_E/0/1/0/all/0/1">Enrico Congiu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Barnes_A/0/1/0/all/0/1">Ashley T. Barnes</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Boquien_M/0/1/0/all/0/1">M&#xe9;d&#xe9;ric Boquien</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chevance_M/0/1/0/all/0/1">M&#xe9;lanie Chevance</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dale_D/0/1/0/all/0/1">Daniel A. Dale</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kruijssen_J/0/1/0/all/0/1">J. M. Diederik Kruijssen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Leroy_A/0/1/0/all/0/1">Adam K. Leroy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pan_H/0/1/0/all/0/1">Hsi-An Pan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pessa_I/0/1/0/all/0/1">Ismael Pessa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schruba_A/0/1/0/all/0/1">Andreas Schruba</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Williams_T/0/1/0/all/0/1">Thomas G. Williams</a>

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 ($Sigma_{Halpha}$). 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
$Sigma_{Halpha}$; 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.

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 ($Sigma_{Halpha}$). 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
$Sigma_{Halpha}$; 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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