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

Abstract

We use integral field spectroscopy from the PHANGS–MUSE survey, which resolves the ionised interstellar medium structure at ∼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 H II regions and then binning the low-surface-brightness areas to achieve significant detections of the key nebular lines in the DIG. A simple model for the leakage and propagation of ionising radiation from H II regions is able to reproduce the observed distribution of Hα in the DIG. This model infers a typical mean free path for the ionising radiation of 1.9 kpc for photons propagating within the disc plane. Leaking radiation from H II regions also explains the observed decrease in line ratios of low-ionisation species ([S II]/Hα, [N II]/Hα, and [O I]/Hα) with increasing Hα surface brightness (ΣHα). 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 of the galaxies in our sample; (2) the observed trends of a flat or decreasing [O III]/Hβ with ΣHα; and (3) the offset of some DIG regions from the typical locus of H II 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 Hα emission), but their harder spectra make them fundamental contributors to [O III] 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.

PHANGS–MUSE: The H II region luminosity function of local star-forming galaxies

“…It is known that, depending on their gas-phase metallicity and ionization parameter, H ii regions can lie outside the boundaries that are commonly adopted to have a clean .4, if we drop the BPT cuts when cleaning our ionized nebula catalogs and rerun the LF fit, we find no significant variation in the measured LF slope. Remarkably, this is true also for NGC 1365 for which there is clear evidence of an AGN ionization cone extending further out the central region (Belfiore et al 2021). As can be seen by comparing the empirical LFs, the regions that are flagged by the BPT cuts tend to populate the fainter end of the LF and thus have negligible effects on the LF shape at the bright end.…”

“…4. Depending on the galaxy, the Hα emission outside the ionized nebulae footprints can represents 20−50% of the total Hα emission (see Belfiore et al 2021).…”

Warm ionized gas in the blue compact galaxy Haro 14 viewed by MUSE

“…The traditional approach distinguishes two sectors here: the AGN and the low-ionization narrow emission line region (LINER) domains (Kewley et al 2001b(Kewley et al , 2006Kauffmann et al 2003). However, it is now well established that other mechanisms can generate line ratios mimicking those of AGNs and LINERs, such as interstellar shocks (Sharp & Bland-Hawthorn 2010;Farage et al 2010;Rich et al 2010Rich et al , 2011Rich et al , 2015, energetic photons leaking out from H ii-regions (Hidalgo-Gámez 2005, 2007Weilbacher et al 2018;Belfiore et al 2022), or hot low-mass evolved stars (HOLMES; Flores-Fajardo et al 2011;Zhang et al 2017;Belfiore et al 2016Belfiore et al , 2022.…”

Nebular abundance gradient in the Cartwheel galaxy using MUSE data

“…The diffuse ionized gas (DIG) component can affect the line measurements. Since DIG is related to young and old stars (Belfiore et al 2022), it is not clear if DIG removal is better for studies like the one presented here. However, we have estimated how much our line estimates can be affected by the DIG.…”