Kavli Affiliate: Roberto Maiolino
| First 5 Authors: William McClymont, Sandro Tacchella, Aaron Smith, Rahul Kannan, Roberto Maiolino
| Summary:
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 $Sigma_{rm
Halpha}$. 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.
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