Abstract
The CII 158$\mu$m emission line can arise in all phases of the ISM,
therefore being able to disentangle the different contributions is an important
yet unresolved problem when undertaking galaxy-wide, integrated CII
observations. We present a new multi-phase 3D radiative transfer interface that
couples Starburst99, a stellar spectrophotometric code, with the
photoionisation and astrochemistry codes Mocassin and 3D-PDR. We model entire
star forming regions, including the ionised, atomic and molecular phases of the
ISM, and apply a Bayesian inference methodology to parametrise how the fraction
of the CII emission originating from molecular regions, $f_CII,mol$,
varies as a function of typical integrated properties of galaxies in the local
Universe. The main parameters responsible for the variations of $f_CII,mol$
are specific star formation rate (sSFR), gas phase metallicity, HII region
electron number density ($n_e$), and dust mass fraction. For example,
$f_CII,mol$ can increase from 60% to 80% when either $n_e$ increases from
10$^1.5$ to 10$^2.5$cm$^-3$, or SSFR decreases from $10^-9.6$ to
$10^-10.6$ yr$^-1$. Our model predicts for the Milky Way that
$f_CII,mol$$=75.8\pm5.9$%, in agreement with the measured value of 75%.
When applying the new prescription to a complete sample of galaxies from the
Herschel Reference Survey (HRS), we find that anywhere from 60 to 80% of the
total integrated CII emission arises from molecular regions.
Description
[1607.03488] Radiative Transfer meets Bayesian statistics: where does your Galaxy's [CII] come from?
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