Photodissociation Region Diagnostics Across Galactic Environments
  We present three-dimensional astrochemical simulations and synthetic
observations of magnetised, turbulent, self-gravitating molecular clouds. We
explore various galactic interstellar medium environments, including cosmic-ray
ionization rates in the range of $\zeta_{\rm CR}=10^{-17}$-$10^{-14}\,{\rm
s}^{-1}$, far-UV intensities in the range of $G_0=1$-$10^3$ and metallicities
in the range of $Z=0.1$-$2\,{\rm Z}_{\odot}$. The simulations also probe a
range of densities and levels of turbulence, including cases where the gas has
undergone recent compression due to cloud-cloud collisions. We examine: i) the
column densities of carbon species across the cycle of CII, CI and CO, along
with OI, in relation to the HI-to-H$_2$ transition; ii) the velocity-integrated
emission of [CII]~$158\mu$m, [$^{13}$CII]~$158\mu$m, [CI]~$609\mu$m and
$370\mu$m, [OI]~$63\mu$m and $146\mu$m, and of the first ten $^{12}$CO
rotational transitions; iii) the corresponding Spectral Line Energy
Distributions; iv) the usage of [CII] and [OI]~$63\mu$m to describe the
dynamical state of the clouds; v) the behavior of the most commonly used ratios
between transitions of CO and [CI]; and vi) the conversion factors for using CO
and CI as H$_2$-gas tracers. We find that enhanced cosmic-ray energy densities
enhance all aforementioned line intensities. At low metallicities, the emission
of [CII] is well connected with the H$_2$ column, making it a promising new
H$_2$ tracer in metal-poor environments. The conversion factors of $X_{\rm CO}$
and $X_{\rm CI}$ depend on metallicity and the cosmic-ray ionization rate, but
not on FUV intensity. In the era of ALMA, SOFIA and the forthcoming CCAT-prime
telescope, our results can be used to understand better the behaviour of
systems in a wide range of galactic and extragalactic environments.
