Photodissociation Region Diagnostics Across Galactic Environments

Abstract

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 ${\zeta }_{\mathrm{CR}}=10^{-17}$-$10^{-14}{\mathrm{s}}^{-1}$, far-UV intensities in the range of $G_0=1$-$10^3$ and metallicities in the range of $Z=0.1$-$2{\mathrm{Z}}_{\odot }$. 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 $X_{\mathrm{CO}}$ and $X_{\mathrm{CI}}$ 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.

33 pages, 24 figures, 3 tables. Submitted to MNRAS. Comments welcome!

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