Photodissociation region diagnostics across galactic environments.

We present three-dimensional astrochemical simulations and synthetic observations of magnetized, turbulent, self-gravitating molecular clouds. We explore various galactic interstellar medium environments, including cosmic ray ionization rates in the range of ζ _CR = 10⁻¹⁷– |$10^{-14}\, {\rm s}^{-1}$|⁠ , far-UV intensities in the range of G ₀ = 1–10³ 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 C  ii , C  i , and CO, along with O  i , in relation to the H  i -to-H₂ transition; (ii) the velocity-integrated emission of [C  ii ] 158 μm, [¹³C  ii ] 158 μm, [C  i ] 609 μm and 370 μm, [O  i ] 63 μm and 146 μm, and of the first ten ¹²CO rotational transitions; (iii) the corresponding Spectral Line Energy Distributions; (iv) the usage of [C  ii ] and [O  i ] 63 μm to describe the dynamical state of the clouds; (v) the behaviour of the most commonly used ratios between transitions of CO and [C  i ]; and (vi) the conversion factors for using CO and C  i as H₂-gas tracers. We find that enhanced cosmic ray energy densities enhance all aforementioned line intensities. At low metallicities, the emission of [C  ii ] is well connected with the H₂ column, making it a promising new H₂ tracer in metal-poor environments. The conversion factors of X _CO and X _{C  i} 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. [ABSTRACT FROM AUTHOR]