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 ζ CR=10-17-10-14\, s-1, far-UV intensities in the range of G0=1-103 and metallicities in the range of Z=0.1-2\, Z. 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-H2 transition; ii) the velocity-integrated emission of [CII]~158μm, [13CII]~158μm, [CI]~609μm and 370μm, [OI]~63μm and 146μm, and of the first ten 12CO rotational transitions; iii) the corresponding Spectral Line Energy Distributions; iv) the usage of [CII] and [OI]~63μm 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 H2-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 H2 column, making it a promising new H2 tracer in metal-poor environments. The conversion factors of X CO and X 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.