Photodissociation region diagnostics across galactic environments
Journal article, 2021

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 zeta(CR) = 10(-17)-10(-14)S(-1), far-UV intensities in the range of G(theta) = 1-10(3) and metallicities in the range of Z = 0.1-2Z(circle dot). 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 H I-to-H-2 transition; (ii) the velocity-integrated emission of [CII] 158 mu m, [C-13 II] 158 mu m, [C I] 609 mu m and 370 mu m, [O I] 63 mu m and 146 mu m, and of the first ten (CO)-C-12 rotational transitions; (iii) the corresponding Spectral Line Energy Distributions; (iv) the usage of [C II] and [O I] 63 mu m to describe the dynamical state of the clouds; (v) the behaviour 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 [C11] 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-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.

galaxies: ISM

photodissociation region (PDR)

methods: numerical

radiative transfer

ISM: abundances

cosmic rays


Thomas G. Bisbas

University of Cologne

Aristotle University of Thessaloniki

Jonathan Tan

University of Virginia

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics


University of Colorado at Boulder

National Astronomical Observatory of Japan

Monthly Notices of the Royal Astronomical Society

0035-8711 (ISSN) 1365-2966 (eISSN)

Vol. 502 2 2701-2732

Massive Star Formation through the Universe (MSTAR)

European Commission (EC) (788829), 2018-09-01 -- 2023-08-31.

Subject Categories

Astronomy, Astrophysics and Cosmology

Atom and Molecular Physics and Optics

Condensed Matter Physics



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