Carbon cycle ratios in evolved molecular clouds: selecting the best H2 tracer

Abstract

This report summarizes the key outcomes of the DFG-funded project “Carbon cycle ratios in evolved molecular clouds: selecting the best H₂ tracer” (Grant No. 424563772). The research investigated the carbon cycle - spanning ionized (CII), atomic (CI), and molecular (CO) phases - in relation to the atomic-to-molecular (HI-to-H₂) transition in various interstellar environments. A central focus was understanding how environmental parameters such as far-ultraviolet (FUV) radiation, cosmic-ray (CR) ionization rates, and metallicity influence the utility of different carbon-bearing species as tracers of H₂ gas, especially in extragalactic contexts. Major achievements include: i) the development of PDFchem, a publicly available tool for rapidly estimating averaged interstellar medium (ISM) properties from probability distributions of visual extinction; ii) the creation of an extensive, public grid of 6,400 photodissociation region (PDR) simulations using the 3d-pdr code, exploring a wide parameter space of density, FUV intensity, CR ionization rate, and metallicity; iii) the detailed analysis of cosmic-ray effects, confirming that elevated CR rates can significantly destroy CO while enhancing CII and CI abundances, thereby altering standard CO-to-H₂ conversion factors and promoting CI and CII as viable alternative tracers in CR-dominated environments; iv) a comparative assessment of H₂ tracers finding that CI is less sensitive to metallicity variations than CO, making it more reliable in low-metallicity systems, while CII becomes a promising tracer in high-CR, high-metallicity regimes due to CR-induced heating and excitation; v) an extension of the models to incorporate CR attenuation, mechanical heating, and applications to high-redshift galaxy dynamics, star formation conditions, and JWST-oriented predictions for H₂ line emission. The project has produced multiple peer-reviewed publications and publicly released computational tools, advancing the understanding of molecular gas tracers and providing refined methods for estimating H₂ masses across diverse galactic environments.