2017ApJ...839...90B

We explore the effects of the expected higher cosmic ray (CR) ionization rates ζ_CR on the abundances of carbon monoxide (CO), atomic carbon (C), and ionized carbon (C⁺) in the H₂ clouds of star-forming galaxies. The study of Bisbas et al. is expanded by (a) using realistic inhomogeneous giant molecular cloud (GMC) structures, (b) a detailed chemical analysis behind the CR-induced destruction of CO, and (c) exploring the thermal state of CR-irradiated molecular gas. CRs permeating the interstellar medium with ζ_CR 10×(Galactic) are found to significantly reduce the [CO]/[H₂] abundance ratios throughout the mass of a GMC. CO rotational line imaging will then show much clumpier structures than the actual ones. For ζ_CR 100 ×(Galactic) this bias becomes severe, limiting the usefulness of CO lines for recovering structural and dynamical characteristics of H₂-rich galaxies throughout the universe, including many of the so-called main-sequence galaxies where the bulk of cosmic star formation occurs. Both C⁺ and C abundances increase with rising ζ_CR, with C remaining the most abundant of the two throughout H₂ clouds, when ζ_CR∼(1-100) ×(Galactic). C⁺ starts to dominate for ζ_CR 10³ ×(Galactic). The thermal state of the gas in the inner and denser regions of GMCs is invariant with T_gas∼10 K for ζ_CR∼(1-10) ×(Galactic). For ζ_CR∼10³ ×(Galactic) this is no longer the case and T_gas∼30–50 K are reached. Finally, we identify OH as the key species whose T_gas-sensitive abundance could mitigate the destruction of CO at high temperatures.