Cosmic-ray Induced Destruction of CO in Star-forming Galaxies

We explore the effects of the expected higher cosmic ray (CR) ionization rates ${\zeta }_{CR}$ 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 ${\zeta }_{CR}\gtrsim 10\times \left(Galactic\right)$ 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 ${\zeta }_{CR}\gtrsim 100\times$ (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 ${\zeta }_{CR}$, with C remaining the most abundant of the two throughout H₂ clouds, when ${\zeta }_{CR}\sim (1-100)\times$ (Galactic). C⁺ starts to dominate for ${\zeta }_{CR}\gtrsim {10}^3\times$ (Galactic). The thermal state of the gas in the inner and denser regions of GMCs is invariant with $T_{gas}\sim 10K$ for ${\zeta }_{CR}\sim (1-10)\times$ (Galactic). For ${\zeta }_{CR}\sim {10}^3\times$ (Galactic) this is no longer the case and $T_{gas}\sim 30\text{--}50K$ are reached. Finally, we identify OH as the key species whose T _gas-sensitive abundance could mitigate the destruction of CO at high temperatures.