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

Abstract

Abstract We explore the effects of the expected higher cosmic ray (CR) ionization rates 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 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 (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 , with C remaining the most abundant of the two throughout H2 clouds, when (Galactic). C+ starts to dominate for (Galactic). The thermal state of the gas in the inner and denser regions of GMCs is invariant with for (Galactic). For (Galactic) this is no longer the case and are reached. Finally, we identify OH as the key species whose T gas-sensitive abundance could mitigate the destruction of CO at high temperatures.