Cosmic-ray induced destruction of CO in star-forming galaxies

Abstract

We explore the effects of the expected higher cosmic ray (CR) ionization rates ${\zeta }_{\mathrm{CR}}$ on the abundances of carbon monoxide (CO), atomic carbon (C), and ionized carbon (C${}^{+}$) in the H${}_2$ clouds of star-forming galaxies. The study of Bisbas et al. (2015) 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 }_{\mathrm{CR}}$$\gtrsim 10\times$(Galactic) are found to significantly reduce the [CO]/[H${}_2$] abundance ratios throughout the mass of a GMC. CO rotational line imaging will then show much clumpier structures than the actual ones. For ${\zeta }_{\mathrm{CR}}$$\gtrsim 100\times$(Galactic) this bias becomes severe, limiting the utility of CO lines for recovering structural and dynamical characteristics of H${}_2$-rich galaxies throughout the Universe, including many of the so-called Main Sequence (MS) galaxies where the bulk of cosmic star formation occurs. Both C${}^{+}$ and C abundances increase with rising ${\zeta }_{\mathrm{CR}}$, with C remaining the most abundant of the two throughout H${}_2$ clouds, when ${\zeta }_{\mathrm{CR}}\sim (1-100)\times$(Galactic). C${}^{+}$ starts to dominate for ${\zeta }_{\mathrm{CR}}$$\gtrsim 10^3\times$(Galactic). The thermal state of the gas in the inner and denser regions of GMCs is invariant with $T_{\mathrm{gas}}\sim 10\mathrm{K}$ for ${\zeta }_{\mathrm{CR}}\sim (1-10)\times$(Galactic). For ${\zeta }_{\mathrm{CR}}$$\sim 10^3\times$(Galactic) this is no longer the case and $T_{\mathrm{gas}}\sim 30-50\mathrm{K}$ are reached. Finally we identify OH as the key species whose $T_{\mathrm{gas}}-$sensitive abundance could mitigate the destruction of CO at high temperatures.

17 pages, 12 figures, accepted by ApJ

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