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

Bisbas, Thomas G., Dishoeck, Ewine F. van, Papadopoulos, Pantelis, Szucs, László, Bialy, Shmuel and Zhang, Zhi-Yu 2017. Cosmic-ray induced destruction of CO in star-forming galaxies. The Astrophysical Journal 839 (2) , 90. 10.3847/1538-4357/aa696d

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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 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 ${\zeta }_{\mathrm{CR}}\gtrsim 10\times (\mathrm{Galactic})$ 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 ${\zeta }_{\mathrm{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 ${\zeta }_{\mathrm{CR}}$, with C remaining the most abundant of the two throughout H2 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 ${T}_{\mathrm{gas}}\sim 10\,{\rm{K}}$ 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 ${T}_{\mathrm{gas}}\sim 30\mbox{--}50\,{\rm{K}}$ are reached. Finally, we identify OH as the key species whose T gas-sensitive abundance could mitigate the destruction of CO at high temperatures.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Publisher: IOP Science
ISSN: 1538-4357
Funders: STFC
Date of First Compliant Deposit: 13 July 2017
Date of Acceptance: 24 March 2017
Last Modified: 14 Jul 2017 09:27
URI: http://orca.cardiff.ac.uk/id/eprint/102373

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