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 }{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}≳ 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 {\zeta }{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 {\zeta }{CR}, with C remaining the most abundant of the two throughout H2 clouds, when {\zeta }{CR}∼ (1-100) × (Galactic). C+ starts to dominate for {\zeta }{CR}≳ {10}3 × (Galactic). The thermal state of the gas in the inner and denser regions of GMCs is invariant with {T}{gas}∼ 10 {{K}} for {\zeta }{CR}∼ (1-10) × (Galactic). For {\zeta }{CR}∼ {10}3 × (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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 2017
- DOI:
- 10.3847/1538-4357/aa696d
- arXiv:
- arXiv:1703.08598
- Bibcode:
- 2017ApJ...839...90B
- Keywords:
-
- astrochemistry;
- cosmic rays;
- galaxies: ISM;
- ISM: abundances;
- methods: numerical;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 17 pages, 12 figures, accepted by ApJ