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

Abstract

We explore the effects of the expected higher cosmic ray (CR) ionization rates CR z 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. 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 CRirradiated molecular gas. CRs permeating the interstellar medium with 10 Galactic CR  z´( ) 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 100 CR  z´(Galactic) this bias becomes severe, limiting the usefulness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 galaxies where the bulk of cosmic star formation occurs. Both C + and C abundances increase with rising CR

Impact of Low-Energy Cosmic Rays on Star Formation

“…Astrochemistry calculations of molecular clouds have typically neglected CR attenuation and instead adopt a constant CR ionisation rate throughout the cloud (Glover and Clark, 2012;Offner et al, 2013;Bisbas et al, 2017). However, the net result of the CR processes described above is that -even in the absence of local, internal CR sources (see Sect.…”

“…The effects of high CR ionisation rates on CO is particularly important for observations of molecular clouds, since the CO emission to H 2 gas mass ratio, or X CO , is ubiquitously used to study molecular clouds and infer their masses (Bolatto et al, 2013). However, chemical models demonstrate that clouds in environments with 10 times the typical Galactic CR ionisation rate exhibit significantly reduced CO/H 2 ratios (Bisbas et al, 2017;Gaches et al, 2019b). The fraction of atomic and ionised carbon rises steadily for CR ionisation rates 10 2 −10 3 times the Galactic mean value, and eventually C + becomes the dominant gas tracer (Bisbas et al, 2017).…”

“…CR heating, produced by the energetic and excited products of CR interactions, can be an important heating source in regions with high CR fluxes, since nearly half of the CR energy goes into this heating mechanism (Glassgold et al, 2012). Clouds with significant CR ionisation are also correspondingly warmer (Bisbas et al, 2017;Gaches and Offner, 2018). The CR pressure is dynamically negligible (Gaches and Offner, 2018); however, the resulting elevated temperatures of 30 − 50 K increase the thermal pressure, which in turn may make it more difficult to form stars.…”

CO line ratios in molecular clouds: the impact of environment

“…A recent paper by Bisbas et al (2017) studied how increasing the CRIR can be important in destroying CO. In Figure 11 of this paper they compare the CO/H2 fraction as a function of number density, for varying CRIR.…”

Giant star-forming clumps?

“…Bright in Hα (Livermore et al 2012(Livermore et al , 2015, these blue clumps are thought to harbour significant star formation 2 (though less than 10 per cent 1 Due to the possibility of excitation effects, or the patchy destruction of CO by cosmic rays (e.g. Bisbas et al 2017). 2 One might ask why, since with adequate spatial resolution we generally find a disconnect between active star formation and blue light, and would anyway expect the dusty gas to be expelled rapidly, post-starburst, such that the ratio of UV to submm clumps may reflect the lifetimes for each phase.…”