Effective destruction of CO by cosmic rays: implications for tracing H${}_2$ gas in the Universe

Abstract

We report on the effects of cosmic rays (CRs) on the abundance of CO in ${\mathrm{H}}_2$ clouds under conditions typical for star-forming galaxies in the Universe. We discover that this most important molecule for tracing H${}_2$ gas is very effectively destroyed in ISM environments with CR energy densities ${\mathrm{U}}_{\mathrm{CR}}\sim (50-10^3)\times {\mathrm{U}}_{\mathrm{CR},\mathrm{Gal}}$, a range expected in numerous star-forming systems throughout the Universe. This density-dependent effect operates volumetrically rather than only on molecular cloud surfaces (i.e. unlike FUV radiation that also destroys CO), and is facilitated by: a) the direct destruction of CO by CRs, and b) a reaction channel activated by CR-produced He${}^{+}$. The effect we uncover is strong enough to render Milky-Way type Giant Molecular Clouds (GMCs) very CO-poor (and thus CO-untraceable), even in ISM environments with rather modestly enhanced average CR energy densities of ${\mathrm{U}}_{\mathrm{CR}}\sim (10-50)\times {\mathrm{U}}_{\mathrm{CR},\mathrm{Gal}}$. We conclude that the CR-induced destruction of CO in molecular clouds, unhindered by dust absorption, is perhaps the single most important factor controlling the CO-visibility of molecular gas in vigorously star-forming galaxies. We anticipate that a second order effect of this CO destruction mechanism will be to make the H${}_2$ distribution in the gas-rich disks of such galaxies appear much clumpier in CO $J$=1--0, 2--1 line emission than it actually is. Finally we give an analytical approximation of the CO/H${}_2$ abundance ratio as a function of gas density and CR energy density for use in galaxy-size or cosmological hydrodynamical simulations, and propose some key observational tests.

Accepted for publication in ApJ, 29 pages

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