Effective Destruction of CO by Cosmic Rays: Implications for Tracing H2 Gas in the Universe

We report on the effects of cosmic rays (CRs) on the abundance of CO in H₂ clouds under conditions typical for star-forming galaxies in the universe. We discover that this most important molecule for tracing H₂ gas is very effectively destroyed in ISM environments with CR energy densities {{U}_CR}∼ (50-{{10}³})× {{U}_CR,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 very CO-poor (and thus CO-untraceable), even in ISM environments with rather modestly enhanced average CR energy densities of {{U}_CR}∼ (10-50)× {{U}_CR,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 H₂ 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/H₂ 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.