Molecular Cloud Properties and CO Line Emission in z >~ 6 Galaxies

We explore molecular cloud properties and the physics of CO transition lines in z >~ 6 Lyman-break galaxies and predict their CO fluxes using an analytic formalism built from global models of star formation in high-redshift galaxies that minimizes our reliance on local observations. Our model includes a new approach to calculating the molecular gas fraction that takes the total gas density, the star formation rate, and the star formation efficiency in clouds as the principle inputs. This method agrees with chemical equilibrium calculations of the molecular fraction based on local chemistry if galaxies at z >~ 6 have metallicities in the range of 3--30 percent of solar. Such low metallicities in turn imply that much of the carbon in these systems exists in ionized form rather than as CO. Moreover, we find that CO transitions will typically be sub-thermally populated, especially those higher-order lines observable at high redshift with ALMA. Ultimately, we expect current facilities will not be able to observe the CO signal from reionization epoch galaxies except with great difficulty. We estimate that ~1000 hours of integration with JVLA will be required to detect the CO(1--0) transition in even the most UV-bright systems at z = 6.