UCLA

We explore molecular cloud properties and the physics of CO transition lines in z > 6 Lymanbreak galaxies and predict their COfluxes 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 principal inputs. This method agrees with chemical equilibrium calculations of the molecular fraction based on local chemistry if galaxies at z>6 have metallicities on the order of a few per cent 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 the higher-order CO transitions observable at high redshift with Atacama Large Millimeter Array (ALMA) will typically be sub-thermally populated but that the details depend sensitively on the presence of turbulent clumps within molecular clouds. Ultimately, we expect current facilities will only be able to observe the CO signal from reionization epoch galaxies with great difficulty. We estimate that at least ~100 h of integration time with ALMA will be required to detect the CO(6-5) transition in z = 6 systems with rest-frame UV magnitudes of -20. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.