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 principal inputs. This method agrees with chemical equilibrium
calculations of the molecular fraction based on local chemistry if galaxies at
z >~ 6 have metallicities of order a few 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 the higher-order CO
transitions observable at high redshift with 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 hours 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.
