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

TL;DR

This paper models molecular cloud properties and CO line emissions in galaxies at redshift z > 6, predicting their observability with ALMA and highlighting the challenges due to low metallicity and sub-thermal excitation.

Contribution

It introduces a new analytic approach to estimate molecular gas fractions in high-redshift galaxies, accounting for low metallicity effects on CO emission.

Findings

High-order CO lines are often sub-thermally excited at high redshift.

Detecting CO(6--5) in z=6 galaxies requires extensive ALMA integration time.

Low metallicity results in most carbon being ionized rather than in CO form.

Abstract

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.