Constraining the properties of the molecular gas in galaxies at the end (z ≈ 6) of the EoR is a compelling step to understand the process of star formation in the first galaxies. Molecular hydrogen, the most abundant molecule in the Universe, lacks of a permanent dipole moment and its first quadrupole line has an excitation temperature significantly higher than the kinetic temperatures of giant molecular clouds (GMCs). This is the reason why molecular gas in galaxies is very often traced through the detection of the rotational transitions the carbon monoxide, CO. The Atacama Large Millimeter/submillimeter Array advent has opened new perspectives for the detection of CO from the EoR.  

To assess the feasibility of detecting CO lines from typical star forming galaxies high-z I developed ISM models by simultaneously capturing the full cosmological context of high-redshift galaxy formation and the radiative transfer from the outer photodissociation regions (PDRs) up to the fully molecular inner part of GMCs.  We then use then to post-process state-ofthe-art zoomed cosmological simulations and produce mock CO maps and CO Sperctral Line Energy Distribution from simulated galaxies.