Abstract
We combine the galaxy formation model GALFORM with the Photon Dominated
Region code UCL_PDR to study the emission from the rotational transitions of
12CO (CO) in galaxies from z=0 to z=6 in the LambdaCDM framework. GALFORM is
used to predict the molecular (H_2) and atomic hydrogen (HI) gas contents of
galaxies using the pressure-based empirical star formation relation of Blitz &
Rosolowsky. From the predicted H_2 mass and the conditions in the interstellar
medium, we estimate the CO emission by applying the UCL_PDR model to each
galaxy. We find that deviations from the Milky-Way CO-H_2 conversion factor
come mainly from variations in metallicity, and in the average gas and star
formation rate surface densities. In the local universe, the model predicts a
CO(1-0) luminosity function (LF), CO-to-total infrared (IR) luminosity ratios
for multiple CO lines and a CO spectral line energy distribution (SLED) which
are in good agreement with observations of luminous and ultra-luminous IR
galaxies. At high redshifts, the predicted CO SLED of the brightest IR galaxies
reproduces the shape and normalization of the observed CO SLED. The model
predicts little evolution in the CO-to-IR luminosity ratio for different CO
transitions, in good agreement with observations up to z~5. We use this new
hybrid model to explore the potential of using colour selected samples of
high-redshift star-forming galaxies to characterise the evolution of the cold
gas mass in galaxies through observations with the Atacama Large Millimeter
Array.
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