PHIBSS: Exploring the dependence of the CO–H₂ conversion factor on total mass surface density at z < 1.5

We present an analysis of the relationship between the CO–H₂ conversion factor (α_CO) and total mass surface density (_tot) in star-forming galaxies at z < 1.5. Our sample, which is drawn from the IRAM Plateau de Bure HIgh-z Blue Sequence Survey (PHIBSS) and the CO Legacy Database for GASS (COLD GASS), includes ‘normal’, massive star-forming galaxies that dominate the evolution of the cosmic star formation rate (SFR) at this epoch and probe the _tot regime where the strongest variation in α_CO is observed. We constrain α_CO via existing CO observations, measurements of the SFR and an assumed molecular gas depletion time (t_dep = M_gas/SFR) – the latter two of which establish the total molecular gas mass independent of the observed CO luminosity. For a broad range of adopted depletion times, we find that α_CO is independent of total mass surface density, with little deviation from the canonical Milky Way value. This runs contrary to a scenario in which α_CO decreases as surface density increases within the extended clouds of molecular gas that potentially fuel clumps of star formation in z ∼ 1 galaxies, similar to those observed in local ultra-luminous infrared galaxies. Instead, our results suggest that molecular gas, at both z ∼ 0 and z ∼ 1, is primarily in the form of self-gravitating molecular clouds. While CO observations suggest a factor of ∼3 reduction in the average molecular gas depletion time between z ∼ 0 and z ∼ 1, we find that, for typical galaxies, the structure of molecular gas and the process of star formation at z ∼ 1 is otherwise remarkably similar to that observed in local star-forming systems.