A survey of the cold molecular gas in gravitationally lensed star-forming galaxies at z > 2

Using the Australia Telescope Compact Array (ATCA), we conducted a survey of COJ = 1-0 and J = 2-1 line emission towards strongly lensed high-redshift dusty star forming galaxies (DSFGs) previously discovered with the South Pole Telescope (SPT). Our sample comprises 17 sources that had CO-based spectroscopic redshifts obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX). We detect all sources with known redshifts in either COJ = 1-0 or J = 2-1. Twelve sources are detected in the 7-mm continuum. The derived CO luminosities imply gas masses in the range (0.5 – 11) X 10¹º Mסּ (K km s⁻¹ pc²)⁻¹. Combining the CO luminosities and dust masses, along with a fixed gas-to-dust ration, we derive αсо factors in the range 0.4 – 1.8 Mסּ (K km s⁻¹ pc²)⁻¹, similar to what is found in other starbursting systems. We find that lensing magnification factors based on the CO linewidth to luminosity relation (µсо) are highly unreliable, but particularly when µ<5. Finally, comparison of the gas and dynamical masses suggest that the average molecular gas fraction stays relatively constant at ʐ = 2 – 5 in the SPT DSFG sample.