A survey of the cold molecular gas in gravitationally lensed star-forming galaxies at<i>z</i>&gt; 2

Abstract

Using the Australia Telescope Compact Array (ATCA), we conducted a survey of\nCO J=1-0 and J=2-1 line emission towards strongly lensed high-redshift dusty\nstar forming galaxies (DSFGs) previously discovered with the South Pole\nTelescope (SPT). Our sample comprises 17 sources that had CO-based\nspectroscopic redshifts obtained with the Atacama Large\nMillimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment\n(APEX). We detect all sources with known redshifts in either CO J=1-0 or J=2-1.\nTwelve sources are detected in the 7-mm continuum. The derived CO luminosities\nimply gas masses in the range (0.5-11)x10^{10} M_sun and gas depletion\ntimescales &lt;200 Myr, using a CO to gas mass conversion factor alpha_CO=0.8\nM_sun (K km/s pc^2)^{-1}. Combining the CO luminosities and dust masses, along\nwith a fixed gas-to-dust ratio, we derive alpha_CO factors in the range\n0.4-1.8, similar to what is found in other starbursting systems. We find small\nscatter in alpha_CO values within the sample, even though inherent variations\nin the spatial distribution of dust and gas in individual cases could bias the\ndust-based alpha_CO estimates. We find that lensing magnification factors based\non the CO linewidth to luminosity relation (mu_CO) are highly unreliable, but\nparticularly when mu&lt;5. Finally, comparison of the gas and dynamical masses\nsuggest that the average molecular gas fraction stays relatively constant at\nz=2-5 in the SPT DSFG sample.\n