Completing the CO spectral line energy distribution for luminous starburst galaxies discovered with the SPT

We propose to use ATCA to observe the CO J=3-2 line emission in three gravitationally lensed, highly magnified dusty star-forming galaxies at z~2.5 discovered by the South Pole Telescope (SPT) millimeter survey. The redshifts of all targets were identified by the detection of several J>6 CO emission lines with APEX/Z-Spec and confirmed with VLT optical spectroscopy. Two of the sources have significant detections of the CO 1-0 line with ATCA, while CO 1-0 observations of the other source are being requested in a companion proposal. The proposed observations are critical to complete the CO spectral energy distribution (SLED) of these sources and thus "fill the gap" between the high-J CO observed with APEX/Z-Spec and the CO 1-0 line detected with ATCA. This will allow us to constrain the physical conditions of the interstellar medium by comparing the line strengths with large velocity gradient models. The strong magnification is key, allowing us to characterize the CO emission in galaxies that would be otherwise hard to detect.