We report the detection of CO ( 1-0 ) line emission in the bright , lensed star-forming galaxies SPT-S 233227-5358.5 ( z = 2.73 ) and SPT-S 053816-5030.8 ( z = 2.78 ) , using the Australia Telescope Compact Array ( ATCA ) .
Both galaxies were discovered in a large-area millimeter survey with the South Pole Telescope ( SPT ) and found to be gravitationally lensed by intervening structures .
The measured CO intensities imply galaxies with molecular gas masses of ( 3.2 \pm 0.5 ) \times 10 ^ { 10 } ( \mu / 15 ) ^ { -1 } ( X _ { \mathrm { CO } } / 0.8 ) M _ { \odot } and ( 1.7 \pm 0.3 ) \times 10 ^ { 10 } ( \mu / 20 ) ^ { -1 } ( X _ { \mathrm { CO } } / 0.8 ) M _ { \odot } , and gas depletion timescales of 4.9 \times 10 ^ { 7 } ( X _ { \mathrm { CO } } / 0.8 ) yr and 2.6 \times 10 ^ { 7 } ( X _ { \mathrm { CO } } / 0.8 ) yr , respectively , where \mu corresponds to the lens magnification and X _ { \mathrm { CO } } is the CO luminosity to gas mass conversion factor .
In the case of SPT-S 053816-5030.8 , we also obtained significant detections of the rest-frame 115.7 and 132.4 GHz radio continuum .
Based on the radio to infrared spectral energy distribution and an assumed synchrotron spectral index , we find that 42 \pm 10 \% and 55 \pm 13 \% of the flux at rest-frame 115.7 and 132.4 GHz arise from free-free emission .
We find a radio-derived intrinsic star formation rate ( SFR ) of 470 \pm 170 ~ { } M _ { \odot } yr ^ { -1 } , consistent within the uncertainties with the infrared estimate .
Based on the morphology of this object in the source plane , the derived gas mass and the possible flattening of the radio spectral index towards low frequencies , we argue that SPT-S 053816-5030.8 exhibits properties compatible with a scaled-up local ultra-luminous infrared galaxy .