Large gas reservoirs and free–free emission in two lensed star-forming galaxies at z = 2.7

Abstract

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. Both galaxies were discovered in a large-area millimetre 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 ± 0.5) × 10^10(μ/15)^(−1)(X_CO/0.8) and (1.7 ± 0.3) × 10^10(μ/20)^(−1)(XCO/0.8) M_⊙, and gas depletion time-scales of 4.9 × 107(X_CO/0.8) and 2.6 × 10^7(X_CO/0.8) yr, respectively, where μ corresponds to the lens magnification and X_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 ± 10 and 55 ± 13 per cent of the flux at rest-frame 115.7 and 132.4 GHz arises from free–free emission. We find a radio-derived intrinsic star formation rate of 470 ± 170 M_⊙ 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 ultraluminous infrared galaxy.

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