We present multi-wavelength detections of nine candidate gravitationally-lensed dusty star-forming galaxies ( DSFGs ) selected at 218 GHz ( 1.4 mm ) from the ACT equatorial survey .
Among the brightest ACT sources , these represent the subset of the total ACT sample lying in Herschel SPIRE fields , and all nine of the 218 GHz detections were found to have bright Herschel counterparts .
By fitting their spectral energy distributions ( SEDs ) with a modified blackbody model with power-law temperature distribution , we find the sample has a median redshift of z = 4.1 ^ { +1.1 } _ { -1.0 } ( 68 per cent confidence interval ) , as expected for 218 GHz selection , and an apparent total infrared luminosity of \log _ { 10 } ( \mu L _ { IR } / { L } _ { \odot } ) = 13.86 ^ { +0.33 } _ { -0.30 } , which suggests that they are either strongly lensed sources or unresolved collections of unlensed DSFGs .
The effective apparent diameter of the sample is \sqrt { \mu } d = 4.2 ^ { +1.7 } _ { -1.0 } kpc , further evidence of strong lensing or multiplicity , since the typical diameter of dusty star-forming galaxies is 1.0 – 2.5 kpc .
We emphasize that the effective apparent diameter derives from SED modelling without the assumption of optically thin dust ( as opposed to image morphology ) .
We find that the sources have substantial optical depth ( \tau = 4.2 ^ { +3.7 } _ { -1.9 } ) to dust around the peak in the modified blackbody spectrum ( \lambda _ { obs } \leq 500 \mu m ) , a result that is robust to model choice .