On the redshift distribution and physical properties of ACT-selected DSFGs

Abstract: We present multi-wavelength detections of nine candidate gravitationally-lensed dusty star-forming galaxies (DSFGs) selected at 218GHz (1.4mm) 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 218GHz 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 218GHz selection, and an apparent total infrared luminosity of $\log_{10}(\mu L_{\rm IR}/{\rm L}\odot) = 13.86^{+0.33}{-0.30}$, 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 ($\lambda_{\rm obs} \le 500$ $\mu$m), a result that is robust to model choice.