2 mm Observations and the Search for High-redshift Dusty Star-forming Galaxies.

Finding high-redshift (z ≫ 4) dusty star-forming galaxies is extremely challenging. It has recently been suggested that millimeter selections may be the best approach since the negative K-correction makes galaxies at a given far-infrared luminosity brighter at z ≳ 4 than those at z = 2–3. Here we analyze this issue using a deep Atacama Large Millimeter/submillimeter Array (ALMA) 2 mm sample obtained by targeting ALMA 870 μ m priors (these priors were the result of targeting SCUBA-2 850 μ m sources) in the GOODS-S. We construct prior-based 2 mm galaxy number counts and compare them with published blank field-based 2 mm counts, finding good agreement down to 0.2 mJy. Only a fraction of the current 2 mm extragalactic background light is resolved, and we estimate what observational depths may be needed to resolve it fully. By complementing the 2 mm ALMA data with a deep SCUBA-2 450 μ m sample, we exploit the steep gradient with a redshift of the 2 mm–450 μ m flux density ratio to estimate redshifts for those galaxies without spectroscopic or robust optical/near-infrared photometric redshifts. Our observations measure galaxies with star formation rates in excess of 250 M _⊙ yr⁻¹. For these galaxies, the star formation rate densities fall by a factor of 9 from z = 2–3 to z = 5–6. [ABSTRACT FROM AUTHOR]