Constraining the Volume Density of Dusty Star-forming Galaxies through the First 3 mm Number Counts from ALMA

We carry out a blind search of 3 mm continuum sources using the ALMA Science Archive to derive the first galaxy number counts at this wavelength. The analyzed data are drawn from observations toward three extragalactic legacy fields: COSMOS, CDF-S, and the UDS comprising more than 130 individual ALMA Band 3 pointings and an effective survey area of ≈200 arcmin2 with a continuum sensitivity that allows for the direct detection of unlensed Dusty Star-forming Galaxies (DSFGs) dust emission beyond the epoch of reionization. We present a catalog of 16 sources detected at >5σ with flux densities ${S}_{3\mathrm{mm}}\approx 60\mbox{--}600\,\mu \mathrm{Jy}$ from which number counts are derived. These number counts are then used to place constraints on the volume density of DSFGs with an empirical backward evolution model. Our measured 3 mm number counts indicate that the contribution of DSFGs to the cosmic star formation rate density at z gsim 4 is non-negligible. This is contrary to the generally adopted assumption of a sharply decreasing contribution of obscured galaxies at z > 4 as inferred by optical and near-infrared surveys. This work demonstrates the power of ALMA-3 mm observations, which can reach outstanding continuum sensitivities during typical spectral line science programs. Further constraints on 3 mm selected galaxies will be essential to refine models of galaxy formation and evolution as well as models of early universe dust production mechanisms