An Analysis of ALMA Deep Fields and the Perceived Dearth of High-z Galaxies

Deep, pencil-beam surveys from ALMA at 1.1–1.3 mm have uncovered an apparent absence of high-redshift dusty galaxies, with existing redshift distributions peaking around z ~ 1.5–2.5. This has led to a perceived dearth of dusty systems at z ≳ 4 and the conclusion, according to some models, that the early universe was relatively dust-poor. In this paper, we extend the backward-evolution galaxy model described by Casey et al. to the ALMA regime (in depth and area) and determine that the measured number counts and redshift distributions from ALMA deep field surveys are fully consistent with constraints of the infrared luminosity function (IRLF) at z 4 may be particularly dust-poor, but we find that their millimeter-wave emission cannot rule out consistency with the Calzetti dust attenuation law, even by assuming relatively typical, cold-dust (T dust ≈ 30 K) spectral energy distributions. Our models suggest that the design of ALMA deep fields requires substantial revision to constrain the prevalence of z > 4 early universe obscured starbursts. The most promising avenue for detection and characterization of such early dusty galaxies will come from future ALMA 2 mm blank-field surveys covering a few hundred arcmin^2 and the combination of existing and future dual-purpose 3 mm data sets.The Aspen Center for Physics is supported by National Science Foundation grant PHY-1066293. C.M.C. thanks the National Science Foundation for support through grant AST-1714528, and additionally C.M.C. and J.A.Z. thank the University of Texas at Austin College of Natural Sciences for support. J.S. thanks the McDonald Observatory at the University of Texas at Austin for support through a Smith Fellowship. E.d.C. gratefully acknowledges the Australian Research Council for funding support as the recipient of a Future Fellowship (FT150100079). J.A.H. acknowledges support of the VIDI research program with project no. 639.042.611, which is (partly) financed by the Netherlands Organisation for Scientific Research (NWO). S.L.F. acknowledges support from an NSF AAG award AST1518183