Mapping Obscuration to Reionization with ALMA (MORA): 2mm Efficiently Selects the Highest-Redshift Obscured Galaxies

Abstract

We present the characteristics of 2 mm-selected sources from the largest Atacama Large Millimeter and submillimeter Array (ALMA) blank-field contiguous survey conducted to-date, the Mapping Obscuration to Reionization with ALMA (MORA) survey covering 184 arcmin 2 at 2 mm. Twelve of thirteen detections above 5σ are attributed to emission from galaxies, eleven of which are dominated by cold dust emission. These sources have a median redshift of z 2mm = 3.6 +0.4 −0.3 primarily based on optical/near-infrared (OIR) photometric redshifts with some spectroscopic redshifts, with 77±11% of sources at z > 3 and 38±12% of sources at z > 4. This implies that 2 mm selection is an efficient method for identifying the highest redshift dusty star-forming galaxies (DSFGs). Lower redshift DS-FGs (z < 3) are far more numerous than those at z > 3 yet likely to drop out at 2 mm. MORA shows that DSFGs with star-formation rates in excess of 300 M yr −1 and relative rarity of ∼ 10 −5 Mpc −3 contribute ∼30% to the integrated star-formation rate density between 3 < z < 6. The volume density of 2 mm-selected DSFGs is consistent with predictions from some cosmological simulations and is

Characterization of Two 2mm-detected Optically-Obscured Dusty Star-Forming Galaxies

“…In total, thirteen robust sources (> 5 σ) were detected in the 2 mm mosaic. We direct the reader to the accompanying MORA papers for a more thorough overview of the survey and complete discussions on 2 mm number counts (Zavala et al 2021) and source characterization (Casey et al 2021). What follows is an explanation of the ALMA data reduction and imaging, including available archival data, as well as the existing ancillary data for MORA-5 and MORA-9, which are the focus of this paper.…”

“…Our current understanding of star formation and galaxy evolution within the first two billion years after the Big Bang is severely limited by a lack of infrared (IR) constraints and associated sample incompleteness at z > 4 (Gruppioni et al 2013;Casey, Narayanan, & Cooray 2014;Madau & Dickinson 2014;Casey et al 2018b). Very recent studies (Zavala et al 2021;Casey et al 2021) have worked to extend IR measurements up to z ∼ 7, but uncertainties are still dominant. The census of cosmic star formation out to the highest redshifts is also biased towards unobscured star formation tracers, relying on rest-frame ultraviolet (UV) continuum measurements to seek out Lyman-break selected galaxies (LBGs).…”

“…The Mapping Obscuration to Reionization ALMA (MORA) Survey is the largest (184 arcmin 2 ) 2 mm mosaic mapped with the Atacama Large Millimeter/submillimeter Array (ALMA) whose aim is to efficiently select z 3 − 4 DSFGs. For a full description of the MORA Survey, see Zavala et al (2021) and Casey et al (2021). Twelve total sources were robustly detected (> 5σ) in the MORA survey.…”

The Nature of 500 Micron Risers II: Sub-mm Faint Dusty Star Forming Galaxies

“…In Figure A1, we present the photometric redshift PDFs obtained from MM for our 500 𝜇m risers and the other SNR > 4 detections in the SCUBA-2 fields. The MM fitting algorithm finds the most likely redshift at which a galaxy resides by determining where its FIR/mm SED is most consistent with the observed 𝐿 IR − 𝜆 peak relation, where 𝜆 peak is the rest-frame wavelength at which the FIR/mm SED peaks (Lee et al 2013;Strandet et al 2016;Casey et al 2018;Casey 2020;Casey et al 2021). We find that the photometric redshifts for Bootes15, Bootes15.SCUBA1 and Bootes15.SCUBA2 are poorly constrained due to poor quality photometry.…”

“…As an independent estimate of the photometric redshift, we additionally run the MM algorithm 7 (Casey 2020) which finds the most likely redshift at which a galaxy resides by determining where its FIR/mm SED is most consistent with the observed 𝐿 IR − 𝜆 peak relation, where 𝜆 peak is the rest-frame wavelength at which the FIR/mm SED peaks (Lee et al 2013;Strandet et al 2016;Casey et al 2018;Casey 2020;Casey et al 2021). We use the integrated FIR photometry for each source, including the Herschel SPIRE and SCUBA-2 photometry, and obtain a photometric redshift PDF for each of our 500 𝜇m risers.…”