Mapping Obscuration to Reionization with ALMA (MORA): 2 mm Efficiently Selects the Highest-redshift Obscured Galaxies
Finkelstein, Steven L.; Casey, Caitlin M.; Long, Arianna S.; Zavala, Jorge A.; Staguhn, Johannes; Fujimoto, Seiji; Caputi, Karina I.; Clements, David L.; Toft, Sune; Magdis, Georgios E.; Spilker, Justin; Aravena, Manuel; Hayward, Christopher C.; Champagne, Jaclyn B.; Béthermin, Matthieu; Weaver, John R.; Kokorev, Vasily; Treister, Ezequiel; Mitsuhashi, Ikki; Lagos, Claudia del P.; Talia, Margherita; Manning, Sinclaire M.; Yun, Min; Popping, Gergö; Man, Allison W. S.; Drew, Patrick; Dekel, Anton M.
United States, Japan, Chile, France, Netherlands, United Kingdom, Denmark, Australia, Canada, Germany, Italy
Abstract
We present the characteristics of 2 mm selected sources from the largest Atacama Large Millimeter/submillimeter Array (ALMA) blank-field contiguous survey conducted to date, the Mapping Obscuration to Reionization with ALMA (MORA) survey covering 184 arcmin2 at 2 mm. Twelve of 13 detections above 5σ are attributed to emission from galaxies, 11 of which are dominated by cold dust emission. These sources have a median redshift of $\langle {z}_{2\,\mathrm{mm}}\rangle ={3.6}_{-0.3}^{+0.4}$ primarily based on optical/near-infrared 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 DSFGs (z < 3) are far more numerous than those at z > 3 yet are likely to drop out at 2 mm. MORA shows that DSFGs with star formation rates in excess of 300 M ⊙ yr-1 and a relative rarity of ~10-5 Mpc-3 contribute ~30% to the integrated star formation rate density at 3 < z < 6. The volume density of 2 mm selected DSFGs is consistent with predictions from some cosmological simulations and is similar to the volume density of their hypothesized descendants: massive, quiescent galaxies at z > 2. Analysis of MORA sources' spectral energy distributions hint at steeper empirically measured dust emissivity indices than reported in typical literature studies, with $\langle \beta \rangle ={2.2}_{-0.4}^{+0.5}$ . The MORA survey represents an important step in taking census of obscured star formation in the universe's first few billion years, but larger area 2 mm surveys are needed to more fully characterize this rare population and push to the detection of the universe's first dusty galaxies.