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

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 arcmin ^2 at 2 mm. Twelve of 13 detections above 5 σ 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 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 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.

Keywords

• Millimeter astronomy
• Submillimeter astronomy
• Dust continuum emission
• High-redshift galaxies
• Active galaxies
• Infrared galaxies