Mapping Obscuration to Reionization with ALMA (MORA): 2mm Efficiently
  Selects the Highest-Redshift Obscured Galaxies
  We present the characteristics of 2mm-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 184arcmin$^2$ at 2mm. Twelve of the thirteen detections
above 5$\sigma$ are attributed to emission from galaxies, eleven of which are
dominated by cold dust emission. These sources have a median redshift of
$\langle z_{\rm 2mm}\rangle=3.6^{+0.4}_{-0.3}$ primarily based on
optical/near-infrared (OIR) photometric redshifts with some spectroscopic
redshifts, with 77$\pm$11% of sources at $z>3$ and 38$\pm$12% of sources at
$z>4$. This implies that 2mm 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 likely to drop out at
2mm. MORA shows that DSFGs with star-formation rates in excess of 300M$_\odot$
yr$^{-1}$ and relative rarity of $\sim$10$^{-5}$ Mpc$^{-3}$ contribute
$\sim$30% to the integrated star-formation rate density between $3<z<6$. The
volume density of 2mm-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 typical literature studies,
with $\langle\beta\rangle=2.2^{+0.5}_{-0.4}$. 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 2mm surveys are needed to more fully
characterize this rare population and push to the detection of the Universe's
first dusty galaxies.
