2017ApJ...840..101C

We present near-infrared and optical spectroscopic observations of a sample of 450 and 850 µm-selected dusty star-forming galaxies (DSFGs) identified in a 400 arcmin² area in the COSMOS field. Thirty-one sources of the 114 targets were spectroscopically confirmed at 0.2 z 4, identified primarily in the near-infrared with Keck MOSFIRE and some in the optical with Keck LRIS and DEIMOS. The low rate of confirmation is attributable both to high rest-frame optical obscuration in our targets and limited sensitivity to certain redshift ranges. The median spectroscopic redshift is <z_spec>=1.55±0.14, comparable to <z_phot>=1.50±0.09 for the larger parent DSFG sample; the median stellar mass is (4.9_–1.4^+2.1)×10¹⁰ M_☉, star formation rate is 160 ± 50 M_☉ yr^–1, and attenuation is A_V=5.0±0.4. The high-quality photometric redshifts available in the COSMOS field allow us to test the robustness of photometric redshifts for DSFGs. We find a subset (11/31~35%) of DSFGs with inaccurate (Δz/(1+z) 0.2) or non-existent photometric redshifts; these have very distinct spectral energy distributions from the remaining DSFGs, suggesting a decoupling of highly obscured and unobscured components. We present a composite rest-frame 4300-7300 A spectrum for DSFGs, and find evidence of 200 ± 30 km s^–1 gas outflows. Nebular line emission for a subsample of our detections indicate that hard ionizing radiation fields are ubiquitous in high-z DSFGs, even more so than typical mass or UV-selected high-z galaxies. We also confirm the extreme level of dust obscuration in DSFGs, measuring very high Balmer decrements and very high ratios of IR to UV and IR to Hα luminosities. This work demonstrates the need to broaden the use of wide bandwidth technology in the millimeter to spectroscopically confirm larger samples of high-z DSFGs, as the difficulty in confirming such sources at optical/near-infrared wavelengths is exceedingly challenging given their obscuration.