Near-infrared MOSFIRE Spectra of Dusty Star-forming Galaxies at 0.2 < z < 4

We present near-infrared and optical spectroscopic observations of a sample of 450 and 850 μ m-selected dusty star-forming galaxies (DSFGs) identified in a 400 arcmin 2 area in the COSMOS field. Thirty-one sources of the 114 targets were spectroscopically confirmed at , 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 , comparable to for the larger parent DSFG sample; the median stellar mass is M , star formation rate is 160 ± 50 M yr −1 , and attenuation is . 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 ( %) of DSFGs with inaccurate ( ) 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 Å 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 H α 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.