We present near-infrared and optical spectroscopic observations of a sample of 450 \mu m and 850 \mu 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 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 \langle z _ { spec } \rangle = 1.55 \pm 0.14 , comparable to \langle z _ { phot } \rangle = 1.50 \pm 0.09 for the larger parent DSFG sample ; the median stellar mass is ( 4.9 ^ { +2.1 } _ { -1.4 } ) \times 10 ^ { 10 } M _ { \odot } , star-formation rate is 160 \pm 50 M _ { \odot } yr ^ { -1 } , and attenuation is A _ { V } = 5.0 \pm 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 \approx 35 % ) of DSFGs with inaccurate ( \Delta z / ( 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 4300–7300Å spectrum for DSFGs , and find evidence of 200 \pm 30 km s ^ { -1 } gas outflows .
Nebular line emission for a sub-sample 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 \alpha 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 .