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

Creators: Casey, Caitlin M.; Cooray, Asantha; Killi, Meghana; Capak, Peter; Chen, Chian-Chou; Hung, Chao-Ling; Kartaltepe, Jeyhan S.; Sanders, D. B.; Scoville, N. Z.

Abstract

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 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 〈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) x 10^(10) 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 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 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.

Additional Information

© 2017 The American Astronomical Society. Received 2016 December 12. Accepted 2017 March 24. Published 2017 May 12. The authors thank the anonymous referee for a constructive and useful review that helped improve the analysis in this paper. This work was supported in part by a NASA Keck PI Data Award, administered by the NASA Exoplanet Science Institute. The data presented herein were obtained at the W.M. Keck Observatory which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This work was performed in part at the Aspen Center for Physics, which is supported by National Science Foundation grant PHY-1066293. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.00118.S, 2013.1.00151.S, and 2011.0.00539.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. COSMOS is based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA Inc, under NASA contract NAS 5-26555; also based on data collected at: the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA; the European Southern Observatory, Chile; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation; the National Radio Astronomy Observatory which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc; and the Canada–France–Hawaii Telescope operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii. CMC thanks the University of Texas at Austin, College of Natural Science for support. AC is supported by NSF ST-1313319 and NASA NNX16AF39G and NNX16AF38G.

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