Spectroscopic characterization of 250-μm-selected hyper-luminous star-forming galaxies

We present near-infrared (near-IR) spectroscopic observations from Very Large Telescope Infrared Spectrometer And Array Camera (ISAAC) of 13 250-mu m luminous galaxies in the Chandra Deep Field-South, seven of which have confirmed redshifts which average to << z >> = 2.0 +/- 0.4. Another two sources of the 13 have tentative z > 1 identifications. Eight of the nine redshifts were identified with H alpha detection in H and K bands, three of which are confirmed redshifts from previous spectroscopic surveys. We use their near-IR spectra to measure H alpha linewidths and luminosities, which average to 415 +/- 20 km s(-1) and 3 x 10(35) W (implying SFR (H alpha) similar to 200 M (circle dot) yr(-1)), both similar to the H alpha properties of submillimetre galaxies (SMGs). Just like SMGs, 250-mu m-luminous galaxies have large H alpha to far-infrared (FIR) extinction factors such that the H alpha star formation rates (SFRs) underestimate the FIR SFRs by approximately eight to 80 times. FIR photometric points observed from 24 to 870 mu m are used to constrain the spectral energy distributions even though uncertainty caused by FIR confusion in the Balloon-borne Large-Aperture Submillimeter Telescope (BLAST) bands is significant. The population has a mean dust temperature of T-d = 52 +/- 6 K, emissivity beta = 1.73 +/- 0.13 and FIR luminosity L-FIR = 3 x 10(13) L-circle dot. Although selection at 250 mu m allows for the detection of much hotter dust-dominated hyper-luminous infrared galaxies (HyLIRGs) than SMG selection (at 850 mu m), we do not find any greater than or similar to 60-K 'hot-dust' HyLIRGs. We have shown that near-IR spectroscopy combined with good photometric redshifts is an efficient way to spectroscopically identify and characterize these rare, extreme systems, hundreds of which are being discovered by the newest generation of IR observatories including the Herschel Space Observatory.