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-μm 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 Hα detection in H and K bands, three of which are confirmed redshifts from previous spectroscopic surveys. We use their near-IR spectra to measure Hα linewidths and luminosities, which average to 415 ± 20 km s ―1 and 3 x 10 35 W (implying SFR Hα ∼ 200 M ⊙ yr ―1 ), both similar to the Hα properties of submillimetre galaxies (SMGs). Just like SMGs, 250-μm-luminous galaxies have large Hα to far-infrared (FIR) extinction factors such that the Hα star formation rates (SFRs) underestimate the FIR SFRs by approximately eight to 80 times. FIR photometric points observed from 24 to 870 μm 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 β = 1.73 ± 0.13 and FIR luminosity L FIR = 3 × 10 13 L ⊙ . Although selection at 250 μm allows for the detection of much hotter dust-dominated hyper-luminous infrared galaxies (HyLIRGs) than SMG selection (at 850 μm), we do not find any ≳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.