Spectroscopic Characterisation of 250um-Selected Hyper-Luminous Star Forming Galaxies

Abstract

We present near-infrared spectroscopic observations from VLT ISAAC of thirteen 250\mu m-luminous galaxies in the CDF-S, seven of which have confirmed redshifts which average to = 2.0 \pm 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} line widths and luminosities, which average to 415 \pm 20 km/s and 3 \times 10^35 W (implying SFR(H{\alpha})~200 M_\odot /yr), both similar to the H{\alpha} properties of SMGs. Just like SMGs, 250 \mu m-luminous galaxies have large H{\alpha} to far-infrared (FIR) extinction factors such that the H{\alpha} SFRs underestimate the FIR SFRs by ~8-80 times. Far-infrared photometric points from observed 24\mu m through 870\mu m are used to constrain the spectral energy distributions (SEDs) even though uncertainty caused by FIR confusion in the BLAST bands is significant. The population has a mean dust temperature of Td = 52 \pm 6 K, emissivity {\beta} = 1.73 \pm 0.13, and FIR luminosity LFIR = 3 \times 10^13 L_\odot. Although selection at 250\mu m allows for the detection of much hotter dust dominated HyLIRGs than SMG selection (at 850\mu m), we do not find any >60 K 'hot-dust' HyLIRGs. We have shown that near-infrared spectroscopy combined with good photometric redshifts is an efficient way to spectroscopically identify and characterise these rare, extreme systems, hundreds of which are being discovered by the newest generation of IR observatories including the Herschel Space Observatory.