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Deep near-infrared spectroscopy of submillimetre-selected galaxies
We present the results of deep near-infrared spectroscopy of sevensubmillimetre-selected galaxies from the Submillimetre Common UserBolometer Array (SCUBA) 8-mJy Survey and Canada-UK Deep SubmillimetreSurvey (CUDSS). These galaxies were selected because they are too faintto be accessible to optical spectrographs on large telescopes. We obtaina spectroscopic redshift for one object, and likely redshifts for twomore, based on a combination of marginal emission-line detections andthe shape of the continuum. All three redshifts broadly agree withestimates from their radio/submm spectral energy distributions. From theemission-line strengths of these objects, we infer star formation ratesof 10-25 Msolar yr-1, while the lack of detectionsin the other objects imply even lower rates. By comparing our resultswith those of other authors, we conclude it is likely that the vastmajority (more than 90 per cent) of the star formation in these objectsis completely extinguished at rest-frame optical wavelengths, and theemission lines originate in a relatively unobscured region. Finally, welook at future prospects for making spectroscopic redshiftdeterminations of submm galaxies.
| Dust and Gas Obscuration in ELAIS Deep X-Ray Survey Reddened Quasars
Hard X-ray surveys have uncovered a large population of heavily obscuredAGNs. They also reveal a population of quasars with moderate obscurationat both visible and X-ray wavelengths. We use Chandra-selected samplesof quasars from the ELAIS Deep X-ray Survey (EDXS) and the Chandra DeepField-North to investigate the obscuration toward the nuclei ofmoderately obscured AGNs. We find an inverse correlation between theoptical-to-X-ray flux ratio and the X-ray hardness ratio that can beinterpreted as being due to obscuration at visible and X-raywavelengths. We present detailed optical and near-infrared data for asample of optically faint (R>23) quasars from the EDXS. These areused to constrain the amount of rest-frame UV/optical reddening towardthese quasars. It is found that optically faint quasars are mostly faintbecause of obscuration, not because they are intrinsically weak. Aftercorrecting for reddening, the optical magnitudes of most of thesequasars are similar to the brighter quasars at these X-ray fluxes.Combining with gas column densities inferred from the X-rayobservations, we consider the gas-to-dust ratios of the obscuringmatter. We find that the quasars generally have higher gas-to-dustabsorption than that seen in the Milky Way, similar to what has beenfound for nearby Seyfert galaxies. We consider the possible existence ofa large population of X-ray sources that have optical properties of type1 (unobscured) quasars but X-ray properties of type 2 (obscured)quasars. However, we find that such sources only contribute about 6% ofthe 0.5-8 keV X-ray background. Finally, we show that the observeddistribution of optical-to-X-ray flux ratios of quasars at z>1 isskewed to low values compared to the intrinsic distribution as a resultof the fact that the observed-frame R-band light is emitted in the UVand is more easily obscured than hard X-rays.
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