Abstract
The results of a large area, ~600 deg^2, K-band flux-limited spectroscopic survey for luminous quasars are presented. The survey utilises the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) in regions of sky within the Sloan Digital Sky Survey (SDSS) footprint. The K-band excess (KX) of all quasars with respect to Galactic stars is exploited in combination with a photometric redshift/classification scheme to identify quasar candidates for spectroscopic follow-up observations. The data contained within this investigation will be able to provide new constraints on the fraction of luminous quasars reddened by dust with E(B-V)<=0.5 mag. The spectroscopic sample is defined using the K-band, 14.0<=K<=16.6, and SDSS i-band limits of i=19.5, 19.7 and 22.0 over sky areas of 287, 150 and 196 deg^2, respectively. The survey includes >3200 known quasars from the SDSS and more than 250 additional confirmed quasars from the KX-selection. A well-defined sub-sample of quasars in the redshift interval 1.0<=z<=3.5 includes 1152 objects from the SDSS and 172 additional KX-selected quasars. The quasar selection is >95 per cent complete with respect to known SDSS quasars and >95 per cent efficient, largely independent of redshift and i-band magnitude. The properties of the new KX-selected quasars confirm the known redshift-dependent effectiveness of the SDSS quasar selection and provide a sample of luminous quasars experiencing intermediate levels of extinction by dust. The catalogue represents an important step towards the assembly of a well-defined sample of luminous quasars that may be used to investigate the properties of quasars experiencing intermediate levels of dust extinction within their host galaxies or due intervening absorption line systems.
Highlights
The Sloan Digital Sky Survey (SDSS, York et al 2000), with some ten thousand square degrees of simultaneous fiveband imaging and extensive spectroscopic follow-up observations has revolutionised studies of optically-selected luminous quasars and active galactic nuclei (AGN).Optical selection of quasars is highly effective at identifying large samples of quasars
The benefit of the K-band excess (KX)-selection is evident from the elimination of the vast majority of Galactic stars with more than 90 per cent of all cross-matched UKIRT Infrared Deep Sky Survey (UKIDSS)-SDSS stellar objects removed in a single step
A number of extra quasars have been found by the NIR Photo-z selection algorithm relying on the K-band excess of quasars with respect to Galactic stars
Summary
Optical selection of quasars is highly effective at identifying large samples of quasars. Between the SDSS, the 2dF Quasar Redshift Survey (2QZ, Croom et al 2004) and the 2dF-SDSS LRG and QSO Survey (2SLAQ, Croom et al 2009), more than 150 000 spectroscopically confirmed quasars have been catalogued between redshifts 0 < z < 5.5. The 2QZ and 2SLAQ exploit the ultraviolet (UV) ‘excess’ shown by many quasars with respect to Galactic stars, and blue quasars with z < 2.2 are confirmed with reasonable efficiency (∼ 50 per cent). At higher redshifts, intergalactic absorption shortward of the Lyα emission line reduces the UV-excess signal. Quasars suffering from even a small amount of dust extinction, whether intrinsic to the quasar or due to intervening absorption systems, show reduced UV flux at all redshifts
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