Luminous K-band selected quasars from UKIDSS

Abstract

Abstract The largest K-band flux-limited sample of luminous quasars to date has been constructed from the UKIRT (UK Infrared Telescope) Infrared Deep Sky Survey (UKIDSS) Large Area Survey Early Data Release, covering an effective area of 12.8 deg2. Exploiting the K-band excess (KX) of all quasars with respect to foreground stars, including quasars experiencing dust reddening and objects with non-standard spectral energy distributions (SEDs), a list of targets suitable for spectroscopic follow-up observations with the AAOmega multi-object spectrograph is constructed, resulting in more than 200 confirmed active galactic nuclei (AGN). KX selection successfully identifies as quasar candidates objects that are excluded from the Sloan Digital Sky Survey (SDSS) quasar selection algorithm due to their colours being consistent with the stellar locus in optical colour space (with the space density of the excluded objects agreeing well with results from existing completeness analyses). Nearly half of the KX-selected quasars with K≤ 17.0 at z < 3 are too faint in the i band to have been targeted by the SDSS quasar selection algorithm, revealing a large population of quasars with red i−K colours. The majority of these objects have significant amounts of host galaxy light contributing to their K-band magnitudes, consistent with previous predictions. The remaining objects are morphologically stellar and have colours consistent with quasars experiencing Small Magellanic Cloud (SMC)-type reddening with 0.10 < E(B−V) < 0.25. The i−K colour distribution indicates that <10 per cent of the quasar population is missing from this K-band selected sample due to dust reddening, and comparisons with simulations strongly favour an obscured fraction of <20 per cent. Photometric redshifts and classifications are computed for the candidates that were not observed spectroscopically. For the extended objects whose colours are consistent with those of a reddened quasar, models of galaxy surface brightness profiles appropriate for each object are used to eliminate the possibility of the presence of a nuclear source bright enough for inclusion in a K≤ 17.0 quasar sample. The effectiveness of near-infrared colour selection of quasars has been demonstrated by this modest-sized sample, and it will only become more apparent as the amount of available data increases.