Luminous K-band Selected Quasars from UKIDSS

TL;DR

This paper presents a large, K-band selected quasar sample from UKIDSS, revealing a significant population of quasars missed by optical surveys due to dust reddening and host galaxy light, with implications for quasar demographics.

Contribution

It introduces a novel K-band selection method that uncovers quasars overlooked by optical surveys, including dust-reddened and host-dominated objects, expanding the known quasar population.

Findings

KX-selection identifies quasars missed by SDSS due to optical colour criteria.

Nearly half of the quasars with K<17.0 at z<3 are too faint in i-band for SDSS targeting.

Less than 10% of quasars are missed due to dust reddening, with an obscured fraction likely below 20%.

Abstract

The largest K-band flux-limited sample of luminous quasars to date has been constructed from the UKIDSS Large Area Survey Early Data Release, covering an effective area of 12.8 deg^2. Exploiting the K-band excess of all quasars with respect to foreground stars, including quasars experiencing dust reddening and objects with non-standard 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 AGN. KX-selection successfully identifies as quasar candidates objects that are excluded from the 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 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. (Abridged)