The large area KX quasar catalogue - I. Analysis of the photometric redshift selection and the complete quasar catalogue

Abstract

International audienceThe results of a large area, 600?deg2, K-band flux-limited spectroscopic survey for luminous quasars are presented. The survey utilizes 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 \textless= K \textless= 16.6, and SDSS i-band limits of i = 19.5, 19.7 and 22.0 over sky areas of 287, 150 and 196?deg2, respectively. The survey includes \textgreater3200 known quasars from the SDSS and more than 250 additional confirmed quasars from the KX selection. A well-defined subsample of quasars in the redshift interval 1.0 \textless= z \textless= 3.5 includes 1152 objects from the SDSS and 172 additional KX-selected quasars. The quasar selection is \textgreater95?per cent complete with respect to known SDSS quasars and \textgreater95?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

The VMC Survey

“…It also provides a clear indication of the location of normal galaxies. We note that our colour selection is analogous to the initial criterion in the KX QSO selection method by Maddox et al (2012) which uses gJK, but with the g band replaced by the Y band. For a similar distribution in the V JK colour-colour diagram see Warren et al (2000).…”

“…A series of papers have described the K excess (KX) method (Warren et al 2000) using UKIDSS JK bands and the SDSS g band (e.g. Maddox et al 2012;Mortlock et al 2012). Findlay et al (2012) based their selection on single epoch VISTA ZY J data.…”

Determining the fraction of reddened quasars in COSMOS with multiple selection techniques from X-ray to radio wavelengths

“…Another approach to identify the missing reddened quasars, regardless of redshift and amount of dust reddening, is to exploit the general K-band excess (KX) shown by all quasars (Warren et al 2000;Croom et al 2001;Maddox et al 2008Maddox et al , 2012 hereafter referred to as KX. This approach is comparable to the HAQ selection criteria with the NIR K-band excess of quasars being the primary quasar/star classifier, although not as tailored for targeting objects with red optical colors.…”

“…Recent studies estimate fractions from below 10% to above 40% (Richards et al 2003(Richards et al , 2006Glikman et al 2004Glikman et al , 2012Gibson et al 2009;Urrutia et al 2009;Allen et al 2011;Maddox et al 2008Maddox et al , 2012. The majority of objects in these samples consist of broad absorption line (BAL) quasars.…”

“…However, they have been found to be systematically biased against It it therefore important to understand to what extent various quasar samples suffer from biases against dust-reddened systems and quasars at high redshifts so that we can build more complete and representative samples. A large number of studies targeting reddened quasars have been executed in the past two decades (see, e.g., Webster et al 1995;Warren et al 2000;Gregg et al 2002;Richards et al 2003;Hopkins et al 2004;Glikman et al 2007Glikman et al , 2012Glikman et al , 2013Maddox et al 2008Maddox et al , 2012Urrutia et al 2009;Banerji et al 2012;Fynbo et al 2013;Krogager et al 2015), all trying to probe this underrepresented subset of quasars using a range of selection techniques that are less sensitive to dust obscuration. With the advent of large-area near-and mid-infrared surveys such as UKIDSS, WISE, and Spitzer (Wright et al 2010;Cutri & et al 2013;Lawrence et al 2007;Warren et al 2007;Werner et al 2004;Fazio et al 2004), the selection of quasars based on infrared (IR) photometry alone has been made possible as well (Donley et al 2008(Donley et al , 2012Peth et al 2011;Stern et al 2012;Mateos et al 2012;Secrest et al 2015).…”