WFCAM, UKIDSS, and z=7 quasars

Abstract

We estimate the number of z~7 quasars that will be discovered in the Large Area Survey (LAS) element of the UKIRT Infrared Deep Sky Survey (UKIDSS). The LAS will cover 4000 sq degs of the northern sky to K=18.4, which is 3 mag. deeper than 2MASS. The Sloan Digital Sky Survey has extended the quasar redshift limit to z=6.3. We demonstrate that to reach higher redshifts z~7, when Lya has passed through the z band, combinations of standard broad-band filters such as zJH and zJK are ineffective. Instead the wavelength range between z and J must be exploited. We introduce the Y passband 0.97-1.07 micron for this purpose. High-redshift quasars up to redshift z=7.2 can be selected from a iYJ or zYJ two-colour diagram, as bluer than L and T dwarfs.