2009A&A...505...97M

Optical and near-infrared (NIR) spectra are presented for ULAS J131911.29+095051.4 (hereafter ULAS J1319+0950), a new redshift z=6.127±0.004 quasar discovered in the Third Data Release (DR3) of the UKIRT Infrared Deep Sky Survey (UKIDSS). The source has Y_Vega=19.10±0.03, corresponding to M_1450,AB=-27.12, which is comparable to the absolute magnitudes of the z≃6 quasars discovered in the Sloan Digital Sky Survey (SDSS). ULAS J1319+0950 was, in fact, registered by SDSS as a faint source with z_AB=20.13±0.12, just below the signal-to-noise ratio limit of the SDSS high-redshift quasar survey. The faint z-band magnitude is a consequence of the weak Lyα/NV emission line, which has a rest-frame equivalent width of ∼20Å and provides only a small boost to the z-band flux. Nevertheless, there is no evidence of a significant new population of high-redshift quasars with weak emission lines from this UKIDSS-based search. The Lyα optical depth to ULAS J1319+0950 is consistent with that measured towards similarly distant SDSS quasars, implying that results from optical- and NIR-selected quasars may be combined in studies of cosmological reionization. Also presented is a new NIR-spectrum of the previously discovered UKIDSS quasar ULAS J020332.38+001229.2, which reveals the object to be a broad absorption line quasar. The new spectrum shows that the emission line initially identified as Lyα is actually Nv, leading to a revised redshift of z=5.72, rather than z=5.86 as previously estimated.