* Astronomy

Title: Discovery of a redshift 6.13 quasar in the UKIRT Infrared Deep Sky Survey
Authors: D. J. Mortlock (1), M. Patel (1), S. J. Warren (1), B. P. Venemans (2), R. G. McMahon (2), P. C. Hewett (2), C. Simpson (3), R. G. Sharp (4), B. Burningham (5), S. Ellis (6), E. A. Gonzales-Solares (2), N. Huelamo ((1) Imperial College London, (2) University of Cambridge, (3) Liverpool John Moores University, (4) Anglo-Australian Observatory, (5) University of Hertfordshire, (6) Cardiff University, (7) University of Sydney, (8) European Space Astronomy Centre)
(Version v3)

Optical and near-infrared (NIR) spectra are presented for ULAS J131911.29+095951.4 (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 SDSS. ULAS J1319+0950 was, in fact, registered by SDSS as a faint source with z_AB = 20.1 ±0.1, 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 Ly alpha/N V emission line, which has a rest-frame equivalent width of only ~20 A and provides only a small boost to the z-band flux. Nevertheless, there is no evidence from this UKIDSS-based search for a significant population of high-redshift quasars with weak emission lines. The Ly alpha 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.
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 previously identified as Ly alpha is actually N V, leading to a revised redshift of z = 5.72, rather than z = 5.86 as previously estimated.

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