Eight new luminous z ≥ 6 quasars discovered via SED model fitting of VISTA, WISE and Dark Energy Survey Year 1 observations

Abstract

We present the discovery and spectroscopic confirmation with the European Southern Observatory New Technology Telescope (NTT) and Gemini South telescopes of eight new, and the rediscovery of two previously known, 6.0 < z < 6.5 quasars with z_AB < 21.0. These quasars were photometrically selected without any morphological criteria from 1533 deg² using spectral energy distribution (SED) model fitting to photometric data from Dark Energy Survey (g, r, i, z, Y), VISTA Hemisphere Survey (J, H, K) and Wide-field Infrared Survey Explorer (W1, W2). The photometric data were fitted with a grid of quasar model SEDs with redshift-dependent Ly α forest absorption and a range of intrinsic reddening as well as a series of low-mass cool star models. Candidates were ranked using an SED-model-based χ²-statistic, which is extendable to other future imaging surveys (e.g. LSST and Euclid). Our spectral confirmation success rate is 100 per cent without the need for follow-up photometric observations as used in other studies of this type. Combined with automatic removal of the main types of non-astrophysical contaminants, the method allows large data sets to be processed without human intervention and without being overrun by spurious false candidates. We also present a robust parametric redshift estimator that gives comparable accuracy to Mg ii and CO-based redshift estimators. We find two z ∼ 6.2 quasars with H ii near zone sizes ≤3 proper Mpc that could indicate that these quasars may be young with ages ≲ 10⁶-10⁷ years or lie in over dense regions of the IGM. The z = 6.5 quasar VDES J0224−4711 has J_AB = 19.75 and is the second most luminous quasar known with z ≥ 6.5.

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