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 ESO NTT and Gemini South telescopes of eight new 6.0 < z < 6.5 quasars with z AB < 21.0. These quasars were photometrically selected without any star-galaxy morphological criteria from 1533 deg 2 using SED model fitting to photometric data from the Dark Energy Survey (g, r, i, z, Y), the VISTA Hemisphere Survey (J, H, K) and the Wide-Field Infrared Survey Explorer (W1, W2). The photometric data was fitted with a grid of quasar model SEDs with redshift dependent Lyman-α forest absorption and a range of intrinsic reddening as well as a series of low mass cool star models. Candidates were ranked using on a SED-model based χ 2 -statistic, which is extendable to other future imaging surveys (e.g. LSST, Euclid). Our spectral confirmation success rate is 100% 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 over run by spurious false candidates. We also present a robust parametric redshift estimating technique that gives compararable accuracy to MgII and CO based redshift estimators. We find two z ∼ 6.2 quasars with HII near zone sizes 3 proper Mpc which could indicate that these quasars may be young with ages 10 6 − 10 7 years or lie in over dense regions of the IGM. The z = 6.5 quasar VDESJ0224−4711 has J AB = 19.75 is the second most luminous quasar known with z 6.5.

Physical Properties of 15 Quasars at z ≳ 6.5

“…The quasar flux at wavelengths shorter than the Lyα emission line (at rest-frame 1215.67 rf l = Å) is absorbed by the intervening neutral medium, causing an extremely red (i − z) or (z − y) color if the source is at z 6  (i-dropouts) or z 6.4  (z-dropouts), respectively. In the past two decades ∼200 quasars have been discovered at z 5.4 6.4 < < , mainly thanks to the advent of large-area surveys, e.g., the Sloan Digital Sky Survey (SDSS; Fan et al 2000, Fan et al 2003, Jiang et al 2016Wang et al 2016, the Canada-France Highredshift Quasar Survey (CFHQS; Willott et al 2007Willott et al , 2009Willott et al , 2010bWillott et al , 2013, the UK Infrared Deep Sky Server (UKIDSS; Venemans et al 2007;Mortlock et al 2009), the Dark Energy Survey (DES; Reed et al 2015, Reed et al 2017, the Very Large Telescope Survey Telescope (VST) ATLAS Survey (Carnall et al 2015), the ESO public Kilo-Degree Survey (KiDS; Venemans et al 2015b), and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1 or PS1; Morganson et al 2012; Bañados et al 2014Bañados et al , 2015bBañados et al , 2016. However, the search for sources at even higher redshift (z 6.4  ; age of the universe <0.80 Gyr) has been extremely challenging, and only a few quasars have been discovered at such a distance prior to the results presented here: three from the VISTA Kilo-Degree Infrared Galaxy Survey (VIKING; Venemans et al 2013), four from PS1 (Venemans et al 2015a;Tang et al 2017), and one from the Subaru Hyper Suprime-Cam-SPP Survey (HSC-SPP; Matsuoka et al 2016, Matsuoka et al 2017; so far, only one quasar has been found at z 7 > in the UKIDSS (Mortlock et al 2011).…”

“…15 We do not consider the quasars VDES J0224-4711 (z 6.50 = ; Reed et al 2017), DELS J104819.09-010940.2 (z 6.63 = ; Wang et al 2017), and J1429-0104 (z 6.80; = Matsuoka et al 2017), which were reported during the final stages of the preparation of this manuscript.…”

A new bright z = 6.82 quasar discovered with VISTA: VHS J0411–0907

“…In this paper we extend a SED classification method presented in Reed et al (2017) to the Pan-STARRS1 survey to select and classify high-z quasar candidates using optical, NIR, and MIR photometry. We report the discovery of the spectroscopically confirmed highest redshift Pan-STARRS (PS1) quasar, VHS J0411-0907 1 , and the brightest object in the NIR J-band among the 9 known quasars at redshift z > 6.7.…”

Formation of massive seed black holes via collisions and accretion

“…More than 100 supermassive black holes have already been detected at z > 6 (Gallerani et al 2017), with the number still continuously increasing, including 32 quasars recently discovered between z = 5.7 and 6.8 via Subaru (Matsuoka et al 2017) and 8 z > 6 quasars via SED model fitting of VISTA, WISE and Dark Energy Survey Year 1 observations (Reed et al 2017). The currently most distant known quasars are at z = 7.085, i.e.…”