Exploring Reionization-era Quasars. IV. Discovery of Six New z ≳ 6.5 Quasars with DES, VHS, and unWISE Photometry

Abstract

This is the fourth paper in a series of publications aiming at discovering quasars at the epoch of reionization. In this paper, we expand our search for z similar to 7 quasars to the footprint of the Dark Energy Survey (DES) Data Release One (DR1), covering similar to 5000 deg(2) of a new area. We select z similar to 7 quasar candidates using deep optical, near-infrared (near-IR) and mid-infrared (mid-IR) photometric data from the DES DR1, the VISTA Hemisphere Survey, the VISTA Kilo-degree Infrared Galaxy survey, the UKIRT InfraRed Deep Sky Surveys-Large Area Survey (ULAS), and the unblurred coadds from the Wide-field Infrared Survey Explore (WISE) images (unWISE). The inclusion of DES and unWISE photometry allows the search to reach similar to 1 mag fainter, comparing to our z greater than or similar to 6.5 quasar survey in the northern sky. We report the initial discovery and spectroscopic confirmation of six new luminous quasars at z > 6.4, including an object at z = 7.02, the fourth quasar yet known at z > 7, from a small fraction of candidates observed thus far. Based on the recent measurement of z similar to 6.7 quasar luminosity function using the quasar sample from our survey in the northern sky, we estimate that there will be greater than or similar to 55 quasars at z > 6.5 at M-1450 < - 24.5 in the full DES footprint.