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  ∼ 7 quasars to the footprint of the Dark Energy Survey (DES) Data Release One (DR1), covering ∼5000 deg 2 of a new area. We select z  ∼ 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 ∼1 mag fainter, comparing to our z ≳ 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  ∼ 6.7 quasar luminosity function using the quasar sample from our survey in the northern sky, we estimate that there will be ≳55 quasars at z  > 6.5 at M 1450  < −24.5 in the full DES footprint.