First Discoveries of z>6 Quasars with the DECam Legacy Survey and UKIRT Hemisphere Survey

Abstract

We present the first discoveries from a survey of $z\gtrsim 6$ quasars using imaging data from the DECam Legacy Survey (DECaLS) in the optical, the UKIRT Deep Infrared Sky Survey (UKIDSS) and a preliminary version of the UKIRT Hemisphere Survey (UHS) in the near-IR, and ALLWISE in the mid-IR. DECaLS will image 9000 deg${}^2$ of sky down to $z_{\mathrm{AB}}\sim 23.0$, and UKIDSS and UHS, which will map the northern sky at $0<DEC<+60^{\circ }$, reaching $J_{\mathrm{VEGA}}\sim 19.6$ (5-$\ddot{I}ƒ$). The combination of these datasets allows us to discover quasars at redshift $z\gtrsim 7$ and to conduct a complete census of the faint quasar population at $z\gtrsim 6$. In this paper, we report on the selection method of our search, and on the initial discoveries of two new, faint $z\gtrsim 6$ quasars and one new $z=6.63$ quasar in our pilot spectroscopic observations. The two new $z\sim 6$ quasars are at $z=6.07$ and $z=6.17$ with absolute magnitudes at rest-frame wavelength 1450 à being $M_{1450}=-25.83$ and $M_{1450}=-25.76$, respectively. These discoveries suggest that we can find quasars close to or fainter than the break magnitude of the Quasar Luminosity Function (QLF) at $z\gtrsim 6$. The new $z=6.63$ quasar has an absolute magnitude of $M_{1450}=-25.95$. This demonstrates the potential of using the combined DECaLS and UKIDSS/UHS datasets to find $z\gtrsim 7$ quasars. Extrapolating from previous QLF measurements, we predict that these combined datasets will yield $\sim 200$ $z\sim 6$ quasars to $z_{\mathrm{AB}}<21.5$, $\sim 1,000$ $z\sim 6$ quasars to $z_{\mathrm{AB}}<23$, and $\sim 30$ quasars at $z>6.5$ to $J_{\mathrm{VEGA}}<19.5$.

8 pages, 5 figures, accepted for publication in The Astrophysical Journal