Discovery of Eight z ~ 6 Quasars in the Sloan Digital Sky Survey Overlap Regions

TL;DR

This study discovered eight new quasars at redshift ~6 in SDSS overlap regions, demonstrating the effectiveness of their selection method and enabling fainter quasar detection than previous single-epoch surveys.

Contribution

The paper introduces a novel approach using SDSS overlap regions to identify fainter high-redshift quasars, expanding the known quasar population at z~6.

Findings

Discovered eight new quasars at z~6 in SDSS overlap regions.

Validated the candidate selection and follow-up procedure.

Revealed quasars fainter than previous magnitude limits.

Abstract

We present the discovery of eight quasars at z~6 identified in the Sloan Digital Sky Survey (SDSS) overlap regions. Individual SDSS imaging runs have some overlap with each other, leading to repeat observations over an area spanning >4000 deg^2 (more than 1/4 of the total footprint). These overlap regions provide a unique dataset that allows us to select high-redshift quasars more than 0.5 mag fainter in the z band than those found with the SDSS single-epoch data. Our quasar candidates were first selected as i-band dropout objects in the SDSS imaging database. We then carried out a series of follow-up observations in the optical and near-IR to improve photometry, remove contaminants, and identify quasars. The eight quasars reported here were discovered in a pilot study utilizing the overlap regions at high galactic latitude (|b|>30 deg). These quasars span a redshift range of 5.86<z<6.06 and a flux range of 19.3<z_AB<20.6 mag. Five of them are fainter than z_AB=20 mag, the typical magnitude limit of z~6 quasars used for the SDSS single-epoch images. In addition, we recover eight previously known quasars at z~6 that are located in the overlap regions. These results validate our procedure for selecting quasar candidates from the overlap regions and confirming them with follow-up observations, and provide guidance to a future systematic survey over all SDSS imaging regions with repeat observations.