DISCOVERY OF EIGHT z ∼ 6 QUASARS IN THE SLOAN DIGITAL SKY SURVEY OVERLAP REGIONS

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 deg² (more than one-fourth of the total footprint). These overlap regions provide a unique data set 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°). 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 they provide guidance to a future systematic survey over all SDSS imaging regions with repeat observations.