The Infrared Medium-deep Survey. III. Survey of Luminous Quasars at 4.7 ≤ z ≤ 5.4

We present the first results of our survey for high-redshift quasars at 5 ≲ z ≲ 5.7. The search for quasars in this redshift range has been known to be challenging due to the limitations of the filter sets used in previous studies. We conducted a quasar survey for two specific redshift ranges, 4.60 ≤ z ≤ 5.40 and 5.50 ≤ z ≤ 6.05, using multiwavelength data that include observations made with custom-designed filters, is and iz. Using these filters and a new selection technique, we were able to reduce the fraction of interlopers. Through optical spectroscopy, we confirmed six quasars at 4.7 ≤ z ≤ 5.4 with -27.4 < M¹⁴⁵⁰ < -26.4 that recently were discovered independently by another group. We estimated black hole masses and Eddington ratios of four of these quasars from optical and near-infrared spectra, and found that these quasars are undergoing nearly Eddington-limited accretion that is consistent with the rapid growth of supermassive black holes in luminous quasars at z ∼ 5.