2018ApJ...863..144S

We present the North Galactic Cap sample of the Extremely Luminous Quasar Survey (ELQS-N), which targets quasars with M₁₄₅₀ < -27 at 2.8 <= z < 5 in an area of ∼7600 deg² of the Sloan Digital Sky Survey (SDSS) footprint with 90° < R.A. < 270°. Based on a near-infrared/infrared JKW2 color cut, the ELQS selection efficiently uses random forest methods to classify quasars and to estimate photometric redshifts; this scheme overcomes some of the difficulties of pure optical quasar selection at z ≃ 3. As a result, we retain a completeness of >70% over z ∼ 3.0-5.0 at m_i <= 17.5, limited toward fainter magnitudes by the depth of the Two Micron All Sky Survey. The presented quasar catalog consists of a total of 270 objects, of which 39 are newly identified in this work with spectroscopy obtained at the Vatican Advanced Technology Telescope and the MMT 6.5 m telescope. In addition to the high completeness, which allowed us to discover new quasars in the already well-surveyed SDSS North Galactic Cap, the efficiency of our selection is relatively high at ∼79%. Using 120 objects of this quasar sample we are able to extend the previously measured optical quasar luminosity function (QLF) by one magnitude toward the bright end at 2.8 <= z <= 4.5. A first analysis of the QLF suggests a relatively steep bright-end slope of β ≃ -4 for this sample. This result contrasts with previous results in the same redshift range, which find a much flatter slope around β ∼ -2.5, but agrees with recent measurements of the bright-end slope at lower and higher redshifts. Our results constrain the bright-end slope at z = 2.8-4.5 to β < -2.94 with a 99% confidence.