The Extremely Luminous Quasar Survey in the Sloan Digital Sky Survey Footprint. II. The North Galactic Cap Sample

Abstract

We present the North Galactic Cap sample of the Extremely Luminous Quasar Survey (ELQS-N), which targets quasars with M-1450 < -27 at 2.8 <= z < 5 in an area of similar to 7600 deg(2) of the Sloan Digital Sky Survey (SDSS) footprint with 90 degrees < R.A. < 270 degrees. 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 approximate to 3. As a result, we retain a completeness of > 70% over z similar to 3.0-5.0 at m(i) less than or similar to 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 similar to 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 beta approximate to -4 for this sample. This result contrasts with previous results in the same redshift range, which find a much flatter slope around beta similar to -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 beta < -2.94 with a 99% confidence.