The Extremely Luminous Quasar Survey (ELQS) in the Sloan Digital Sky Survey footprint. II. The North Galactic Cap Sample

TL;DR

This paper presents the North Galactic Cap sample of the ELQS, a survey targeting extremely luminous quasars at redshifts 2.8 to 5, using innovative infrared/optical classification methods to improve completeness and extend the quasar luminosity function.

Contribution

The study introduces a new infrared/optical quasar selection method using random forest classification, achieving high completeness and efficiency, and extends the bright-end quasar luminosity function at high redshift.

Findings

Discovered 39 new quasars in the North Galactic Cap.

Extended the quasar luminosity function by one magnitude at z=2.8-4.5.

Estimated a steep bright-end slope of approximately -4 for the quasar luminosity function.

Abstract

We present the North Galactic Cap sample of the Extremely Luminous Quasar Survey (ELQS-N), which targets quasars with $M_{1450}<-27$ at $2.8 \leq z < 5$ in an area of $\sim7600\,\rm{deg}^2$ of the Sloan Digital Sky Survey (SDSS) footprint with $90\text{\textdegree}<\rm{RA}<270\text{\textdegree}$. Based on a near-infrared/infrared \textit{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 $z\approx3$. As a result, we retain a completeness of $>70\%$ over $z\sim3.0-5.0$ at $m_{i}\lesssim17.5$, limited toward fainter magnitudes by the depth of the Two Micron All Sky Survey (2MASS). 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 $6.5\,\rm{m}$ 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 $\sim79\%$. 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 $2.8 \leq z \leq 4.5$. A first analysis of the QLF suggests a relatively steep bright-end slope of $\beta\approx-4$ for this sample. This result contrasts with previous results in the same redshift range, which find a much flatter slope around $\beta\sim-2.5$, 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.