# The Extremely Luminous Quasar Survey in the Sloan Digital Sky Survey footprint. III. The South Galactic Cap Sample and the Quasar Luminosity Function at Cosmic Noon

## TL;DR

This paper presents the discovery of 70 new extremely luminous quasars, doubling the known population in a large sky area, and measures the bright-end quasar luminosity function at cosmic noon, revealing a steep slope and redshift evolution.

## Contribution

It introduces a novel selection strategy for quasars, increasing completeness and discovering 109 new quasars, and extends the bright-end quasar luminosity function to brighter magnitudes.

## Key findings

- Discovered 70 new quasars in the South Galactic Cap sample.
- Measured the bright-end quasar luminosity function with a steep slope of approximately -4.1.
- Found a moderate decrease in quasar density with redshift, modeled by exponential evolution.

## Abstract

We have designed the Extremely Luminous Quasar Survey (ELQS) to provide a highly complete census of unobscured UV-bright quasars during the cosmic noon, $z=2.8-5.0$. Here we report the discovery of 70 new quasars in the ELQS South Galactic Cap (ELQS-S) quasar sample, doubling the number of known extremely luminous quasars in $4,237.3\,\rm{deg}^2$ of the SDSS footprint. These observations conclude the ELQS and we present the properties of the full ELQS quasar catalog, containing 407 quasars over $11,838.5\,\rm{deg}^2$. Our novel ELQS quasar selection strategy resulted in unprecedented completeness at the bright end and allowed us to discover 109 new quasars in total. This marks an increase of $\sim36\%$ (109/298) to the known population at these redshifts and magnitudes, while we further are able to retain a selection efficiency of $\sim80\%$. On the basis of 166 quasars from the full ELQS quasar catalog, who adhere to the uniform criteria of the 2MASS point source catalog, we measure the bright-end quasar luminosity function (QLF) and extend it one magnitude brighter than previous studies. Assuming a single power law with exponential density evolution for the functional form of the QLF, we retrieve the best fit parameters from a maximum likelihood analysis. We find a steep bright-end slope of $\beta\approx-4.1$ and we can constrain the bright-end slope to $\beta\leq-3.4$ with $99\%$ confidence. The density is well modeled by the exponential redshift evolution, resulting in a moderate decrease with redshift ($\gamma\approx-0.4$).