Pure Density Evolution of the Ultraviolet Quasar Luminosity Function at $2\lesssim z \lesssim6$

TL;DR

This paper demonstrates that the ultraviolet quasar luminosity function at redshifts 2 to 6 can be modeled by pure density evolution, linking it to halo mass function evolution and highlighting the role of outshining in faint AGN detection.

Contribution

It introduces a model explaining the UV QLF evolution as pure density evolution based on halo mass functions and outshining effects, providing a unified framework for high-redshift quasar demographics.

Findings

UV QLF has a universal shape across redshifts 2-6.

The model reproduces the observed flat faint-end slope (~-1.1).

Outshining effects suggest a hidden population of faint AGNs.

Abstract

Quasar luminosity function (QLF) shows the active galactic nucleus (AGN) demography as a result of the combination of the growth and the evolution of black holes, galaxies, and dark matter halos along the cosmic time. The recent wide and deep surveys have improved the census of high-redshift quasars, making it possible to construct reliable ultraviolet (UV) QLFs at $2\lesssim z\lesssim6$ down to $M_{1450}=-23$ mag. By parameterizing these up-to-date observed UV QLFs that are the most extensive in both luminosity and survey area coverage at a given redshift, we show that the UV QLF has a universal shape, and their evolution can be approximated by a pure density evolution (PDE). In order to explain the observed QLF, we construct a model QLF employing the halo mass function, a number of empirical scaling relations, and the Eddington ratio distribution. We also include the outshining of AGN over its host galaxy, which made it possible to reproduce a moderately flat shape of the faint end of the observed QLF (slope of $\sim-1.1$). This model successfully explains the observed PDE behavior of UV QLF at $z>2$, meaning that the QLF evolution at high redshift can be understood under the framework of halo mass function evolution. The importance of the outshining effect in our model also implies that there could be a hidden population of faint AGNs ($M_{1450}\gtrsim-24$ mag), which are buried under their host galaxy light.