Probing the faint end of the quasar luminosity function at z ~ 4 in the COSMOS field

TL;DR

This study investigates the faint end of the quasar luminosity function at redshift around 4 by identifying low-luminosity quasars in the COSMOS field, revealing a shallower faint-end slope than previous surveys.

Contribution

It provides new measurements of the faint end of the quasar luminosity function at z ~ 4, using deep optical data and spectroscopic follow-up, with detailed completeness analysis.

Findings

Faint-end slope beta = -1.67^{+0.11}_{-0.17} at z ~ 4.

Identification of 8 low-luminosity quasars at z ~ 4.

Results support downsizing evolution of active galactic nuclei.

Abstract

We searched for quasars that are ~ 3 mag fainter than the SDSS quasars in the redshift range 3.7 < z < 4.7 in the COSMOS field to constrain the faint end of the quasar luminosity function. Using optical photometric data, we selected 31 quasar candidates with 22 < i' < 24 at z ~ 4. We obtained optical spectra for most of these candidates using FOCAS on the Subaru telescope, and identified 8 low-luminosity quasars at z ~ 4. In order to derive the quasar luminosity function (QLF) based on our spectroscopic follow-up campaign, we estimated the photometric completeness of our quasar survey through detailed Monte Carlo simulations. Our QLF at z ~ 4 has a much shallower faint-end slope beta = -1.67^{+0.11}_{-0.17} than that obtained by other recent surveys in the same redshift. Our result is consistent with the scenario of downsizing evolution of active galactic nuclei inferred by recent optical and X-ray quasar surveys at lower redshifts.