The Faint End of the Quasar Luminosity Function at z~4: Implications for Ionization of the Intergalactic Medium and Cosmic Downsizing

TL;DR

This study refines the faint end of the quasar luminosity function at z~4, revealing a flatter slope and implications for the ionization of the intergalactic medium, with improved measurement techniques and larger sample data.

Contribution

It provides an updated, more accurate measurement of the faint end of the z~4 quasar luminosity function using spectrum-based magnitudes and expanded sample data.

Findings

Faint-end slope of the QLF is flatter at z~4, similar to z~3.

QSOs contribute about half of the ionizing radiation at z~4.

K-corrections based on templates overestimate low-luminosity QSO brightness.

Abstract

We present an updated determination of the z ~ 4 QSO luminosity function (QLF), improving the quality of the determination of the faint end of the QLF presented in Glikman et al. (2010). We have observed an additional 43 candidates from our survey sample, yielding one additional QSO at z = 4.23 and increasing the completeness of our spectroscopic follow-up to 48% for candidates brighter than R = 24 over our survey area of 3.76 deg2. We study the effect of using K-corrections to compute the rest-frame absolute magnitude at 1450A compared with measuring M1450 directly from the object spectra. We find a luminosity-dependent bias: template-based K-corrections overestimate the luminosity of low-luminosity QSOs, likely due to their reliance on templates derived from higher luminosity QSOs. Combining our sample with bright quasars from the Sloan Digital Sky Survey and using spectrum-based M1450 for all the quasars, we fit a double-power-law to the binned QLF. Our best fit has a bright-end slope, {\alpha} = 3.3\pm0.2, and faint-end slope, {\beta} = 1.6(+0.8/-0.6). Our new data revise the faint-end slope of the QLF down to flatter values similar to those measured at z ~ 3. The break luminosity, though poorly constrained, is at M* = -24.1(+0.7/-1.9), approximately 1 - 1.5 mag fainter than at z ~ 3. This QLF implies that QSOs account for about half the radiation needed to ionize the IGM at these redshifts.