The Spectral Slope and Escape Fraction of Bright Quasars at $z \sim 3.8$: the Contribution to the Cosmic UV Background

TL;DR

This study analyzes the spectral slope and escape fraction of bright quasars at redshift around 3.8, estimating their contribution to the cosmic UV background and showing that quasars dominate the UV ionizing background up to z~3, with galaxies contributing more at higher redshifts.

Contribution

It provides new measurements of the spectral slope and escape fraction of quasars at z~3.8, and models their contribution to the cosmic UV background across redshifts.

Findings

Median spectral slope γ=1.30 with no redshift dependence.

Mean escape fraction f_esc=0.75, rising to 0.82 with a double power-law model.

Quasars dominate the UV background up to z~3, galaxies contribute more at higher redshifts.

Abstract

We use a sample of 1669 QSOs ($r<20.15$, $3.6<z<4.0$) from the BOSS survey to study the intrinsic shape of their continuum and the Lyman continuum photon escape fraction (f$_{esc}$), estimated as the ratio between the observed flux and the expected intrinsic flux (corrected for the intergalactic medium absorption) in the wavelength range 865-885 \AA\ rest-frame. Modelling the intrinsic QSO continuum shape with a power-law, $F_{\lambda}\propto\lambda^{-\gamma}$, we find a median $\gamma=1.30$ (with a dispersion of $0.38$, no dependence on the redshift and a mild intrinsic luminosity dependence) and a mean f$_{esc}=0.75$ (independent of the QSO luminosity and/or redshift). The f$_{esc}$ distribution shows a peak around zero and a long tail of higher values, with a resulting dispersion of $0.7$. If we assume for the QSO continuum a double power-law shape (also compatible with the data) with a break located at $\lambda_{\rm br}=1000$ \AA\ and a softening $\Delta\gamma=0.72 $ at wavelengths shorter than $\lambda_{\rm br}$, the mean f$_{esc}$ rises to $=0.82$. Combining our $\gamma$ and f$_{esc}$ estimates with the observed evolution of the AGN luminosity function (LF) we compute the AGN contribution to the UV ionizing background (UVB) as a function of redshift. AGN brighter than one tenth of the characteristic luminosity of the LF are able to produce most of it up $z\sim 3$, if the present sample is representative of their properties. At higher redshifts a contribution of the galaxy population is required. Assuming an escape fraction of Lyman continuum photons from galaxies between $5.5$ and $7.6\%$, independent of the galaxy luminosity and/or redshift, a remarkably good fit to the observational UVB data up to $z\sim 6$ is obtained. At lower redshift the extrapolation of our empirical estimate agrees well with recent UVB observations, dispelling the so-called Photon Underproduction Crisis.