The spectral slope and escape fraction of bright quasars at z ~ 3.8: the contribution to the cosmic UV background.

We use a sample of 1669 quasars (r < 20.15, 3.6 < z < 4.0) from the Baryon Oscillation Spectroscopic Survey to study the intrinsic shape of their continuum and the Lyman continuum photon escape fraction (f_esc,q), 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 Å rest frame. Modelling the intrinsic quasar (QSO) continuum shape with a power law, F_λ ∝ λ^-γ, we find a median γ = 1.30 (with a dispersion of 0.38, no dependence on the redshift and a mild intrinsic luminosity dependence) and a mean f_esc,q = 0.75 (independent of the QSO luminosity and/or redshift). The f_esc,q 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 λbr = 1000 Å and a softening Δγ = 0.72 at wavelengths shorter than λbr, the mean f_esc,q rises to 0.82. Combining our γ and f_esc,q estimates with the observed evolution of the active galactic nucleus (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 to z ~ 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 per cent, independent of the galaxy luminosity and/or redshift, a remarkably good fit to the observational UVB data up to z ~ 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. [ABSTRACT FROM AUTHOR]