1 research outputs found
The Spectral Slope and Escape Fraction of Bright Quasars at : the Contribution to the Cosmic UV Background
We use a sample of 1669 QSOs (, ) from the BOSS survey to
study the intrinsic shape of their continuum and the Lyman continuum photon
escape fraction (f), 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,
, we find a median (with a
dispersion of , no dependence on the redshift and a mild intrinsic
luminosity dependence) and a mean f (independent of the QSO
luminosity and/or redshift). The f distribution shows a peak around
zero and a long tail of higher values, with a resulting dispersion of . If
we assume for the QSO continuum a double power-law shape (also compatible with
the data) with a break located at \AA\ and a softening
at wavelengths shorter than , the mean
f rises to . Combining our and f 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 , 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 and , independent of the galaxy luminosity and/or
redshift, a remarkably good fit to the observational UVB data up to
is obtained. At lower redshift the extrapolation of our empirical estimate
agrees well with recent UVB observations, dispelling the so-called Photon
Underproduction Crisis.Comment: 8 pages, 9 figures, MNRAS accepte