Radiative transfer in a clumpy universe: IV. New synthesis models of the
  cosmic UV/X-ray background
  We present improved synthesis models of the evolving spectrum of the UV/X-ray
diffuse background, updating and extending our previous results. Five new main
components are added to our radiative transfer code CUBA: (1) the sawtooth
modulation of the background intensity from resonant line absorption in the
Lyman series of cosmic hydrogen and helium; (2) the X-ray emission from
obscured and unobscured quasars; (3) a piecewise parameterization of the
distribution in redshift and column density of intergalactic absorbers that
fits recent measurements of the mean free path of 1 ryd photons; (4) an
accurate treatment of the photoionization structure of absorbers; and (5) the
UV emission from star-forming galaxies at all redshifts. We provide tables of
the predicted HI and HeII photoionization and photoheating rates for use, e.g.,
in cosmological hydrodynamics simulations of the Lya forest, and a new
metallicity-dependent calibration to the UV luminosity density-star formation
rate density relation. A "minimal cosmic reionization model" is also presented
in which the galaxy UV emissivity traces recent determinations of the cosmic
history of star formation, the luminosity-weighted escape fraction of
hydrogen-ionizing radiation increases rapidly with lookback time, the clumping
factor of the high-redshift intergalactic medium evolves following the results
of recent hydrodynamic simulations, and Population III stars and miniquasars
make a negligible contribution to the metagalactic flux. The model provides a
good fit to the hydrogen-ionization rates inferred from flux decrement and
proximity effect measurements, predicts that cosmological HII (HeIII) regions
overlap at redshift 6.7 (2.8), and yields an optical depth to Thomson
scattering that is in agreement with WMAP results. (Abridged)
