A Cosmic UV/X-ray Background Model Update
  We present an updated model of the cosmic ionizing background from the UV to
the X-rays. Relative to our previous model (Faucher-Giguere et al. 2009), the
new model provides a better match to a large number of up-to-date empirical
constraints, including: 1) new galaxy and AGN luminosity functions; 2) stellar
spectra including binary stars; 3) obscured and unobscured AGN; 4) a
measurement of the non-ionizing UV background; 5) measurements of the
intergalactic HI and HeII photoionization rates at z~0-6; 6) the local X-ray
background; and 7) improved measurements of the intergalactic opacity. In this
model, AGN dominate the HI ionizing background at z<~3 and star-forming
galaxies dominate it at higher redshifts. Combined with the steeply declining
AGN luminosity function beyond z~2, the slow evolution of the HI ionization
rate inferred from the high-redshift HI Lya forest requires an escape fraction
from star-forming galaxies that increases with redshift (a population-averaged
escape fraction of ~1% suffices to ionize the intergalactic medium at z=3 when
including the contribution from AGN). We provide effective photoionization and
photoheating rates calibrated to match the Planck 2018 reionization optical
depth and recent constraints from the HeII Lya forest in hydrodynamic
simulations.
