A cosmic UV/X-ray background model update

Abstract

We present an updated model of the cosmic ionizing background from the UV to the X-rays. Relative to our previous model, 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 H I and He II 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 H I 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 H I ionization rate inferred from the high-redshift HI Ly α forest requires an escape fraction from star-forming galaxies that increases with redshift (a population-averaged escape fraction of ≈ 1 per cent 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 He II Ly α forest in hydrodynamic simulations.