# A Cosmic UV/X-ray Background Model Update

## TL;DR

This paper updates the cosmic UV/X-ray background model to better match recent empirical data, revealing the dominant sources of ionization at different redshifts and implications for galaxy escape fractions.

## Contribution

The new model incorporates recent observational constraints and improves upon previous models, providing a more accurate depiction of the cosmic ionizing background across redshifts.

## Key findings

- AGN dominate the HI ionizing background at z<~3.
- Star-forming galaxies dominate at higher redshifts.
- An escape fraction of ~1% from galaxies suffices for reionization at z=3.

## Abstract

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.