Are There Enough Ionizing Photons to Reionize the Universe by z=6?

TL;DR

This paper estimates the ionizing photon budget from galaxies to assess if they can reionize the universe by redshift 6, considering different cosmological models and escape fractions.

Contribution

It provides a detailed analysis of the ionizing photon production and escape fraction needed for reionization, based on galaxy luminosity functions and cosmological parameters.

Findings

Low escape fraction in low-mass galaxies challenges reionization by z=6 in WMAP 3rd year cosmology.

Reionization is marginally possible under WMAP 1st year cosmology.

Constant escape fraction across galaxy masses is required for reionization in the favored cosmology.

Abstract

An estimate for the number of ionizing photons per baryon as a function of redshift is computed based on the plausible extrapolation of the observed galaxy UV luminosity function and the latest results on the properties of the escape fraction of ionizing radiation. It is found that, if the escape fraction for low mass galaxies (Mtot<10^{11}Msun) is assumed to be negligibly small, as indicated by numerical simulations, then there are not enough ionizing photons to reionize the universe by z=6 for the cosmology favored by the WMAP 3rd year results, while the WMAP 1st year cosmology is marginally consistent with the reionization requirement. The escape fraction as a function of galaxy mass would have to be constant to within a factor of two for the whole mass range of galaxies for reionization to be possible within the WMAP 3rd year cosmology.