AGNs as main contributors to the UV ionizing emissivity at high redshifts: predictions from a Lambda-CDM model with linked AGN/galaxy evolution

TL;DR

This study uses a semi-analytic Lambda-CDM model linking AGN and galaxy evolution to evaluate AGNs' role in cosmic reionization, finding they could be significant contributors especially at certain redshifts.

Contribution

It introduces a physically motivated model connecting black hole growth, AGN activity, and UV photon escape, providing new insights into AGNs' impact on the universe's ionization history.

Findings

Model matches observed AGN UV luminosity functions at low and intermediate redshifts.

Predicted hydrogen photoionization rates align with observational data.

At high redshifts, the model overestimates faint AGN luminosities, prompting further investigation.

Abstract

We have evaluated the contribution of the AGN population to the ionization history of the Universe based on a semi-analytic model of galaxy formation and evolution in the CDM cosmological scenario. The model connects the growth of black holes and of the ensuing AGN activity to galaxy interactions. In the model we have included a self consistent physical description of the escape of ionizing UV photons; this is based on the blast-wave model for the AGN feedback we developed in a previous paper to explain the distribution of hydrogen column densities in AGNs of various redshifts and luminosities, due to absorption by the host galaxy gas. The model predicts UV luminosity functions for AGNs which are in good agreement with those derived from the observations especially at low and intermediate redshifts (z=3). At higher redshifts (z>5) the model tends to overestimate the data at faint luminosities. Critical biases both in the data and in the model are discussed to explain such apparent discrepancies. The predicted hydrogen photoionization rate as a function of redshift is found to be consistent with that derived from the observations. All that suggests to reconsider the role of the AGNs as the main driver of the ionization history of the Universe.