Constraining the UV emissivity of AGN throughout cosmic time via X-ray surveys

TL;DR

This study uses deep X-ray surveys to accurately measure the UV emissivity of AGN across cosmic time, providing constraints that suggest AGN played a minor role in hydrogen reionization at high redshifts.

Contribution

It offers the first comprehensive X-ray based measurement of AGN UV emissivity up to z~6, reducing reliance on extrapolations and challenging the idea of AGN-dominated reionization.

Findings

AGN contribute only 1-7% to reionization at z=6.

X-ray and optical luminosity functions are consistent across redshifts.

AGN are unlikely to have driven the reionization process.

Abstract

The cosmological process of hydrogen (HI) reionization in the intergalactic medium is thought to be driven by UV photons emitted by star-forming galaxies and ionizing active galactic nuclei (AGN). The contribution of QSOs to HI reionization at $z>4$ has been traditionally believed to be quite modest. However, this view has been recently challenged by new estimates of a higher faint-end UV luminosity function (LF). To set firmer constraints on the emissivity of AGN at $z<6$, we here make use of complete X-ray selected samples including deep Chandra and new COSMOS data, capable to efficiently measure the 1 ryd comoving AGN emissivity up to $z\sim5-6$ and down to five magnitudes fainter than probed by current optical surveys, without any luminosity extrapolation. We find good agreement between the logN$\rm{_H}\lesssim21-22$ cm$^{-2}$ X-ray LF and the optically-selected QSO LF at all redshifts for $M_{1450}\leq -23$. The full range of the logN$\rm{_H}\lesssim21-22$ cm$^{-2}$ LF ($M_{1450} \leq -17$) was then used to quantify the contribution of AGN to the photon budget critical value needed to keep the Universe ionized. We find that the contribution of ionizing AGN at $z = 6$ is as small as 1\% - 7\%, and very unlikely to be greater than 30\%, thus excluding an AGN-dominated reionization scenario.