The Goldilocks problem of the quasar contribution to reionization

TL;DR

This paper investigates a quasar-dominated reionization scenario using hydrodynamical simulations and compares it with observational data, revealing both promising features and inconsistencies with current constraints.

Contribution

It provides a detailed simulation-based analysis of QSO-driven reionization, highlighting its potential to explain certain observations and identifying key areas for future observational constraints.

Findings

QSO-dominated reionization can produce an extended high optical depth tail.

The scenario aligns with some observational data but conflicts with IGM temperature and HeII optical depth constraints.

Additional HeII spectra at high redshift are crucial for constraining QSO contributions.

Abstract

The detection of an unexpectedly large number of faint candidate quasars (QSO) at $z \gtrsim 4$ has motivated Madau and Haardt to investigate a reionization scenario in which active galactic nuclei dominate the photon budget at all times. Their analytical study reveals that this picture is in agreement with the evolution of the HI volume fraction in the intergalactic medium (IGM) and the optical depth to Thomson scattering of the cosmic microwave background. We employ a suite of hydrodynamical simulations post-processed with a radiative transfer code to further investigate the properties of such a reionization history. In particular, we generate synthetic absorption-line spectra for HI and HeII that we compare with observational data. Although we confirm the analytical results mentioned above, we also find that the evolution of the IGM temperature and of the HeII optical depth are at odds with current observational constraints. Nevertheless, the QSO-dominated scenario presents the attractive feature that it naturally generates an inhomogeneous IGM and thus produces an extended tail at high values in the distribution function of the HI effective optical depth, in agreement with recent observations. We find tentative evidence that considering some QSO emission at redshift $z \gtrsim 5$ could reconcile such observations with numerical simulations that have so far failed to reproduce this feature. Finally, we show that collecting more HeII-absorption spectra at $z \gtrsim 3$ and studying their distinctive characteristics will be key to precisely constraining the QSO contribution to reionization.