Constraints on ionising photon production from the large-scale Lyman-alpha forest

TL;DR

This paper forecasts how large-scale Lyman-alpha forest data can constrain the sources of ionising photons, distinguishing between galaxies and quasars, and assesses the impact on dark energy constraints.

Contribution

First forecasts for using Lyman-alpha forest data to differentiate ionising photon sources and estimate source properties with high precision.

Findings

Potential to determine dominant ionising sources with sub-10% accuracy

Constraints on dark energy remain robust when accounting for inhomogeneous ionisation

Forecasts for current and future surveys like DESI

Abstract

Recent work has shown that the z~2.5 Lyman-alpha forest on large scales encodes information about the galaxy and quasar populations that keep the intergalactic medium photoionized. We present the first forecasts for constraining the populations with data from current and next-generation surveys. At a minimum the forest should tell us whether galaxies or, conversely, quasars dominate the photon production. The number density and clustering strength of the ionising sources might be estimated to sub-10% precision with a DESI-like survey if degeneracies (e.g., with the photon mean-free-path, small-scale clustering power normalization and potentially other astrophysical effects) can be broken by prior information. We demonstrate that, when inhomogeneous ionisation is correctly handled, constraints on dark energy do not degrade.