Dominant galaxy population driving cosmic reionisation

Determine whether the major contributors to cosmic reionisation were massive, bright galaxies or more numerous faint galaxies with ultraviolet magnitudes fainter than M_UV = -18.

Background

The paper frames the Epoch of Reionisation as a period whose timescale and physical drivers depend on the sources of hydrogen-ionising photons. Two competing possibilities are identified: massive, luminous galaxies, which produce substantial ionising output individually, and numerous faint galaxies, whose high number density could yield a larger integrated ultraviolet luminosity density.

The paper investigates ionising photon production efficiency and Lyman-continuum escape fractions in a z approximately 2 control sample, providing empirical context for studies of high-redshift galaxies. However, the relative contribution of bright and faint galaxy populations to reionisation remains unresolved in the stated passage.

References

For example, it is still unclear whether the relative contribution to reionization is from the brighter and more massive galaxies \citep[e.g.,][]{naidu_rapid_2020}, or from the more common faint and low-mass systems \citep[e.g.,][]{atek_extreme_2018,finkelstein_conditions_2019,atek_most_2024,simmonds_low-mass_2024}.

— Chasing Cosmic Reionization: An Extremely Faint Highly Magnified Source at $z=5.66$ with high $ξ_{\rm ion}$  (2609.09281 - Rojas-Ruiz et al., 8 Sep 2026) in Section 1, Introduction

It is yet unclear if the major contributors to reionisation were massive bright galaxies \citep{Sharma2018, Naidu2020} or more numerous faint ones (M$_{UV}>-18$) \citep{Ciardi2003,Bouwens2012, Robertson2010}.

— OutThere Survey: Addressing $\mathrm{ξ_{ion}}$ and $\mathrm{f_{esc}}$ with a population of average galaxies at z$\sim$2  (2608.19687 - Jaiswar et al., 20 Aug 2026) in Section 1, Introduction

As pointed out in , the degeneracy between the contributions of high-mass and fainter neighbouring haloes cannot be fully resolved using this geometric approach, meaning that source clustering likely plays an important role in boosting the apparent ionisation bias near [\ion{O}{III}] emitters.

— Exploring the influence of reionisation-era galaxies on their local intergalactic medium using the Sherwood-Relics simulations  (2609.26483 - Anderson et al., 22 Sep 2026) in Section 5, Conclusions