Can galaxy evolution mimic cosmic reionization?

TL;DR

This paper investigates whether changes in galaxy properties, such as neutral hydrogen content and outflow velocity, can mimic the observed drop in Ly$ extalpha$ emission at high redshift, potentially confounding cosmic reionization measurements.

Contribution

It demonstrates that galactic evolution can produce effects similar to increased IGM neutrality, emphasizing the need to consider galaxy properties when interpreting Ly$ extalpha$ data.

Findings

Increased galactic neutral hydrogen can mimic reionization signals.

Spectral differences can distinguish galactic effects from IGM neutrality.

Current data favor minimal galactic evolution as the cause of Ly$ extalpha$ drop.

Abstract

Lyman-$\alpha$ (Ly$\alpha$) emitting galaxies are powerful tools to probe the late stages of cosmic reionization. The observed sudden drop in Ly$\alpha$ fraction at $z>6$ is often interpreted as a sign of reionization, since the intergalactic medium (IGM) is more neutral and opaque to Ly$\alpha$ photons. Crucially, this interpretation of the observations is only valid under the assumption that galaxies themselves experience a minimal evolution at these epochs. By modelling Ly$\alpha$ radiative transfer effects in and around galaxies, we examine whether a change in the galactic properties can reproduce the observed drop in the Ly$\alpha$ fraction. We find that an increase in the galactic neutral hydrogen content or a reduction in the outflow velocity toward higher redshift both lead to a lower Ly$\alpha$ escape fraction, and can thus mimic an increasing neutral fraction of the IGM. We furthermore find that this change in galactic properties leads to systematically different Ly$\alpha$ spectra which can be used to differentiate the two competing effects. Using the CANDELSz7 survey measurements which indicate slightly broader lines at $z\sim 6$, we find that the scenario of a mere increase in the galactic column density towards higher $z$ is highly unlikely. We also show that a decrease in outflow velocity is not ruled out by existing data but leads to more prominent blue peaks at $z>6$. Our results caution the use of Ly$\alpha$ observations to estimate the IGM neutral fraction without accounting for the potential change in the galactic properties, e.g., by mapping out the evolution of Ly$\alpha$ spectral characteristics.