Quasar Ionization Front Ly{\alpha} Emission in an Inhomogeneous Intergalactic Medium

TL;DR

This study models Lyman-alpha emission from quasar ionization fronts in an inhomogeneous intergalactic medium, revealing it to be weaker and more challenging to observe than previously thought, with implications for understanding reionization.

Contribution

It provides the first detailed 1D radiative transfer calculations of Lyman-alpha emission in an inhomogeneous medium, improving upon previous homogeneous models.

Findings

Lyman-alpha emission is weaker than earlier estimates.

Inhomogeneities reduce emission by over a factor of 3.

Current instruments may not detect this faint emission.

Abstract

The conditions within the ionization front of a quasar during reionization (T ~ 30,000 K, neutral hydrogen fraction ~ 0.5) are ideal for producing Lyman-alpha emission via collisional excitation of hydrogen atoms. Observations of this emission, which could subtend >10 arcmin$^2$ on the sky, would definitively demonstrate the presence of a neutral intergalactic medium at the observed epoch, placing valuable constraints on the progress of reionization. We find that the expected Lyman-alpha surface brightness is significantly weaker than previously determined and may be impossible to observe with current and near-future instruments. Past work calculated the Lyman-alpha emission from a quasar ionization front in a homogeneous medium with a clumping factor approximation to account for inhomogeneities. We find using 1D radiative transfer calculations that this approximation overestimates the emission by a factor of >3. Our calculations model the propagation of ionizing photons and compute the Lyman-alpha emission from quasar ionization fronts on sightlines from a hydrodynamic cosmological simulation at z = 7.1. To better understand the physical properties of the emission, we also develop an analytic model that accurately describes the results of the full radiative transfer calculation.