Quasar ionization front Lyα emission in an inhomogeneous intergalactic medium

The conditions within the ionization front of a quasar during reionization (T similar to 30 000 K, neutral hydrogen fraction x(HI) similar to 0.5) are ideal for producing Ly alpha emission via collisional excitation of hydrogen atoms. Observations of this emission, which could subtend greater than or similar to 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 Ly 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 Ly 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 greater than or similar to 3. Our calculations model the propagation of ionizing photons and compute the Ly 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.