Quasar ionization front Lyα emission in an inhomogeneous intergalactic medium
The conditions within the ionization front of a quasar during reionization (T � 30,000 K, neutral hydrogen fraction xH I � 0.5) are ideal for producing Lyα emission via collisional ex- citation of hydrogen atoms. Observations of this emission, which could subtend > 10 arcmin 2 on the sky, would definitively demonstrate the presence of a n eutral intergalactic medium at the observed epoch, placing valuable constraints on the progress of reionization. We find that the expected Lyα surface brightness is significantly weaker than previously determined and may be impossible to observe with current and near-future instruments. Past work calculated the Lyα emission from a quasar ionization front in a homogeneous medium with a clump- ing 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 calcu- lations model the propagation of ionizing photons and compute the Lyα 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.