The conditions within the ionization front of a quasar during reionization ( T \sim 30 , 000 K , neutral hydrogen fraction x _ { \mathrm { H I } } \sim 0.5 ) are ideal for producing Ly \alpha emission via collisional excitation of hydrogen atoms .
Observations of this emission , which could subtend \ga 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 \ga 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 .