Smooth boundaries to cosmological HII regions from galaxy clustering

The H II regions around quasars and galaxies at redshifts beyond the epoch of reionization will provide prime targets for upcoming 21-cm campaigns using a new generation of low-frequency radio observatories. Here we show that the boundaries of these H II regions will not be sharp. Rather, the clustering of sources near massive galaxies results in a neutral fraction that rises gradually towards large radii from an interior value near zero. A neutral fraction corresponding to the global background value is typically reached at a distance of 2-5 times the radius of the H II region around the central massive galaxy.