Quasar Proximity Zones and Patchy Reionization

Lyman-alpha forest absorption spectra towards quasars at z ~ 6 show regionsof enhanced transmission close to their source. Several authors have arguedthat the apparently small sizes of these regions indicate that quasarionization fronts at z >~ 6 expand into a largely or partly neutralintergalactic medium (IGM). Assuming that the typical region in the IGM isreionized by z <= 6, as is suggested by Ly-a forest observations, we argue thatat {\em least} 50% of the volume of the IGM was reionized before the highestredshift quasars turned on. Further, even if the IGM is as much as 50% neutralat quasar turn-on, the quasars are likely born into large galaxy-generated HIIregions. The HII regions during reionization are themselves clustered, andusing radiative transfer simulations, we find that long skewers through the IGMtowards quasar progenitor halos pass entirely through ionized bubbles, evenwhen the IGM is half neutral. These effects have been neglected in mostprevious analyses of quasar proximity zones, which assumed a spatially {\emuniform} neutral fraction. We model the subsequent ionization from a quasar,and construct mock Ly-a forest spectra. Our mock absorption spectra are moresensitive to the level of small-scale structure in the IGM than to thevolume-averaged neutral fraction, and suggest that existing proximity-zone sizemeasurements are compatible with a fully ionized IGM. However, we mentionseveral improvements in our modeling that are necessary to make more definitiveconclusions.Comment: 23 pages, 13 figures, submitted to Ap