The Str\"omgren sphere, the environment and the reionization in the   local universe of the highest redshift QSOs
  In this paper we investigate the environment and reionization process around the highest redshift QSOs having Gunn-Peterson troughs (z>6.1). Starting with the cosmic density perturbation and structure formation theory and the fact that the highest redshift QSOs are located in rare overdense regions, we show that the halo formation, gas distribution, and star formation around QSOs are biased from those of the cosmic average. We argue that a significant fraction of hydrogen in the Str\"omgren sphere around QSOs is ionized by photons from stars and that only about several percent to at most 10%--20% of the total hydrogen is left (e.g., in minihalos, halos, or high-density subregions) to be ionized by QSO photons. The cosmic average neutral hydrogen fraction at z~6.2-6.4 should also be smaller than the upper limit of 10%--20% and may be only a few percent. We analyze the clumping property of the hydrogen ionized by QSOs and study the evolution of the Str\"omgren sphere. We find that the expected Str\"omgren radii from our models are consistent with observations if the lifetime of the highest redshift QSOs is about or longer than a few times 10^7 yr (as is the lifetime of the main population of QSOs; with comoving number density peaked at z~2-3). With such a QSO lifetime, the ages of most of the observed QSOs are long enough that the QSO photon emission is balanced by the recombination of the hydrogen ionized by QSO photons in their Str\"omgren spheres, and the expected Str\"omgren radii from the balance are independent of the detailed values of the QSO ages. (abridged) 
Qingjuan Yu, Youjun Lu (UC Berkeley)