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K. Wood, A. Loeb (Harvard-Smithsonian Center for Astrophysics)
We use a three-dimensional radiative transfer code to calculate the steady-state escape fraction of ionizing photons from disk galaxies as a function of redshift and galaxy mass. The gaseous disks are assumed to be isothermal (with a sound speed of 10 km/s) and radially exponential. Their scale-radius is related to the characteristic spin parameter and virial radius of their host halos, and their vertical structure is dictated by their self-gravity. The sources of radiation are taken to be either stars embedded in the disk, or a central quasar. The predicted increase in the disk density with redshift results in an overall decline of the escape fraction with redshift. For typical parameters we find that the escape fraction at z~10 is < 1% for stars but > 30% for mini-quasars. Unless the smooth gas content of high-redshift disks was depleted by 1 to 2 orders of magnitude due to supernove-driven outflows or fragmentation, the reionization of the universe was most likely dominated by mini-quasars rather than by stars.
This work was supported by NASA grants NAG 5-6039 (KW) and NAG 5-7039 and NAG 5-7768 (AL).
The author(s) of this abstract have provided an email address for comments about the abstract: kenny@claymore.harvard.edu