Effective optical depth from the spectrum of CFHQS J1509-1749

Effective optical depth from the spectrum of CFHQS J1509-1749 (Lyman alpha: red symbols, Lyman beta: blue symbol). The small black symbols show data from SDSS quasars (Fan et al. 2006). The solid line is the extrapolation of optical depth evolution at lower redshift. For J1509-1749 and the SDSS quasars, there is a more rapid evolution at z > 5.4 than indicated in this extrapolation. The dotted and dot-dashed lines are the effective optical depth evolution for two theoretical models.