A feature at z similar to 3.2 in the evolution of the Ly alpha forest optical depth

The effective optical depth in the Lyalpha forest region of 1061 low-resolution QSO spectra drawn from the SDSS database decreases with decreasing redshift over the range 2.5 less than or equal to z less than or equal to 4. Although the evolution is relatively smooth, tau(eff) proportional to (1 + z)(3.8+/-0.2), at z similar to 3.2 the effective optical depth decreases suddenly by about 10% with respect to this smoother evolution. It climbs back to the original smooth scaling again by z similar to 2.9. We describe two techniques, one of which is new, for quantifying this evolution that give consistent results. A variety of tests show that the feature is not likely to be a consequence of how the QSO sample was selected nor the result of flux calibration or other systematic effects. Other authors have argued that, at this same epoch, the temperature of the IGM also shows a departure from an otherwise smooth decrease with time. These features in the evolution of the temperature and the optical depth are signatures of the reionization of He II.