TRUSTEES OF THE UNIVERSITY OF PENNSYLVANIA
In 2003, the PI showed that the effective optical depth in the Ly-alpha forest region of 1061 low-resolution QSO spectra drawn from the SDSS database decreases with decreasing redshift over the range 2.5 z 4. Although the evolution is relatively smooth, at z ~ 3.2 the effective optical depth decreases suddenly, by about ten percent with respect to this smoother evolution.
It climbs back to the original smooth scaling again by z ~ 2.9.
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, nor is it an artifact of the novel technique which was developed to make the measurement.
The strength and duration of the feature in the effective optical depth is consistent with numerical simulations of the effect of He II reionization. Moreover, the temperature of the IGM is known to show a departure from an otherwise smooth decrease with time at this same epoch; this is also consistent with the reionization of He II at z ~ 3.2.
Although the validity of this feature in the SDSS dataset was initially questioned, a similar feature has now been seen in two other very different datasets -- about 90 high-resolution, high S/N spectra obtained from the Keck and Magellan telescopes, and about 40 VLT spectra -- for which the analysis techniques and systematic effects are completely different, show almost exactly the same feature in the effective optical depth. Meantime, more recent models of the reionization signal are unable to reproduce the observed redshift dependence. This has motivated the present study.
The SDSS dataset is now 8 times larger than it was in 2003, making it 60 times larger than the Keck-Magellan-VLT high-resolution sample; it will remain the largest available dataset with which to study He II reionization for at least 10 years. This larger dataset will be used to provide a precision determination of the evolution of the effective optical depth. It is large enough to also study the patchiness of reionization, an outstanding question whose answer will constrain models of the sources of the ionizing photons.