Constraining the reionization history with QSO absorption spectra

We use a semi-analytical approach to simulate absorption spectra of QSOs at high redshifts with the aim of constraining the cosmic reionization history. We consider two physically motivated and detailed reionization histories: (i) an early reionization model (ERM) in which the intergalactic medium is reionized by Pop III stars at z approximate to 14, and (ii) a more standard late reionization model (LRM) in which overlapping, induced by QSOs and normal galaxies, occurs at z approximate to 6. From the analysis of current Ly alpha forest data at z < 6, we conclude that it is impossible to disentangle the two scenarios, which fit equally well the observed Gunn-Peterson optical depth, flux probability distribution function and dark gap width distribution. At z > 6, however, clear differences start to emerge which are best quantified by the dark gap and peak width distributions. We find that 35 (0) per cent of the lines of sight (LOS) within 5.7 < z < 6.3 show dark gaps of widths > 50 angstrom in the rest frame of the QSO if reionization is not (is) complete at z greater than or similar to 6. Similarly, the ERM predicts peaks of width similar to 1 angstrom in 40 per cent of the LOS in the redshift range 6.0-6.6; in the same range, LRM predicts no peaks of width > 0.8 angstrom. We conclude that the dark gap and peak width statistics represent superb probes of cosmic reionization if about ten QSOs can be found at z > 6. We finally discuss strengths and limitations of our method.