Reionization constraints using principal component analysis

Using a semi-analytical model developed by Choudhury & Ferrara we study the observational constraints on reionization via a principal component analysis (PCA). Assuming that reionization at z > 6 is primarily driven by stellar sources, we decompose the unknown function Nion(z), representing the number of photons in the intergalactic medium per baryon in collapsed objects, into its principal components and constrain the latter using the photoionization rate, ΓPI, obtained from Lyα forest Gunn-Peterson optical depth, the 7 yr Wilkinson Microwave Anisotropy Probe (WMAP7) electron scattering optical depth τel and the redshift distribution of Lyman-limit systems dNLL/dz at z˜ 3.5. The main findings of our analysis are as follows. (i) It is sufficient to model Nion(z) over the redshift range 2 < z < 14 using five parameters to extract the maximum information contained within the data. (ii) All quantities related to reionization can be severely constrained for z < 6 because of a large number of data points whereas constraints at z > 6 are relatively loose. (iii) The weak constraints on Nion(z) at z > 6 do not allow to disentangle different feedback models with present data. There is a clear indication that Nion(z) must increase at z > 6, thus ruling out reionization by a single stellar population with non-evolving initial mass function, and/or star-forming efficiency, and/or photon escape fraction. The data allow for non-monotonic Nion(z) which may contain sharp features around z˜ 7. (iv) The PCA implies that reionization must be 99 per cent completed between 5.8 < z < 10.3 (95 per cent confidence level) and is expected to be 50 per cent complete at z≈ 9.5-12. With future data sets, like those obtained by Planck, the z > 6 constraints will be significantly improved.