Joining Bits and Pieces of Reionization History.

Cosmic microwave background (CMB) temperature and polarization anisotropies from Planck have estimated a lower value of the optical depth to reionization (τ) compared to WMAP. A significant period in the reionization history would then fall within 6<redshift(z)<10, where detection of galaxies with Hubble frontier fields program and independent estimation of neutral hydrogen in the inter galactic medium by Lyman-α observations are also available. This overlap allows an analysis of cosmic reionization which utilizes a direct combination of CMB and these astrophysical measurements and potentially breaks degeneracies in parameters describing the physics of reionization. For the first time we reconstruct reionization histories by assuming photoionization and recombination rates to be free-form and by allowing underlying cosmological parameters to vary with CMB (temperature and polarization anisotropies and lensing) data from Planck 2018 release and a compilation of astrophysical data. We find an excellent agreement between the low-ℓ Planck 2018 High Frequency Instrument polarization likelihood and astrophysical data in determining the integrated optical depth. By combining both data, we report for a minimal reconstruction τ=0.051_{-0.0012-0.002}^{+0.001+0.002} at 68% and 95% C.L., which, for the errors in the current astrophysical measurements quoted in the literature, is nearly twice better than the projected cosmic variance limited CMB measurements. For the duration of reionization, redshift interval between 10%, and complete ionization, we get 2.9_{-0.16-0.26}^{+0.12+0.29} at 68% and 95% C.L., which improves significantly on the corresponding result obtained by using Planck 2015 data. By a Bayesian analysis of the combined results we do not find evidence beyond monotonic reionization histories, therefore a multiphase reionization scenario such as a first burst of reionization followed by recombination plateau and thereafter complete reionization is disfavored compared to minimal alternatives.