Cosmic Reionization May Still Have Started Early and Ended Late: Confronting Early Onset with Cosmic Microwave Background Anisotropy and 21 cm Global Signals

Abstract

Abstract The global history of reionization was shaped by the relative amounts of starlight released by three halo mass groups: the first two groups are atomic-cooling halos (ACHs) with virial temperatures T vir > 104 K, either (1) massive enough to form stars even after reionization (high-mass ACHs, ≳ 109 M ⊙) or (2) less massive (low-mass ACHs), subject to star formation suppression when overtaken by reionization, and the third group comprises (3) H2-cooling mini-halos (MHs) with T vir < 104 K, whose star formation is predominantly suppressed by the H2-dissociating Lyman–Werner background. Our previous work showed that including MHs caused two-stage reionization—early rise to x ≲ 0.1, driven by MHs, followed by a rapid rise, late, to x ∼ 1, driven by ACHs—with a signature in cosmic microwave background (CMB) polarization anisotropy predicted to be detectable by the Planck satellite. Motivated by this prediction, we model global reionization semi-analytically for comparison with Planck CMB data and the Experiment to Detect the Global Epoch of Reionization (EDGES) global 21 cm absorption feature, for models with: (1) ACHs, no feedback; (2) ACHs, self-regulated; and (3) ACHs and MHs, self-regulated. Model (3) agrees well with Planck E-mode polarization data, even with a substantial tail of high-redshift ionization, beyond the limit proposed by the Planck Collaboration. No model reproduces the EDGES feature. For model (3), across the EDGES trough, an order of magnitude too shallow, and absorption starts at higher z but is spectrally featureless. Early onset reionization by Population III stars in MHs is compatible with current constraints, but only if the EDGES interpretation is discounted, or else other processes we did not include account for it.