Redshift evolution of the electron scattering optical depth (left panel) and the volume-filling fraction of ionizied hydrogen (right panel). We show the volume-filling fraction weighted values from star-forming galaxies and AGN as well as the contribution deconstructed into galaxies with stellar masses above and below 10⁹ M_⊙, as marked. In the left panel, the dot-dashed horizontal line and the shaded region show the electron-scattering optical depth and its associated error bars from Planck Collaboration VI (2020). In the right panel, points show compilations of Q_II results from a number of works (Fan et al. 2006; Davies et al. 2018; Yang et al. 2020a; Jung et al. 2020; Lu et al. 2020; Bosman et al. 2022; Gaikwad et al. 2023; Nakane et al. 2024).