2017ApJ...836..217C

An analysis of the physics-rich endgame of reionization at z = 5.7 is performed, jointly utilizing the observations of the Lyα forest, the mean free path (mfp) of ionizing photons, the luminosity function of galaxies, and new physical insight. We find that an upper limit on τ_e provides a constraint on the minimum mfp (of ionizing photons) that is primarily due to dwarf galaxies, which in turn yields a new, yet strongest constraint on the matter power spectrum on 10⁶-10⁹ M_☉ scales. With the latest Planck measurements of τ_e = 0.055 ± 0.009, we can place an upper limit of (8.9 x 10⁶, 3.8 x 10⁷, 4.2 x 10⁸) M_☉ on the lower cutoff mass of the halo-mass function, or equivalently, a lower limit on warm dark matter particle mass m_x >= (15.1, 9.8, 4.6) keV or on sterile neutrino mass m_s >= (161, 90, 33) keV at the (1, 1.4, 2.2)σ confidence level, respectively.