An analysis of the physics-rich endgame of reionization at $z=5.7$ is performed, utilizing jointly the observations of the Ly$alpha$ forest, the mean free path of ionizing photons, the luminosity function of galaxies and new physical insight. We find that an upper limit on ${rm tau_e}$ provides a constraint on the minimum mean free path (of ionizing photons) that is primarily due to dwarf galaxies, which in turn yields a new and yet the strongest constraint on the matter power spectrum on $10^6-10^9M_odot$ scales. With the latest Planck measurements of ${rm tau_e = 0.055 pm 0.009}$, we can place an upper limit of $(8.9times 10^6, 3.8times 10^7, 4.2times 10^8)M_odot$ on the lower cutoff mass of the halo mass function, or equivalent a lower limit on warm dark matter particle mass ${rm m_x ge (15.1, 9.8, 4.6)keV}$ or on sterile neutrino mass ${rm m_s ge (161, 90, 33)keV}$, at $(1, 1.4, 2.2)sigma$ confidence level, respectively.
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