Cosmic Reionization and Early Star-Forming Galaxies: A Joint Analysis of
  New Constraints from Planck and Hubble Space Telescope
  We discuss new constraints on the epoch of cosmic reionization and test the
assumption that most of the ionizing photons responsible arose from high
redshift star-forming galaxies. Good progress has been made in charting the end
of reionization through spectroscopic studies of z~6-8 QSOs, gamma-ray bursts
and galaxies expected to host Lyman-alpha emission. However, the most stringent
constraints on its duration have come from the integrated optical depth, tau,
of Thomson scattering to the cosmic microwave background. Using the latest data
on the abundance and luminosity distribution of distant galaxies from Hubble
Space Telescope imaging, we simultaneously match the reduced value tau=0.066
+/- 0.012 recently reported by the Planck collaboration and the evolving
neutrality of the intergalactic medium with a reionization history within 6 <~
z <~ 10, thereby reducing the requirement for a significant population of very
high redshift (z>>10) galaxies. Our analysis strengthens the conclusion that
star-forming galaxies dominated the reionization process and has important
implications for upcoming 21cm experiments and searches for early galaxies with
James Webb Space Telescope.
