COSMIC REIONIZATION AND EARLY STAR-FORMING GALAXIES: A JOINT ANALYSIS OF NEW CONSTRAINTS FROM PLANCK AND THE 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 similar or equal to 6-8 QSOs, gamma-ray bursts, and galaxies expected to host Ly 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 less than or similar to z less than or similar to 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 21 cm experiments and searches for early galaxies with the James Webb Space Telescope.