Observational Constraints of Reionization History in the JWST Era

The epoch of reionization is a crucial phase of cosmic evolution, when the UV photons from the first generation of galaxies and quasars ionized the neutral hydrogen in the IGM, ending the cosmic dark ages. In this Chapter, I will first review the techniques and current results on constraining reionization history. Measurement of CMB polarization indicates the peak of reionization activity at z similar to 10. Observations of IGM transmission in high-redshift quasar spectra show that the Universe was ionized by z similar to 6, while rapid increase of Gunn-Peterson optical depth towards high redshift suggests that reionization is ending at that epoch. Reionization is likely a prolonged and complex process, requiring detailed observations of high-redshift Sources to completely unveal its history. JWST and concurrent ground-based facilities will allow dramatic progress in high-redshift observations. New high-redshift quasar surveys will provide JWST ideal luminous sources to map IGM evolution at z > 7; galaxy surveys using Lyman Break galaxies and Ly alpha emitters as tracers will not only detect the dwarf galaxies that are likely the primary sources of reionization, but also allow detailed measurements of IGM topology during reionization era. Aided by further development in theoretical modeling and better understanding of high-redshift galaxy population, JWST will play a central goal in understanding the history of reionization.