Galaxy Evolution during the Epoch of Reionization
Steve Finkelstein, University of Texas at Austin
Abstract: The advent of the Wide Field Camera 3 on the Hubble Space Telescope has heralded a new era in our ability to study the earliest phases of galaxy formation and evolution. The number of candidates for galaxies now known at redshifts greater than six has grown to be in the thousands. This allows us to move beyond mere counting of galaxies, to endeavor to understand the detailed physics regulating the growth of galaxies. I will review the recent progress our group in Texas has made in this arena using the exquisite datasets from the CANDELS and Frontier Fields programs. Specifically, our detailed new measurements of both the evolution of the stellar mass function and rest-frame UV luminosity function now allow us to probe the effect of feedback on low-mass galaxies, the star-formation efficiency in high-mass galaxies, and the contribution of galaxies to the reionization of the universe. Our most recent result comes from the Frontier Fields, where we have used an advanced technique to remove the light from the cluster galaxies to uncover z > 6 galaxies as faint as M_UV=-13. Our updated luminosity functions show no sign of a turnover down to these extremely faint levels, providing the first empirical test of reionization models which require such faint galaxies, and is in modest tension with simulations which predict a turnover at brighter levels. I will also discuss our spectroscopic followup efforts, which have yielded two of the four highest redshift confirmed galaxies, and also provide further insight into reionization, by the scattering of Lyman alpha emission by neutral gas in the intergalactic medium. I will conclude with a look ahead to the problems we can expect to tackle with ALMA, JWST, and even more future facilities.
Understanding How Galaxies Reionized the Universe
Sanchayeeta Borthakur, Arizona State University
Identifying the population of galaxies that was responsible for the reionization of the universe is a long-standing quest in astronomy. While young stars can produce large amounts of ionizing photons, the mechanism behind the escape of Lyman continuum photons (wavelength < 912 A) from star-forming regions has eluded us. To identify such galaxies and to understand the process of the escape of Lyman continuum, we present an indirect technique known as the residual flux technique. Using this technique, we identified (and later confirmed) the first low-redshift galaxy that has an escape fraction of ionizing flux of 21%. This leaky galaxy provides us with valuable insights into the physics of starburst-driven feedback. In addition, since direct detection of ionizing flux is impossible at the epoch of reionization, the residual flux technique presents a highly valuable tool for future studies to be conducted with the upcoming large telescopes such as the JWST.