Galaxies in the First Billion Years After the Big Bang

In the past five years, deep imaging campaigns conducted with the Hubble Space Telescope (HST) and ground-based observatories have delivered large samples of galaxies at 6.5 < z < 10, providing our first glimpse of the census of star formation activity in what is thought to be the heart of the reionization era. The space density of luminous galaxies has been shown to decrease by 15-20x over 4 < z < 8. Over this same redshift interval, the faint-end slope of the UV luminosity function becomes steeper (alpha similar or equal to -2.0 at z similar or equal to 7-8), revealing a dominant population of low-luminosity galaxies. Analysis of multiwavelength imaging from HST and the Spitzer Space Telescope demonstrates that z > 6 UV-selected galaxies are relatively compact with blue UV continuum slopes, low stellar masses, and large specific star formation rates. In the last year, ALMA (the Atacama Large Millimeter Array) and ground-based infrared spectrographs have begun to complement this picture, revealing minimal dust obscuration and hard radiation fields, and providing evidence for metal-poor ionized gas. Weak low-ionization absorption lines suggest a patchy distribution of neutral gas surrounds O and B stars, possibly aiding in the escape of ionizing radiation. Gamma ray burst afterglows and Lyman-alpha surveys have provided evidence that the intergalactic medium (IGM) evolves from mostly ionized at z similar or equal to 6-6.5 (x(HI) < 0.2) to considerably neutral at z similar or equal to 7-8 (x(HI) similar or equal to 0.3-0.8). The reionization history that emerges from considering the UV output of galaxies over 6 < z < 10 is consistent with these constraints on the IGM ionization state. The latest measurements suggest that galaxies can complete reionization by z similar or equal to 6 and reproduce the Thomson scattering optical depth faced by cosmic microwave background photons if the luminosity function extends similar or equal to 4 mag below current surveys and a moderate fraction (f(esc) similar or equal to 0.2) of ionizing radiation escapes from galaxies.