Galaxies in the First Billion Years After the Big Bang

Abstract

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 15–20× over 4<z<8. Over this same redshift interval, the faint-end slope of the UV luminosity function becomes steeper (α≃−2.0 at z≃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-α surveys have provided evidence that the intergalactic medium (IGM) evolves from mostly ionized at z≃6−6.5 ([Formula: see text]) to considerably neutral at z≃7−8 ([Formula: see text]). 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≃6 and reproduce the Thomson scattering optical depth faced by cosmic microwave background photons if the luminosity function extends ≃4 mag below current surveys and a moderate fraction ([Formula: see text]) of ionizing radiation escapes from galaxies.