AA54CH18-Stark ARI 25 August 2016 20:52 ANNUAL Galaxies in the First Billion REVIEWS Further Click here to view this article's online features: Years After the Big Bang • Download figures as PPT slides • Navigate linked references • Download citations • Explore related articles • Search keywords Daniel P. Stark Steward Observatory, University of Arizona, Tucson, Arizona 85721; email: [email protected] Annu. Rev. Astron. Astrophys. 2016. 54:761–803 Keywords The Annual Review of Astronomy and Astrophysics is cosmology, galaxy formation, galaxy evolution, reionization online at astro.annualreviews.org This article’s doi: Abstract 10.1146/annurev-astro-081915-023417 In the past five years, deep imaging campaigns conducted with the Hubble Copyright c 2016 by Annual Reviews. Space Telescope (HST) and ground-based observatories have delivered large All rights reserved 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 −2.0at z 7−8), revealing a dominant population of low-luminosity galaxies. Anal- ysis 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 for- mation 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 ab- sorption lines suggest a patchy distribution of neutral gas surrounds O and Access provided by California Institute of Technology on 01/11/17. For personal use only. B stars, possibly aiding in the escape of ionizing radiation. Gamma ray burst Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org afterglows and Lyman-α surveys have provided evidence that the intergalac- tic medium (IGM) evolves from mostly ionized at z 6−6.5(xHI < 0.2) to considerably neutral at z 7−8(xHI 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 6and reproduce the Thomson scattering optical depth faced by cosmic microwave background photons if the luminosity function extends 4 mag below cur- rent surveys and a moderate fraction ( fesc 0.2) of ionizing radiation escapes from galaxies. 761 AA54CH18-Stark ARI 25 August 2016 20:52 Contents 1. INTRODUCTION . 762 2.IDENTIFYINGEARLYSTAR-FORMINGGALAXIES........................ 764 2.1.LymanBreakSelection..................................................... 764 2.2. Lyman-α Emitters.......................................................... 766 2.3.DustyStar-FormingGalaxies............................................... 768 2.4.GammaRayBursts......................................................... 768 3.THECENSUSOFGALAXIESINTHEFIRSTBILLIONYEARS............. 769 3.1. The Ultraviolet Luminosity Function at 6 < z < 8........................... 769 3.2. The Census of z 9–10 UV-Selected Dropouts . 774 3.3. The Lyman-α LuminosityFunction......................................... 774 3.4. The Contribution of Dusty Star-Forming Galaxies . 776 4.EARLYSTAR-FORMINGGALAXYPROPERTIES............................ 777 4.1. UV Continuum Slopes . 777 4.2.StellarMassesandSpecificStarFormationRates............................. 780 4.3. Insight into Massive Stellar Populations, Neutral Gas Covering Fractions, and Lyman-α VelocityOffsetsfromRest-FrameUVSpectroscopy............. 782 4.4. Interstellar Medium Conditions and Dust Properties from the Rest-Frame Submillimeter . 784 4.5. Metallicity and Chemical Abundance Patterns . 786 4.6. Galaxy Sizes and Morphologies . 787 5. LYMAN-α EMITTERS AND GAMMA RAY BURSTS AS PROBES OFREIONIZATION.......................................................... 787 5.1. Redshift Evolution of Narrowband-Selected Lyman-α Emitting Galaxies over 5.7 < z < 8.8.................................................. 787 5.2. The Lyman-α Emitter Fraction in z > 6LymanBreakGalaxies............... 790 5.3.GammaRayBurstAfterglows............................................... 792 6. EARLY GALAXY GROWTH AND CONTRIBUTION TOREIONIZATION......................................................... 793 6.1. Cosmic Evolution of Star Formation Rate Density and Stellar Mass Density . 793 6.2. Contribution to Reionization . 794 1. INTRODUCTION Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org One of the longstanding goals of extragalactic astronomy is to construct a coherent picture of cosmic history, tracking the universe from its origins to the present day. The cosmic microwave background (CMB) provides our first snapshot, revealing the universe just after the epoch of re- combination roughly 400,000 years after the Big Bang. This period marks the beginning of the cosmic dark ages with neutral hydrogen filling most of the universe. Deep images from the Hubble Space Telescope (HST) provide our next picture nearly one billion years later, revealing a dramat- ically different landscape. Star-forming galaxies are abundant; some appear to have already built 10 up 10 M of stellar mass. The absorption lines in quasar spectra highlight another fundamental change: the hydrogen that filled the universe after recombination has become highly ionized in the intergalactic medium (IGM) by a redshift z 6, one billion years after the Big Bang. The current observational frontier in the study of cosmic history aims to piece together these two radically different views of the universe, addressing when and how galaxies first formed and 762 Stark AA54CH18-Stark ARI 25 August 2016 20:52 built up their stellar content, and determining whether the emergence of the first galaxies pro- vided the ionizing radiation necessary to reionize intergalactic hydrogen. The existing theoretical framework suggests that the process begins following the emergence of the first generation of chemically pristine stars formed in minihalos at z 20−30. The formation of these so-called Pop- ulation III (Pop III) stars terminates the cosmic dark ages and initiates the gradual enrichment of the universe with metals. The nature of the first stars remains a matter of debate. Theoretical work initially suggested a top-heavy mass function (see Bromm & Yoshida 2011 for a review), but more recent simulations have indicated that the Pop III stars could be much lower in mass (e.g., Stacy & Bromm 2014). Dwarf galaxies eventually form in low-mass halos containing gas that has been polluted by earlier generations of stars, and are thus above the metallicity threshold necessary for forming normal low-mass Population II stars. Over the next billion years, galaxies are expected to rapidly accrete cold gas, growing considerably in mass. The Lyman continuum (LyC) radiation released from these early galaxies ionizes their intergalactic surroundings. Over time, bubbles of ionized hydrogen grow and overlap around overdensities in the matter distribution, leading to the reionization of hydrogen. This phase transition marks the important point at which structure formation has impacted every baryon in the universe. In the past five years, the cosmic frontier has been pushed back to a redshift z 10, revolu- tionizing our view of early galaxies and allowing the first direct tests of the theoretical picture described above. Much of this progress has been achieved thanks to deep imaging campaigns con- ducted with the Wide Field Camera 3 (WFC3) onboard HST following its installation in 2009. The unprecedented near-IR sensitivity of WFC3 has enabled more than a thousand galaxies to be identified within the first billion years, providing a census of star formation activity in the redshift range 6 < z < 10, and allowing investigation of their contribution to reionization. A variety of multiwavelength constraints targeting UV-selected galaxies, quasar metal absorption lines, (sub)millimeter selected galaxies, and gamma ray bursts (GRBs) have provided new insight into the physical nature of these early star-forming systems. Meanwhile, Lyman-α (Lyα) surveys and GRB afterglow spectra are constraining the timescale of reionization, complementing new constraints on the electron scattering optical depth faced by CMB photons (Planck Collaboration 2015, 2016) and a growing number of quasar absorption line spectra at z > 6.5 (e.g., see Becker et al. 2015 for a review). The rapid progress promises to accelerate in the coming years following the completion of a suite of ambitious new facilities, including the James Webb Space Telescope ( JWST), the Atacama Large Millimeter Array (ALMA), the Wide Field Infrared Survey Telescope (WFIRST), and the next generation of ground-based optical/infrared Extremely Large Telescopes. With these facilities on the near horizon, it is a particularly valuable time to summarize the physical picture that has emerged following the past five years of observations. This manuscript builds on the Access provided by California Institute of Technology