TitleAuthorsAbstractProbes of the reionization epochA first census of early galaxiesThe escape fraction of ionizing photonsCharting the end of reionizationFuture prospectsReferencesFigure 1 Cosmic reionization.Figure 2 Ionizing flux from high-redshift galaxies.Figure 3 Distant star-forming galaxies and reionization.Figure 4 Tracing the end of reionization with the Lya line.Box 1 The physics of reionizationBox 2 Observational probes of the reionization epochBox 3 The discovery and study of galaxies at redshifts z>7REVIEWdoi:10.1038/nature09527Early star-forming galaxies and thereionization of the UniverseBrant E. Robertson1, Richard S. Ellis1, James S. Dunlop2, Ross J. McLure2& Daniel P. Stark3Star-forming galaxies trace cosmic history. Recent observational progress with the NASA Hubble Space Telescope hasled to the discovery and study of the earliest known galaxies, which correspond to a period when the Universe was only800 million years old. Intense ultraviolet radiation from these early galaxies probably induced a major event in cosmichistory: the reionization of intergalactic hydrogen.The frontier in completing the physical story of cosmic history is tounderstand cosmic reionization—the transformation of neutralhydrogen, mostly located outside galaxies in the intergalacticmedium (IGM), into an ionized state. Neutral hydrogen first formed370,000 years after the Big Bang and released the radiation presentlyobserved as the cosmic microwave background (CMB)1. Initially devoidof sources of light, the Universe then entered a period termed the ‘DarkAges’2, which lasted until the first stars formed from overdense clouds ofhydrogen gas that cooled and collapsed within early cosmic structures.Observations of distant quasars3demonstrate that the IGM has beenhighly ionized since the Universe was ,1 billion years (Gyr) old, and thetransition from a neutral medium is popularly interpreted as beingcaused by ionizing photons with energies greater than 13.6 eV (wave-length, l , 91.2 nm), generated by primitive stars and galaxies4(Fig. 1).Astronomers wish to confirm the connection between early galaxiesand reionization because detailed studies of this period of cosmic historywill reveal the physical processes that originally shaped the galaxies ofvarious luminosities and masses we see around us today. Alternativesources of reionizing photons include material collapsing into earlyblack holes that power active galactic nuclei, and decaying elementaryparticles. Verifying that star-forming galaxies were responsible for cosmicreionization requires understanding how many energetic ultravioletphotons were produced by young stars at early times and what fractionof photons capable of ionizing hydrogen outside galaxies escaped withoutbeing intercepted by clouds of dust and hydrogen within galaxies.Astronomers desire accurate measurements of the abundance of earlygalaxies and the distribution of their luminosities to quantify the numberof sources producing energetic photons, as well as a determination of themixture of stars, gas and dust in galaxies, to determine the likelihood thatthe ultraviolet radiation can escape to ionize the IGM5,6. The Lyman-a(Lya) emission line, which is detectable using spectrographs on largeground-based telescopes, is a valuable additional diagnostic given that itis easily erased by neutral gas outside galaxies7–12. Its observed strength indistant galaxies is therefore a sensitive gauge of the latest time whenreionization was completed.In this primarily observational Review, we discuss substantial pro-gress that now points towards a fundamental connection between earlygalaxies and reionization. Recent observations with the Hubble SpaceTelescope (HST) have provided the first detailed constraints on theabundance and properties of galaxies in the first 1 Gyr of cosmic history.With some uncertainties, these data indicate that sufficient ultravioletradiation was produced to establish and maintain an ionized Universeby redshift z < 7, corresponding to ,800 million years (Myr) after theBig Bang. Further observations of these early systems using currentfacilities will produce a more robust census and clarify what fractionof the ionizing radiation escaped primitive galaxies. The rapid progressnow being made will pave the way for ambitious observations of theearliest known galaxies using future facilities.1Department of Astronomy, California Institute of Technology, MC 249-17, Pasadena, California 91125, USA.2Institute for Astronomy, University of Edinburgh, Edinburgh EH9 3HJ, UK.3Institute ofAstronomy, University of Cambridge, Cambridge CB3 0HA, UK.Inflation/Big BangRecombination/CBR emittedPresent day, z ≈ 0t = 13.75 Gyr Neutral IGMFirst stars(z ≈ 15–30?)Rare sources formionized bubblesIonized bubblesoverlapModern galaxies formDense, neutral pocketsCosmic Dark Agesz > 15–30?t < 100–270 MyrReionizationz ≈ 6–15?t < 1 GyrIGM mostly ionizedz = 0–6, t > 1 GyrFirst galaxies(z ≈ 10–30?)z ≈ 1,100 t ≈ 370,000 yearsFigure 1|Cosmic reionization. The transition from the neutral IGM left afterthe Universe recombined, at z < 1,100, to the fully ionized IGM observed todayis termed cosmic reionization. After recombination, when the CMB radiationwas released, hydrogen in the IGM remained neutral until the first stars andgalaxies2,4formed, at z < 15–30. These primordial systems released energeticultraviolet photons capable of ionizing local bubbles of hydrogen gas. As theabundance of these early galaxies increased, the bubbles increasinglyoverlapped and progressively larger volumes became ionized. This reionizationprocess ended at z < 6–8, ,1 Gyr after the Big Bang. At lower redshifts, theIGM remains highly ionized by radiation provided by star-forming galaxiesandthe gas accretion onto supermassive black holes that powers quasars.4 NOVEMBER 2010 | VOL 468 | NATURE | 49Macmillan Publishers Limited. All rights reserved©2010Probes of the reionization epochCosmological reionization involves a complex interplay between thestrength, distribution and spectrum of photoionizing sources and thedensity and spatial structure of intergalactic gas. Theoretical calculationsusing both analytical methods13–17and sophisticated numerical simula-tions18–28to model these complexities have reached broad agreement onhow reionization is expected to proceed (Fig. 1). Sources of ionizingphotons, such as star-forming galaxies, are associated with dark matterhalos forged through