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THE JAMES WEBB SPACE TELESCOPE
JONATHAN P. GARDNER1, , JOHN C. MATHER1 , MARK CLAMPIN2 , RENE DOYON3 , MATTHEW A. GREENHOUSE1 , HEIDI B. HAMMEL4 , JOHN B. HUTCHINGS5 , PETER JAKOBSEN6 , SIMON J. LILLY7 , KNOX S. LONG8 , JONATHAN I. LUNINE9 , MARK J. MCCAUGHREAN10,11 , MATT MOUNTAIN8 , JOHN NELLA12 , GEORGE H. RIEKE13 , MARCIA J. RIEKE13 , HANS-WALTER RIX14 , ERIC P. SMITH15 , GEORGE SONNEBORN1 , MASSIMO STIAVELLI8 , H. S. STOCKMAN8 , ROGIER A. WINDHORST16 and GILLIAN S. WRIGHT17
1

Laboratory for Observational Cosmology, Code 665, Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. 2 Laboratory for Exoplanet and Stellar Astrophysics, Code 667, Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. 3 Departement de Physique, Universite de Montreal, C.P. 6128 Succ. Centre-ville, Montreal, ´ Quebec, Canada H3C 3J7 4 Space Science Institute, 4750 Walnut Avenue, Suite 205, Boulder CO 80301, U.S.A. 5 Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, British Columbia, Canada V9E 2E7 6 Astrophysics Division, RSSD, European Space Agency, ESTEC, 2200 AG Noordwijk, The Netherlands 7 Department of Physics, Swiss Federal Institute of Technology (ETH-Zurich), ETH Honggerberg, ¨ CH-8093 Zurich, Switzerland 8 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, U.S.A. 9 Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721, U.S.A. 10 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany 11 School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, U.K. 12 Northrop Grumman Space Technology, 1 Space Park, Redondo Beach, CA 90278, U.S.A. 13 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, U.S.A. 14 Max-Planck-Institut fur Astronomie, Konigstuhl 17, Heidelberg D-69117, Germany ¨ ¨ 15 NASA Headquarters, 300 E Street Southwest, Washington, DC 20546, U.S.A. 16 Department of Physics and Astronomy, Arizona State University, Box 871504, Tempe, AZ 85287, U.S.A. 17 Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, U.K. ( Author for correspondence, E-mail: jonathan.p.gardner@nasa.gov) (Received: 8 March 2006; Accepted in final form: 15 May 2006)

Abstract. The James Webb Space Telescope (JWST) is a large (6.6 m), cold (<50 K), infrared (IR)optimized space observatory that will be launched early in the next decade into orbit around the second Earth­Sun Lagrange point. The observatory will have four instruments: a near-IR camera, a near-IR multiobject spectrograph, and a tunable filter imager will cover the wavelength range, 0.6 < < 5.0 m, while the mid-IR instrument will do both imaging and spectroscopy from 5.0 < < 29 m. The JWST science goals are divided into four themes. The key objective of The End of the Dark Ages: First Light and Reionization theme is to identify the first luminous sources to form and to determine the ionization history of the early universe. The key objective of The Assembly of Galaxies theme is to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, Space Science Reviews (2006) 123: 485­606 DOI: 10.1007/s11214-006-8315-7 Springer 2006

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and active nuclei within them evolved from the epoch of reionization to the present day. The key objective of The Birth of Stars and Protoplanetary Systems theme is to unravel the birth and early evolution of stars, from infall on to dust-enshrouded protostars to the genesis of planetary systems. The key objective of the Planetary Systems and the Origins of Life theme is to determine the physical and chemical properties of planetary systems including our own, and investigate the potential for the origins of life in those systems. Within these themes and objectives, we have derived representative astronomical observations. To enable these observations, JWST consists of a telescope, an instrument package, a spacecraft, and a sunshield. The telescope consists of 18 beryllium segments, some of which are deployed. The segments will be brought into optical alignment on-orbit through a process of periodic wavefront sensing and control. The instrument package contains the four science instruments and a fine guidance sensor. The spacecraft provides pointing, orbit maintenance, and communications. The sunshield provides passive thermal control. The JWST operations plan is based on that used for previous space observatories, and the majority of JWST observing time will be allocated to the international astronomical community through annual peer-reviewed proposal opportunities. Keywords: galaxies: formation, infrared: general, planetary systems, space vehicles: instruments, stars: formation

1. Introduction The James Webb Space Telescope (JWST; Table I) will be a large, cold, infrared(IR)optimized space telescope designed to enable fundamental breakthroughs in our understanding of the formation and evolution of galaxies, stars, and planetary systems. It is a project led by the United States National Aeronautics and Space Administration (NASA), with major contributions from the European and Canadian Space Agencies (ESA and CSA). It will have an approximately 6.6 m diameter aperture, will be passively cooled to below 50 K, and will carry four scientific instruments: a Near-IR Camera (NIRCam), a Near-IR Spectrograph (NIRSpec), a near-IR Tunable Filter Imager (TFI), and a Mid-IR Instrument (MIRI). It is planned for launch early in the next decade on an Ariane 5 rocket to a deep space orbit around the Sun­Earth Lagrange point L2, about 1.5 â 106 km from Earth. The spacecraft will carry enough fuel for a 10 yr mission. In this paper, we describe the scientific capabilities and planned implementation of JWST. The scientific planning is based on current theoretical understanding, interpretation of Hubble Space Telescope (HST) and Spitzer Space Telescope observations (e.g., Werner et al., 2004 and additional papers in that volume), and results from studies using ground-based and other space-based facilities. Many classes of targets for JWST have never been observed before, so the scientific plans are necessarily derived from theoretical predictions and extrapolations from known objects. In these cases, we outline the basis of the predictions and describe the observatory capabilities needed to verify them. Additional results from HST and Spitzer, and other advances in theory and observation will continue to refine the observational plans for JWST. The scientific


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TABLE I Table of acronyms Acronym AB AGN AI&T ALMA AMSD AU AURA CDM CMB CSA DHS DWS DSN EGG ESA ESO EW FGS FOR FORS1 FOV FPA FSM FUSE FWHM GOODS GSC GSFC HDF HgCdTe HST IFU IGM IMF IR IRAS ISIM Definition Absolute Bolometric Active Galactic Nuclei Assembly, Integration and Test Atacama Large Millimeter Array Advanced Mirror System Demonstrator Astronomical Unit Association of Universities for Research in Astronomy Cold Dark Matter Cosmic Microwave Background Canadian Space Agency Dispersed Hartmann Sensor Deep-Wide Survey Deep-Space Network Evaporating Gaseous Globule European Space Agency European Southern Observatory Equivalent Width Fine Guidance Sensor Field of Regard Focal Reducer and Low Dispersion Spectrograph 1 Field of View Focal-Plane Array Fine Steering Mirror Far-Ultraviolet Spectroscopic Explorer Full-Width Half-Maximum Great Observatories Origins Deep Survey Guide-Star Catalog Goddard Space Flight Center Hubble Deep Field Mercury Cadmium Telluride Hubble Space Telescope Integral Field Unit Intergalactic Medium Initial Mass Function Infrared Infrared Astronomical Satellite Integrated Science Instrument Module (Continued on next page)


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TABLE I (Continued) Acronym ISM ISOCAM ISO LWS ISO SWS JPL JWST KBO L2 LF LMC LRS MIRI MSA MJy/sr NASA NGST NIRCam NIRSpec nJy NTT OPR OS OTA OTE PAH PSF PM PMSA QA QPM QSO rms RoC SDSS SED SFR SI Definition Interstellar Medium Infrared Space Observatory Camera Infrared Space Observatory Long Wavelength Spectrograph Infrared Space Observatory Short Wavelength Spectrograph Jet Propulsion Laboratory James Webb Space Telescope Kuiper Belt Object Second Lagrange Point Luminosity Function Large Magellanic Cloud Low-Resolution Spectrograph Mid-Infrared Instrument Micro-shutter Assembly Mega-Jansky per steradian National Aeronautics and Space Agency Northrop Grumman Space Technology Near-Infrared Camera Near-Infrared Spectrograph nano-Jansky New Technology Telescope Ortho-Para Ratio Operating System Optical Telescope Assembly Optical Telescope Element Polycyclic Aromatic Hydrocarbon Point-Spread Function Primary Mirror Primary Mirror Segment Assembly Quality Assurance Quarter Phase Mask Quasi-Stellar Object Root Mean Squared Radius of Curvature Sloan Digital Sky Survey Spectral Energy Distribution Star-Formation Rate Science Instrument (Continued on next page)


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TABLE I (Continued) Acronym SM SMC SN SNe SOFI SSM STIS STScI SWG S&OC TAC TBD TFI TMA UDF UDS ULIRG UV VLA VLT WFPC2 WFE WFS&C WMAP YSO Definition Secondary Mirror Small Magellanic Cloud Supernova Supernovae Son of ISAAC (Infrared Spectrograph and Imaging Camera) Space Support Module Space Telescope Imaging Spectrograph Space Telescope Science Institute Science Working Group Science and Operations Center Time Allocation Committee To Be Determined Tunable Filter Imager Three Mirror Anastigmat Ultra-Deep Field Ultra-Deep Survey Ultra-Luminous Infrared Galaxy Ultraviolet Very Large Array Very Large Telescope Wide-Field Planetary Camera 2 Wavefront Error Wavefront Sensing and Control Wilkinson Microwave Anisotropy Probe Young Stellar Object

investigations we describe here define the measurement capabilities of the telescope, but they do not imply that those particular observations will be made. In this paper, we do not list all potential applications of JWST. Instead, the scientific programs we discuss here are used to determine the key parameters of the mission such as collecting area, spatial and spectral resolution, wavelength coverage, etc. A mission which provides these capabilities will support a wide variety of astrophysical investigations. JWST is a facility-class mission, so most of the observing time will be allocated to investigators from the international astronomical community through competitively-selected proposals. The plans for JWST reflect scientific and engineering discussions, studies, and development over the last 16 yr. At a workshop held in 1989, the astronomical community began discussions of a scientific successor to HST (Bely et al., ´


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1989). In the mid-1990s, the "HST and Beyond" committee recommended that NASA build an IR-optimized telescope to extend HST discoveries to higher redshift and longer wavelength (Dressler, 1996). Initial studies of the mission, then called the Next Generation Space Telescope, were reported by Stockman et al. (1997). Its scientific program was given top priority by the National Academy of Sciences survey "Astronomy and Astrophysics in the New Millennium" (McKee and Taylor, 2001). In 2002, the Next Generation Space Telescope was renamed the James Webb Space Telescope in honor of the Administrator of NASA during the Apollo era (Lambright, 1995). This paper provides an update to these previous studies. The capabilities and performance specifications for JWST given here are preliminary. The mission is currently in its detailed design phase and has not yet been given official permission to proceed beyond that stage. NASA missions receive authority, and the accompanying budget, to proceed to launch only after they pass a nonadvocate review and transition from detailed design into development phases. Hence, the specifications given in this paper are subject to change based upon technical progress of individual elements of the observatory and available budgets. JWST is expected to receive final approval and make the transition to the development phase in 2008. The scientific objectives of JWST fall into four broad themes: The End of the Dark Ages: First Light and Reionization; The Assembly of Galaxies; The Birth of Stars and Protoplanetary Systems; and Planetary Systems and the Origins of Life. We organize this paper as follows. The remainder of this section provides a scientific introduction to these themes. In Sections 2­5, we expand on the themes, and describe the scientific capabilities JWST will use to address them. In Section 6 we describe the planned implementation of JWST.

1.1. T

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Theory and observation have given us a simple picture of the early universe. The Big Bang produced (in decreasing order of present mass­energy density): dark energy (the cosmic acceleration force), dark matter, hydrogen, helium, cosmic microwave and neutrino background radiation, and trace quantities of lithium, beryllium, and boron. As the universe expanded and cooled, hydrogen molecules formed, enabling the formation of the first individual stars, at about 180 Myr after the Big Bang (Barkana and Loeb, 2001). According to theory, the first stars were 30­300 times as massive as the Sun and millions of times as bright, burning for only a few million years before meeting a violent end (Bromm and Larson, 2004). Each one produced either a core-collapse supernova (SN) (type II) or a black hole. The supernovae (SNe) enriched the surrounding gas with the chemical elements produced in their interiors, and future generations of stars contained these heavier elements. The black holes started to swallow gas and other stars to become mini-quasars, which


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grew and merged to become the huge black holes now found at the centers of nearly all massive galaxies (Magorrian et al., 1998). The SNe and the mini-quasars could be individually observable by JWST. Some time after the appearance of the first sources of light, hydrogen in the intergalactic medium (IGM) was reionized. Results from the Wilkinson Microwave Anisotropy Probe (WMAP; Kogut et al., 2003; Page et al., 2006; Spergel et al., 2006) combined with data on quasars at z 6 from the Sloan Digital Sky Survey (SDSS; Fan et al., 2002) show that this reionization had a complex history (Cen, 2003b). Although there are indications that galaxies produced the majority of the ultraviolet (UV) radiation which caused the reionization, the contribution of quasars could be significant. JWST will address several key questions in this theme: What are the first galaxies? When and how did reionization occur? What sources caused reionization? JWST will conduct ultra-deep near-IR surveys with spectroscopic and mid-IR follow-up to find and identify the first galaxies to form in the early universe. It will determine the processes that caused reionization through spectroscopy of high-redshift quasars or galaxies, studies of the properties of galaxies during that epoch.

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Theory predicts that galaxies are assembled through a process of the hierarchical merging of dark matter concentrations (e.g., White and Frenk, 1991; Cole et al., 1994). Small objects formed first, and were drawn together to form larger ones. This dynamical buildup of massive systems is accompanied by chemical evolution, as the gas (and dust) within the galaxies are processed through successive generations of stars. The interaction of these luminous components with the invisible dark matter produces the diverse properties of present-day galaxies, organized into the Hubble Sequence of galaxies. This galaxy assembly process is still occurring today, as the Magellanic Clouds fall into the Milky Way, and as the Andromeda Galaxy heads toward the Milky Way for a possible future collision. To date, galaxies have been observed back to times about one billion years after the Big Bang. While most of these early galaxies are smaller and more irregular than present-day galaxies, some early galaxies are very similar to those seen nearby today. Despite all the work done to date, many questions are still open. We do not really know how galaxies are formed, what controls their shapes, and what makes them form stars. We do not know how the chemical elements are generated and redistributed through the galaxies, and whether the central black holes exert great influence over the galaxies. We do not know the global effects of violent events as small and large parts join together in collisions. JWST will address several key questions in this theme: When and how did the Hubble Sequence form? How did the heavy elements form? What physical


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processes determine galaxy properties? What are the roles of starbursts and black holes in galaxy evolution? To answer these questions, JWST will observe galaxies back to their earliest precursors, so that we can understand their growth and evolution. JWST will conduct deep-wide imaging and spectroscopic surveys of thousands of galaxies to study morphology, composition, and the effects of environment. It will conduct detailed studies of individual ultraluminous IR galaxies (ULIRGs) and active galactic nuclei (AGN) to investigate what powers these energetic sources. 1.3. T
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YSTEMS

While stars have been the main topic of astronomy for thousands of years, only in recent times have we begun to understand them with detailed observations and computer simulations. A hundred years ago, we did not know that stars are powered by nuclear fusion, and 50 yr ago we did not know that stars are continually being formed. We still do not know the details of how stars are formed from clouds of gas and dust, why most stars form in groups, or how planetary systems form. Young stars within a star-forming region interact with each other chemically, dynamically, and radiatively in complex ways. The details of how they evolve and liberate the "metals" back into space for recycling into new generations of stars and planets remains to be determined through a combination of observation and theory. We also know that a substantial fraction of stars, solar-type and later, have gasgiant planets, although the discovery of large numbers of these planets in very close orbits around their stars was a surprise. The development of a full theory of planet formation requires substantially more observational input, including observations of young circumstellar disks and older debris disks in which the presence of planets can be traced. JWST will address several key questions in this theme: How do protostellar clouds collapse? How does environment affect star formation and vice versa? What is the initial mass function (IMF) of stars at substellar masses? How do protoplanetary systems form? How do gas and dust coalesce to form planetary systems? JWST will observe stars at all phases of their evolution, from infall onto dust-enshrouded protostars, through the formation of planetary systems, penetrating the dust to determine the physical processes that produce stars, planets, and debris disks. 1.4. P
LANETARY

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IFE

Understanding the origin of the Earth and its ability to support life is an important objective for astronomy. Key parts of the story include understanding the formation of planetesimals, and how they combine to form larger objects. We do not know how planets reach their present orbits, and how the large planets affect the smaller ones in solar systems like our own. We want to learn about the chemical and physical history of the small and large objects that formed the Earth and delivered


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the necessary chemical precursors for life. The cool objects and dust in the outer Solar System are evidence of conditions in the early Solar System, and are directly comparable to cool objects and dust observed around other stars. JWST will address several key questions in this theme: How do planets form? How are circumstellar disks like our Solar System? How are habitable zones established? JWST will determine the physical and chemical properties of planetary systems including our own, and investigate the potential for the origins of life in those systems. JWST will use coronagraphy to investigate extrasolar planets and debris disks, and will compare these observations to objects within our own Solar System.

2. The End of the Dark Ages: First Light and Reionization The key objective of The End of the Dark Ages: First Light and Reionization theme is to identify the first luminous sources to form and to determine the ionization history of the early universe. The emergence of the first individual sources of light in the universe marks the end of the "Dark Ages" in cosmic history, a period characterized by the absence of discrete sources of light (Rees, 1997). Understanding these first sources is critical, since they greatly influenced subsequent structures. The current leading models for structure formation predict a hierarchical assembly of galaxies and clusters. The first sources of light act as seeds for the successive formation of larger objects, and by studying these objects we will learn the processes that formed the nuclei of present-day giant galaxies. This epoch is currently under intense theoretical investigation. The formation of structure in the Dark Ages is easier to study theoretically than similar processes occurring at other epochs because: (i) the formation of the first structures is directly linked to the growth of linear perturbations, and (ii) these objects have known elemental abundances set by the end-product of the primordial nucleosynthesis. By studying this epoch, it is possible to probe the power spectrum of density fluctuations emerging from recombination at scales smaller than those accessible by current cosmic microwave background (CMB) experiments. Some time after the appearance of the first sources of light, hydrogen in the universe is reionized. We do not know the time lag between these two events, nor whether reionization is brought about by the first light sources themselves or by subsequent generations of objects. Reionization is by itself a period in cosmic history that is as interesting as the emergence of first light. The epoch of reionization is the most recent global phase transition undergone by the universe after the Big Bang. Before the epoch of first light, ionizing photons and metals are essentially absent. Thus, hydrogen molecules can form and survive to become the primary cooling agent of the first perturbation to collapse. We expect the stars that formed


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from this process to be very massive and very hot. Historically, this primordial stellar population has been given the name of population III stars. The most overdense peaks in the perturbations emerging from recombination will collapse first. Hierarchical-clustering models predict that these overdense peaks sit in larger overdense regions that have larger mass but lower contrast. We may expect the first stars to be markers of, and possibly reside in, star clusters or even small galaxies. At the end of their short lives, some very massive first stars will leave black holes as remnants. These black holes will begin accreting gas and form mini-AGN. In more exotic models, black holes can form directly from the collapse of perturbations instead of from stellar remnants. Thus, in these models, the first sources of light would be powered by gravitational accretion, rather than by nuclear fusion. Soon after the first light sources appear, both high-mass stars and accretion onto black holes become viable sources of ionizing radiation. We do not know which are primarily responsible for reionizing hydrogen in the surrounding IGM. AGN produce a highly energetic synchrotron power spectrum, and would reionize helium as well as hydrogen. Because observational evidence reveals that helium is reionized at a much later time, hydrogen was probably reionized by starlight at earlier epochs. However, it is possible that helium recombines after being reionized for the first time together with hydrogen. A second reionization of helium would occur during the epoch when quasar activity peaks. More recently, a combination of observations by the Wilkinson Microwave Anisotropy Probe (WMAP) of the CMB polarization (Kogut et al., 2003; Page et al., 2006; Spergel et al., 2006) with spectra of z > 6 quasars found by the SDSS (Fan et al., 2001, 2002) has revealed the possibility that there were two reionization epochs for hydrogen (Cen, 2003a,b). In these models, the completion of the reionization epoch that is seen at z 6 would be that of the second reionization, with the first reionization taking place during or after the peak of the first light epoch at higher redshifts. Although the observations allow for other possibilities, in general, there is evidence that the reionization history of the universe was complex (e.g., Gnedin, 2004). 2.1. W
HAT A RE THE

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IRST

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?

When did the first luminous sources arise and what was their nature? What were their clustering properties? In standard Cold Dark Matter (CDM) cosmology, galaxies are assembled through hierarchical merging of building blocks with smaller mass. The first such building blocks, with M 104 M form in these models at z 15 (see Figure 1; Couchman and Rees, 1986; Haiman et al., 1996; Ostriker and Gnedin, 1996; Haiman and Loeb, 1997; Abel et al., 1998, 2000; Barkana and Loeb, 2001). While we do not know whether the first sources of light are powered by nuclear energy from fusion reactions in stars, or by gravitational accretion (Haiman and Loeb, 1999), it is possible that population III stars are responsible for the reionization of hydrogen at z > 6 (Madau and Shull, 1996; see also Gnedin and


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Figure 1. The mass of collapsing dark matter halos in the early universe. The red solid curves show the mass of collapsing halos corresponding to 1, 2, and 3 fluctuations (in order from bottom to top.) The blue dashed curves show the mass corresponding to the minimum temperature required for efficient cooling with primordial atomic species only (upper curve) or with the addition of molecular hydrogen (lower curve). The intersection of these lines indicates that the epoch of formation for the first galaxies is likely to be 10 < z < 20 (From Barkana and Loeb, 2001).

Ostriker, 1997; Haiman and Loeb, 1999; Chiu and Ostriker, 2000). Efficient cooling by H2 molecules and an early, vigorous formation of massive objects could result in reionization at redshifts as early as z 20 (Cen, 2003b; Haiman and Holder, 2003). The very first stars (population III) have zero metallicity. In the absence of any metals, cooling is dominated by the less effective H2 cooling process, which leads to the formation of very massive objects, with masses exceeding 100 M (Bromm et al., 1999, 2002) and possibly going as high as 500 M . The spectral energy distribution (SED) of these massive stars resembles a blackbody with an effective temperature around 105 K (Bromm et al., 2001). Due to their high temperatures, these stars are very effective at ionizing both hydrogen and helium. It should be noted that, even at lower mass, zero-metallicity stars are expected to be much hotter than their solar metallicity analogs (Tumlinson and Shull, 2000). Two consequences of the high effective temperature of zero-metallicity stars are their effectiveness in ionizing hydrogen (and helium) and their low optical-toUV fluxes. Both tend to make the direct detection of the stellar continuum much


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harder than the detection of the associated HII region. In the surrounding HII region, electron temperatures exceed 20,000 K and 45% of the total luminosity is emitted through the Lyman line, resulting in a Lyman equivalent width (EW) ° of 3000 A (Bromm et al., 2001). The helium lines are also strong, with the intensity of HeII 1640 comparable to that of H (Tumlinson et al., 2001; Panagia et al., 2003). An interesting feature of these models is that the HII region emission longward of Lyman is dominated by a strong two-photon nebular continuum. The H /H ratio for these models is 3.2. Both the red continuum and the high H /H ratio could be incorrectly interpreted as a consequence of dust extinction, even though no dust is present in these systems. By estimating the brightness of the sources that enriched the IGM to 10-2 Z (Miralda-Escude and Rees, 1998), one finds that a combined surface brightness of ´ about AB = 32 mag arcsec-2 is needed. (The AB magnitude system is defined to be AB = 31.4­2.5 log( f ), where f is in nJy, Oke, 1974.) This surface brightness is about two orders of magnitude brighter than the surface brightness derived later for reionization (see Section 2.2). For reasonable luminosity functions (LFs), these sources would be either detected directly, or by exploiting amplification by gravitational lensing from an intervening cluster of galaxies. Their large number offers the promising prospect of identifying first light by observing a decrease in the number of sources seen at increasing redshifts (after properly accounting for the effects of sample completeness.) The deepest images of the universe include the Hubble Ultra-Deep Field (UDF) in the optical (Beckwith et al., 2003), which reaches AB = 29.0 mag in the I band, HST near-IR images of the UDF, which reach AB = 28.5 in the J and H bands (Bouwens et al., 2005a), and the Spitzer Great Observatories Origins Deep Survey (Dickinson, 2004), which reaches AB = 26.6 mag at 3.6 m. Galaxies are detected in these observations at 6 < z < 7 (e.g., Yan et al., 2005) with potential candidates at even higher redshift. The rest-frame UV LF of z 6 galaxies is intrinsically fainter than that at z 3 (Dickinson et al., 2004; Bouwens et al., 2005b), showing that the global star-formation rate (SFR) is climbing. However, the detection of galaxies with stellar populations as old as 400­500 Myr at z 6.5 (Egami et al., 2005; Eyles et al., 2005; Mobasher et al., 2005) indicate that the first galaxies formed much earlier, perhaps in the range 7.5 < z < 13.5. The Spitzer detections point to the importance of using mid-IR observations for galaxy age determinations through stellar population model fitting. The number of SNe expected before reionization also strongly depends on the assumptions made about the nature of the ionizing sources. Based on relatively normal stell