The Advanced Camera for Surveys (ACS) observations of the Hubble Ultra Deep Field (HUDF) provide the highest sensitivity optical observations of galaxies and stars ever achieved. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) observations in the central portion of the field extend the wavelength coverage by a factor of two to beyond 1.6 microns. Although not as sensitive as the ACS images due to a much smaller field and less observing time, the NICMOS observations extend the redshift range of the HUDF to redshifts as high as 13. Even though the observations are sensitive to redshift 13 objects, we confine our investigation to objects between redshifts of 7 and 9 where there is flux in both the F110W and F160W bands. Candidate sources in this redshift region are identified by requiring a non-detection in the ACS bands and a detection in both the F110W and F160W bands. All of the candidates have an almost flat or blue color in the F110W and F160W bands. The extremely high sensitivity of the ACS observations make this a very stringent criterion. We identify five candidates for objects in this redshift range and discuss tests of the reality of these sources. Although the sources are selected to have flux in both NICMOS bands and none in the ACS bands, we also present the results of a photometric redshift analysis of the candidates. This shows them to be very blue galaxies with redshifts between 7.3 and 7.9. One source yielded an anomalous redshift and spectral type due to flux from an adjacent galaxy falling in the photometric aperture.

The Hubble Space Telescope is uniquely able to study planets that are observed to transit their parent stars. The extremely stable platform afforded by an orbiting spacecraft, free from the contaminating effects of the Earth's atmosphere, enables HST to conduct ultra-high precision photometry and spectroscopy of known transiting extrasolar planet systems. Among HST's list of successful observations of the first such system, HD 209458, are (1) the first detection of the atmosphere of an extrasolar planet, (2) the determination that gas is escaping from the planet, and (3) a search for Earth-sized satellites and circumplanetary rings. Numerous widefield, ground-based transit surveys are poised to uncover a gaggle of new worlds for which HST may undertake similar studies, such as the newly-discovered planet TrES-1. With regard to the future of Hubble, it must be noted that it is the only observatory in existence capable of confirming transits of Earth-like planets that may be detected by NASA's Kepler mission. Kepler could reveal Earth-like transits by the year 2010, but without a servicing mission it is very unlikely that HST would still be in operation.

We discuss currently available observational constraints on the reionization history of the intergalactic medium (IGM), and the extent to which accreting black holes (BHs) and stars can help account for these observations. We argue, based on the combined statistics of Lyman α and β absorption in quasar spectra, that the IGM contains a significant amount of neutral hydrogen with nH I/nH ≳ 0.1. On the other hand, we argue, based on the lack of a strong evolution in the observed abundance of Lyman α emitting galaxies beyond z ∼ 5.5, that the mean neutral hydrogen fraction cannot exceed nH I/nH ≈ 0.3 at the same redshift. We conclude that the IGM is experiencing rapid ionization at redshift z ∼ 6.

We find that quasar BHs, including faint ones that are individually below the detection thresholds of existing optical and X-ray surveys, are unlikely to drive the evolution of the neutral fraction around this epoch, because they would over-produce the present-day soft X-ray background. On the other hand, the seeds of the z ∼ 6 quasar BHs likely appeared at much earlier epochs (z ∼ 20), and produced hard ionizing radiation by accretion. These early BHs are promising candidates to account for the high redshift (z ∼ 15) ionization implied by the recent cosmic microwave anisotropy data from WMAP.

Using a model for the growth of BHs by accretion and mergers in a hierarchical cosmology, we suggest that the early growth of quasars must include a super-Eddington growth phase, and that, although not yet optically identified, the FIRST radio survey may have already detected several thousand > 108 M⊙ BHs at z > 6.

The Hubble Space Telescope has shown us the homes of nearby quasars in revealing detail, and has dealt us surprising answers to some of our long-standing questions about quasar host galaxy morphology. However, like all cutting-edge instruments, HST has taught us that the very questions we were asking were not necessarily the most interesting ones. Exploring the latter will require a combination of ground- and space-based work over the remaining lifetime of HST, and beyond. Such studies promise to give us insight into the formation and evolution of galaxies like our own over the whole history of the Universe.

The Hubble Space Telescope is very well tailored for observations of extragalactic star clusters. Obvious reasons are HST's ability to recognize clusters as extended objects and measure sizes out to distances of several Mpc. Equally important is the wavelength range offered by the instruments on board HST—in particular the blue and near-UV coverage—which is essential for age-dating young clusters. HST observations have helped establish the ubiquity of young massive clusters (YMCs) in a wide variety of star-forming environments, ranging from dwarf galaxies and spiral disks to nuclear starbursts and mergers. These YMCs have masses and structural properties similar to those of old globular clusters in the Milky Way and elsewhere, and the two may be closely related. Several lines of evidence suggest that a large fraction of all stars are born in clusters, but most clusters disrupt rapidly and their stars disperse to become part of the field population. In most cases studied to date, the luminosity functions of young cluster systems are well fit by power laws dN(L)/dL ∝ L−α with α ≈ 2, and the luminosity of the brightest cluster can (with few exceptions) be predicted from simple sampling statistics. Mass functions have only been constrained in a few cases, but appear to be well approximated by similar power laws. The absence of any characteristic mass scale for cluster formation suggests that star clusters of all masses form by the same basic process, without any need to invoke special mechanisms for the formation of “massive” clusters. It is possible, however, that special conditions can lead to the formation of a few YMCs in some dwarfs where the mass function is discontinuous. […]

New surveys with the Spitzer Space Telescope identify distant star-forming and active galaxies by their strong emission at far-infrared wavelengths, which provides strong constraints on these galaxies' bolometric energy. Using early results from Spitzer surveys at 24 μm, we argue that the faint sources correspond to the existence of a population of infrared-luminous galaxies at z ≳ 1 that are not expected from predictions based on previous observations from ISO and IRAS. Combining Spitzer images with deep ground-based optical and Hubble Space Telescope imaging, we discuss the properties of galaxies selected at 24 μm in the region of the Chandra Deep Field South, including redshift and morphological distributions. Galaxies with z ≲ 1 constitute roughly half of the faint 24 μm sources. Infrared-luminous galaxies at these redshifts span a wide variety of normal to strongly interacting/merging morphologies, which suggests that a range of mechanisms produce infrared activity. Large-area, joint surveys between Spitzer and HST are needed to understand the complex relation between galaxy morphology, structure, environment, and activity level, and how this evolves with cosmic time. We briefly discuss strategies for constructing surveys to maximize the legacy of these missions.

In a ΛCDM Universe, galaxies grow in mass both through star formation and through the addition of already-formed stars in galaxy mergers. Because of this partial decoupling of these two modes of galaxy growth, I discuss each separately in this biased and incomplete review of galaxy assembly—first giving an overview of the cosmic-averaged star formation history, and then moving on to discuss the importance of major mergers in shaping the properties of present-day massive galaxies. The cosmic-averaged star-formation rate, when integrated, is in reasonable agreement with the build-up of stellar mass density. Roughly 2/3 of all stellar mass is formed during an epoch of rapid star formation prior to z ∼ 1, with the remaining 1/3 formed in the subsequent 9 Gyr during a period of rapidly-declining star-formation rate. The epoch of important star formation in massive galaxies is essentially over. In contrast, a significant fraction of massive galaxies undergo a major merger at z ≲ 1, as evidenced by close-pair statistics, morphologically-disturbed galaxy counts, and the build-up of stellar mass in morphologically early-type galaxies. Each of these methods is highly uncertain; yet, taken together, it is not implausible that the massive galaxy population is strongly affected by late galaxy mergers, in excellent qualitative agreement with our understanding of galaxy evolution in a ΛCDM Universe.

In this review, we describe our surveys of low column density (Lyα) absorbers (NHI = 1012.5−16 cm−2), which show that the warm photoionized IGM contains ∼30% of all baryons at z ≤ 0.1. This fraction is consistent with cosmological hydrodynamical simulations, which also predict that an additional 20–40% of the baryons reside in much hotter 105−7 K gas, the warm-hot IGM (WHIM). The observed line density of Lyα absorbers, dN/dz ≈ 170 for NHI ≥ 1012.8 cm−2, is dominated by low-NHI systems that exhibit slower redshift evolution than those with NHI ≥ 1014 cm−2. HST/FUSE surveys of OVI absorbers, together with recent detections of OVII with Chandra and XMM/Newton, suggest that 10–40% of all baryons could reside in the WHIM, depending on its assumed abundance (O/H ≈ 10% solar). We also review the relationship between the various types of Lyα absorbers and galaxies. At the highest column densities, NHI ≥ 1020.3 cm−2, the damped Lyα (DLA) systems are often identified with gas-rich disks of galaxies over a large range in luminosities (0.03–1 L*) and morphologies. Lyman-limit systems (NHI ≥ 1017.3−20.3 cm−2) appear to be associated with bound bright (≥ 0.1–0.3 L*) galaxy halos. The Lyα absorbers with NHI = 1013−17 cm−2 are associated with filaments of largescale structure in the galaxy distribution, although some may arise in unbound winds from dwarf galaxies. Our discovery that ∼20% of low-z Lyα absorbers reside in galaxy voids suggests that a substantial fraction of baryons may be entirely unrelated to galaxies. In the future, HST can play a crucial role in a precise accounting of the local baryons and the distribution of heavy elements in the IGM. […]

Within the last few years, a number of public and legacy projects have generated very deep photometric datasets. The Hubble Space Telescope (HST) leads the way in this field, with the high spatial resolution and ability to detect very faint galaxies essential for this challenging work. The Advanced Camera for Surveys (ACS) on HST has now carried out several large deep surveys, including the Great Observatories Origins Deep Survey (GOODS) and the Hubble Ultra Deep Field (HUDF). These have been designed to allow the systematic broadband selection of very high redshift galaxies (z > 5) using the SDSS-i′ and z′ filters. This endeavor to identify faint and distant galaxies has been complemented by advances in spectroscopy. The current generation of spectrographs on 8m-class telescopes and the development of new techniques such as Nod & Shuffle have allowed the spectroscopic limit to be pushed to ever fainter magnitudes. The Gemini Lyman-Alpha at Reionization Era (GLARE) project is a spectroscopic campaign which aims to obtain 100-hour Gemini/GMOS spectra for a large number of z ≈ 6 galaxy candidates in and around the Ultra Deep Field. We describe the use of the i′-drop photometric technique to identify very high-redshift candidates in the data of the public GOODS and HUDF surveys. We comment on confirmed high-redshift galaxies discovered using this technique. We then discuss the photometric and spectroscopic characteristics of the galaxy sample resulting from the first 7.5 hours of GLARE observations.

We examine the process of feedback in star-forming galaxies at 2 ≤ z ≤ 3. Large-scale outflows of interstellar material are observed in starburst galaxies in the nearby universe, and have long been invoked as a means to address important shortcomings in current models of galaxy formation. At z ∼ 3, superwinds appear to be a generic feature of color-selected star-forming galaxies with spectroscopic information, and may explain both the apparent lack of neutral hydrogen near star-forming galaxies, and also the strong cross-correlation between galaxies and CIV metalabsorption systems. Another type of star-formation feedback is the leakage of hydrogen-ionizing radiation from galaxies, which may also have a profound effect on the physical state of the intergalactic medium (IGM), especially as the number density of QSOs drops off at z > 2.5. Between z = 3 and z = 2, there is strong evolution in the number density of HI absorption systems in the Lyα forest. Therefore, it is also of interest to trace how the effect of galactic superwinds on the IGM evolves from z = 3 to z = 2. We show preliminary results that many properties of superwinds are similar in star-forming galaxies at z ∼ 2, and direct evidence that enriched gas reaches radii of at least ∼100 kpc. Finally, we discuss future directions for the study of outflows in the high-redshift universe. Specifically, we highlight the unique combination of existing deep HST/ACS imaging in the GOODS-N field with high signal-to-noise rest-frame UV spectra. Using the morphological information provided by the HST/ACS will enable us to probe a complementary, spatial dimension of feedback at high redshift, which has been unexplored until now.

The study of star formation is currently benefiting from a wealth of new observational data, exploiting the high-sensitivity, wide-field, high-resolution capabilities of a diverse range of space and ground-based instrumentation. In parallel with this, high performance computing is enabling theorists to tackle key problems which—due to their complex geometry and non-linear nature—had long been recognized to be beyond the reach of analytical theory. In this review, rather than reporting progress in each of these areas, I will instead set out some scientific questions that one would expect to be answered before one would regard star formation as a topic that was largely solved. I have accordingly selected three areas: 1) molecular clouds and their relationship to the stars they form and to the wider galactic disk, 2) the question of the determinants of stellar mass (i.e., the IMF), and 3) the issue of protostellar disk dispersal and its relation to planet formation). For each topic, I outline areas of consensus, recent results, and discuss the key problems that can plausibly be addressed in the next five years.