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Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least 4 m. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the 6.5 m James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 yr, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.

We report ultraviolet spectra of the high-redshift (z em [?] 2.9) quasar, HE 2347 - 4342, taken by the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope. Spectra in the G130M (medium resolution, 1135-1440 A) and G140L (low resolution, 1030-2000 A) gratings exhibit patchy Gunn-Peterson absorption in the 303.78 A Lya line of He II between z = 2.39-2.87 (G140L) and z = 2.74-2.90 (G130M). With COS, we obtain better spectral resolution, higher signal-to-noise ratio (S/N), and better determined backgrounds than previous studies, with sensitivity to abundance fractions x He II [?] 0.01 in filaments of the cosmic web. The He II optical depths from COS are higher than those with the Far Ultraviolet Spectroscopic Explorer and range from tHe II [?] 0.02 to tHe II [?] 5, with a slow recovery in mean optical depth to [?]tHe II [?] [?] 2 at z < 2.7. The He II/H I optical-depth ratio varies (e[?] 10-100 for 2.4 < z < 2.73 and e = 5-500 for 2.75 < z < 2.89) on scales Dz [?] 0.01 (10.8 Mpc in comoving radial distance at z = 2.8), with numerous flux-transmission windows between 1135 and 1186 A. The He II absorption extends to 1186.26 A (z = 2.905), including associated absorbers with z abs [?] z QSO and minimal "proximity effect" of flux transmission at the He II edge. We propose a QSO systemic redshift z QSO = 2.904 +- 0.002, some Dz = 0.019 higher than that derived from O I l1302 emission. Three long troughs (4-10 A or 25-60 Mpc comoving distance) of strong He II absorption between z = 2.75and2.90 are uncharacteristic of the intergalactic medium if He II reionized at zr [?] 3. Contrary to recent indirect estimates (zr = 3.2 +- 0.2) from H I optical depths, the epoch of He II reionization may extend to z [?] 2.7.

This paper presents theoretical star formation and chemical enrichment histories for the stellar halo of the Milky Way (MW) based on new chemodynamical modeling. The goal of this study is to assess the extent to which metal-poor stars in the halo reflect the star formation conditions that occurred in halo progenitor galaxies at high redshift, before, and during the epoch of re-ionization. Simple prescriptions that translate dark-matter (DM) halo mass into baryonic gas budgets and star formation histories yield models that resemble the observed MW halo in its total stellar mass, metallicity distribution, and the luminosity function and chemical enrichment of dwarf satellite galaxies. These model halos in turn allow an exploration of how the populations of interest for probing the epoch of re-ionization are distributed in physical and phase space, and of how they are related to lower-redshift populations of the same metallicity. The fraction of stars dating from before a particular time or redshift depends strongly on radius within the galaxy, reflecting the "inside-out" growth of cold DM halos, and on metallicity, reflecting the general trend toward higher metallicity at later times. These results suggest that efforts to discover stars from z>6-10 should select for stars with [Fe/H] [?]-3 and favor stars on more tightly bound orbits in the stellar halo, where the majority are from z>10 and 15%-40% are from z>15. The oldest, most metal-poor stars--those most likely to reveal the chemical abundances of the first stars--are most common in the very center of the Galaxy's halo: they are in the bulge, but not of the bulge. These models have several implications for the larger project of constraining the properties of the first stars and galaxies using data from the local universe.

We propose a simple theoretical model for star formation in which the local star formation rate (SFR) in a galaxy is determined by three factors. First, the interplay between the interstellar radiation field and molecular self-shielding determines what fraction of the gas is in molecular form and thus eligible to form stars. Second, internal feedback determines the properties of the molecular clouds that form, which are nearly independent of galaxy properties until the galactic interstellar medium (ISM) pressure becomes comparable to the internal giant molecular cloud (GMC) pressure. Above this limit, galactic ISM pressure determines molecular gas properties. Third, the turbulence driven by feedback processes in GMCs makes star formation slow, allowing a small fraction of the gas to be converted to stars per free-fall time within the molecular clouds. We combine analytic estimates for each of these steps to formulate a single star formation law, and show that the predicted correlation between SFR, metallicity, and surface densities of atomic, molecular, and total gas agree well with observations.

Gas in galactic disks is collected by gravitational instabilities into giant atomic-molecular complexes, but only the inner, molecular parts of these structures are able to collapse to form stars. Determining what controls the ratio of atomic-to-molecular hydrogen in complexes is, therefore, a significant problem in star formation and galactic evolution. In this paper, we use the model of H2 formation, dissociation, and shielding developed in the previous paper in this series to make theoretical predictions for atomic-to-molecular ratios as a function of galactic properties. We find that the molecular fraction in a galaxy is determined primarily by its column density and secondarily by its metallicity, and is to a good approximation independent of the strength of the interstellar radiation field. We show that the column of atomic hydrogen required to shield a molecular region against dissociation is [?]10 M pc-2 at solar metallicity. We compare our model to data from recent surveys of the Milky Way and of nearby galaxies, and show that the both the primary dependence of molecular fraction on column density and the secondary dependence on metallicity that we predict are in good agreement with observed galaxy properties.

In this series of papers, we study the structure of the atomic-to-molecular transition in the giant atomic-molecular complexes that are the repositories of most molecular gas in galaxies, with the ultimate goal of attaining a better understanding of what determines galaxies' molecular content. Here we derive an approximate analytic solution for the structure of a photodissociation region (PDR) in a cloud of finite size that is bathed in an external dissociating radiation field. Our solution extends previous work, which with few exceptions has been restricted to a one-dimensional treatment of the radiation field. We show that our analytic results compare favorably to exact numerical calculations in the one-dimensional limit. However, our more general geometry provides a more realistic representation than a semi-infinite slab for atomic-molecular complexes exposed to the interstellar radiation field, particularly in environments such as low-metallicity dwarf galaxies, where the curvature and finite size of the atomic envelope cannot be neglected. For clouds that are at least 20% molecular, we obtain analytic expressions for the molecular fraction in terms of properties of the gas and radiation field that are accurate to tens of percent, while for clouds of lower molecular content we obtain upper limits. As a side benefit, our analysis helps to clarify when self-shielding is the dominant process in H2 formation, and under what circumstances shielding by dust makes a significant contribution.

Recent analysis of five gamma-ray burst (GRB) afterglow spectra reveals the absence of molecular hydrogen absorption lines, a surprising result in light of their large neutral hydrogen column densities and the detection of H2 in similar, more local star-forming regions such as 30 Doradus in the Large Magellanic Cloud (LMC). Observational evidence further indicates that the bulk of the neutral hydrogen column in these sight lines lies more than 100 pc from the GRB progenitor and that H2 was likely absent prior to the burst, suggesting that direct flux from the star, FUV background fields, or both suppressed its formation. We present one-dimensional radiation hydrodynamical models of GRB host galaxy environments, including self-consistent radiative transfer of both ionizing and Lyman-Werner (L-W) photons, nine-species primordial chemistry with dust formation of H2, and dust extinction of UV photons. We find that a single GRB progenitor is sufficient to ionize neutral hydrogen to distances of 50-100 pc, but that a galactic L-W background is required to dissociate molecular hydrogen in the ambient ISM. Intensities of 0.1-100 times the Galactic mean are necessary to destroy H2 in the cloud, depending on its density and metallicity. The minimum radii at which neutral hydrogen will be found in afterglow spectra is insensitive to the mass of the progenitor or the initial mass function (IMF) of its cluster, if present.

Recent observations have revealed that high surface brightness, metal-rich debris is present over large regions of the Andromeda (M31) stellar halo. We use a set of numerical models to determine whether extended metal-rich debris is expected to exist in galaxy halos formed in a hierarchical LCDM universe. We identify tidal debris in the simulations according to the current best surface brightness detection limits in M31 and demonstrate that bright features in the outer halo regions usually originate from single satellites, with very little contamination from other sources due to the low probability of tidal streams from two overlapping accretion events. In contrast, high surface brightness features in the inner halo often originate from multiple progenitors. We also compare the age and metallicity distribution of the debris with the well-mixed stellar halos in which they reside. We find that high surface brightness tidal debris is produced almost exclusively by relatively high mass progenitors ( M* [?] 107-109M) and thus is expected to be of moderate to high metallicity. Similarly, in our models the smooth inner halo is expected to be metal-rich, as this region has been built up mainly from massive satellites. Our results imply that the stellar populations of substructure observed around external galaxies with current techniques should not resemble old and metal-poor dwarf spheroidal satellites, nor the underlying component of the stellar halo.

We examine the abundance of molecular hydrogen (H2) in the spectra of gamma ray burst afterglows (GRBs). In nearby galaxies, H2 traces the cold neutral medium (CNM) and dense molecular star-forming interstellar gas. Although H2 is detected in at least half of all sight lines toward hot stars in the Magellanic Clouds and in [?]25% of damped Lya systems toward quasars, it is not detected in any of the five GRB environments with a similar range of neutral hydrogen column density and metallicity. We detect no vibrationally excited H2 that would imply that the GRB itself has photodissociated its parent molecular cloud, so such models are ruled out unless the parent cloud was [?]4 pc in radius and was fully dissociated prior to the spectroscopic observations, or the star escaped its parent cloud during its main-sequence lifetime. The low molecular fractions for the GRBs are mysterious in light of their large column densities of neutral H and expectations based on local analogs, i.e., 30 Doradus in the LMC. This surprising lack of H2 in GRB damped Lya absorbers indicates that the destruction processes that suppress molecule formation in the LMC and SMC are more effective in the GRB hosts, most probably due to a combination of low metallicity and an FUV radiation field 10-100 times the Galactic mean field. These inferred conditions place strong constraints on the star-forming regions in these early galaxies.

The two known "hyper-metal-poor" (HMP) stars, HE 0107-5240 and HE 1327-2326, have extremely high enhancements of the light elements C, N, and O relative to Fe and appear to represent a statistically significant excess population relative to the halo metallicity distribution extrapolated from [Fe/H] > -3. This study weighs the available evidence for and against three hypothetical origins for these stars: (1) that they formed from gas enriched by a primordial "faint supernova," (2) that they formed from gas enriched by core-collapse supernovae and C-rich gas ejected in rotation-driven winds from massive stars, and (3) that they formed as the low-mass secondaries in binary systems at Z ~ 10-5.5 Z and acquired their light-element enhancements from an intermediate-mass companion as it passed through an asymptotic giant branch (AGB) phase. The observations interpreted here, especially the depletion of lithium seen in HE 1327-2326, favor the binary mass-transfer hypothesis. If HE 0107-5240 and HE 1327-2326 formed in binary systems, the statistically significant absence of isolated and/or C-normal stars at similar [Fe/H] implies that low-mass stars could form at this metallicity, but that masses M [?] 1.4 M were disfavored in the IMF. This result is also explained if the abundance-derived top-heavy IMF for primordial stars persists to [Fe/H] ~ -5.5. This finding indicates that low-mass star formation was possible at extremely low metallicity and that the typical stellar mass may have had a complex dependence on metallicity rather than a sharp transition driven solely by gas cooling.

We combine a FUSE sample of O VI absorbers (z < 0.15) with a database of 1.07 million galaxy redshifts to explore the relationship between absorbers and galaxy environments. All 37 absorbers with Nimg1.gif [?] 1013.2 cm-2 lie within 800 himg2.gif kpc of the nearest galaxy, with no compelling evidence for O VI absorbers in voids. The O VI absorbers often appear to be associated with environments of individual galaxies. Gas with 10% +- 5% solar metallicity (O VI and C III) has a median spread in distance of 350-500 himg2.gif kpc around L[?] galaxies and 200-270 himg2.gif kpc around 0.1L[?] galaxies (ranges reflect uncertain metallicities of gas undetected in Lya absorption). In order to match the O VI line frequency, (d[?] /dz) [?] 20 for Nimg1.gif [?] 1013.2 cm-2, galaxies with L [?] 0.1L[?] must contribute to the cross section. Lya absorbers with Nimg3.gif [?] 1013.2 cm-2 cover ~50% of the surface area of typical galaxy filaments. Two-thirds of these show O VI and/or C III absorption, corresponding to a 33%-50% covering factor at 0.1 Z and suggesting that metals are spread to a maximum distance of 800 himg2.gif kpc, within typical galaxy supercluster filaments. Approximately 50% of the O VI absorbers have associated Lya line pairs with separations (Dv)Lya = 50-200 km s-1. These pairs could represent shocks at the speeds necessary to create copious O VI, located within 100 himg2.gif kpc of the nearest galaxy and accounting for much of the two-point correlation function of low-z Lya forest absorbers.

I present a new Galactic chemical evolution model motivated by and grounded in the hierarchical theory of galaxy formation, as expressed by a halo merger history of the Galaxy. This model accurately reproduces the "metallicity distribution function" (MDF) for Population II stars residing today in the Galactic halo. Model MDFs are calculated for a fiducial Galaxy formation scenario and a range of assumptions about the astrophysics of star formation and chemical enrichment at early times. The observed MDF and the apparent absence of true Population III stars from the halo strongly imply that there is some critical metallicity, Zcr [?] 10-4 Z, below which low-mass star formation is inhibited and perhaps impossible. The observed constraints from the halo MDF, relative metal abundances from extremely metal-poor Galactic halo stars, and the ionizing photon budget needed to reionize the IGM together imply a stellar IMF below Zcr that is peaked in the range of massive stars that experience core-collapse supernovae, with mean mass [?]M[?] = 8-42 M. This mass range is similar to the masses predicted by models of primordial star formation that account for formation feedback. A set of five plausible IMF cases is presented, ranging from broadly peaked with mean mass ~15 M to narrowly peaked at mean mass ~70 M. These IMF cases cannot be distinguished formally by the available constraints, but the models with lower characteristic mass produce overall better fits to the available data. The model also implies that metal-poor halo stars below [Fe/H] [?] -3 had only 1-10 metal-free stars as their supernova precursors, such that the relative abundances in these halo stars exhibit IMF-weighted averages over the intrinsic yields of the first supernovae. This paper is the first part of a long-term project to connect the high-redshift in situ indicators of early star formation with the low-z, old remnants of the first stars.

We report results from a Far Ultraviolet Spectroscopic Explorer (FUSE) survey of interstellar molecular hydrogen (H2) along 45 sight lines to AGNs at high Galactic latitudes (img1.gif > 20deg). Most (39 out of 45) of the sight lines show detectable Galactic H2 absorption from Lyman and Werner bands between 1000 and 1126 A, with column densities ranging from Nimg2.gif = 1014.17 to 1019.82 cm-2. In the northern Galactic hemisphere, we identify many regions of low Nimg2.gif ([?]1015 cm-2) between l = 60deg and 180deg and at b > 54deg. These "H2 holes" provide valuable, uncontaminated sight lines for extragalactic UV spectroscopy, and a few may be related to the "Northern Chimney" (low Na I absorption) and the "Lockman Hole" (low Nimg3.gif). A comparison of high-latitude H2 with 139 OB star sight lines surveyed in the Galactic disk suggests that high-latitude and disk H2 clouds may have different rates of heating, cooling, and UV excitation. For rotational states J = 0 and 1, the mean excitation temperature at high latitude, [?]Timg4.gif[?] = 124 +- 8 K, is somewhat higher than that in the Galactic disk, [?]Timg5.gif[?] = 86 +- 20 K. For J [?] 2, the mean [?]Texc[?] = 498 +- 28 K, and the column-density ratios, N(3)/N(1), N(4)/N(0), and N(4)/N(2), indicate a comparable degree of UV excitation in the disk and low halo for sight lines with Nimg2.gif [?] 1018 cm-2. The distribution of molecular fractions at high latitude shows a transition at lower total hydrogen column density (log Nimg6.gif [?] 20.38 +- 0.13) than in the Galactic disk (log Nimg7.gif [?] 20.7). If the UV radiation fields are similar in disk and low halo, this suggests an enhanced H2 (dust-catalyzed) formation rate in higher density, compressed clouds, which could be detectable as high-latitude, sheetlike infrared cirrus.

We present new H I 21 cm absorption data and ultraviolet spectroscopy from Hubble Space Telescope/Space Telescope Imaging Spectrograph of the QSO/galaxy pair 3C 232/NGC 3067. The QSO sight line lies near the minor axis and 1.'8 (11 himg1.gif kpc) above the plane of NGC 3067, a nearby luminous (cz = 1465 +- 5 km s-1, L = 0.5 L*) starburst galaxy with a moderate star formation rate of 1.4 M yr-1. The UV spectra show that the Si IV and C IV doublets have the same three velocity components at cz = 1369, 1417, and 1530 km s-1 found in Ca II H and K, Na I D, Mg I, Mg II, and Fe II, implying that the low- and high-ionization gas are both found in three distinct absorbing clouds (only the strongest component at 1420 km s-1 is detected in H I 21 cm). The new Lya observation allows the first measurements of the spin and kinetic temperatures of halo gas: Ts = 435 +- 140 K and Tk/Ts [?] 1. However, while a standard photoionization model can explain the low ions, the C IV and Si IV are explained more easily as collisionally ionized boundary layers of the photoionized clouds. Because of their small inferred space velocity offsets (Dv = -260, -130, and +170 km s-1) relative to the nucleus of NGC 3067 and the spatial coincidence of low- and high-ionization gas, we propose that these absorbers are analogous to Galactic high-velocity clouds (HVCs). A comparison of the NGC 3067 clouds and Galactic HVCs finds similar H I column densities, kinematics, metallicities, spin temperatures, and inferred sizes. We find no compelling evidence that any halo gas along this sight line is escaping the gravitational potential of NGC 3067, despite its modest starburst.

We present Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) and Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of the QSO PG 1211+143 (zem = 0.081) and a galaxy survey of the surrounding field. This sight line shows two strong intergalactic absorption systems at cz [?] 15,300 and 19,300 km s-1. This sight line addresses the nature and origin of the O VI absorbers and their connection to galaxies. We explore the relationship of these absorbers to the nearby galaxies and compare them to other O VI-bearing absorbers in diverse environments. At 15,300 km s-1, we find four distinct H I components and associated C II, C III, C IV, Si II, Si III, Si IV, N V, and O VI, lying near a spiral-dominated galaxy group with a bright member galaxy 137 himg1.gif kpc from the sight line. The observed ions of C, Si, and N are likely to be photoionized, but the O VI is more consistent with collisional ionization. The ion ratios in this absorber resemble the highly ionized Galactic high-velocity clouds (HVCs); it may also trace the hot intragroup medium gas or the unbound wind of an undiscovered dwarf galaxy. At 19,300 km s-1, we find five H I components and associated C III, Si III, and collisionally ionized O VI lying 146 himg1.gif kpc from an isolated galaxy. The properties of the O VI-bearing gas are consistent with an origin in strong shocks between low-metallicity gas ([?]2%-6% solar) and one or more of the warm photoionized components. It is likely that these absorbers are related to the nearby galaxies, perhaps by outflows or gas stripped from unseen satellite galaxies by interactions. However, we cannot reject completely the hypothesis that they reside in the same large-scale structure in which the galaxies are embedded but are otherwise not directly related.

We critique the hypothesis that the first stars were very massive stars (VMSs; M > 140 M). We review the two major lines of evidence for the existence of VMSs: (1) that the relative metal abundances of extremely metal-poor Galactic halo stars show evidence of VMS enrichment and (2) that the high electron-scattering optical depth (te) to the cosmic microwave background found by the Wilkinson Microwave Anisotropy Probe (WMAP) requires VMSs for reionization in a concordance LCDM cosmology. The yield patterns of VMSs exploding as pair-instability supernovae are incompatible with the Fe-peak and r-process abundances in halo stars. Models including Type II supernovae and/or "hypernovae" from zero-metallicity progenitors with M = 8-40 M can better explain the observed trends. We use the nucleosynthesis results and stellar evolution models to construct an initial mass function (IMF) for reionization. With a simple metal transport model, we estimate that halo enrichment curtails metal-free star formation after ~108 yr at z ~ 20. Because the lifetime-integrated ionizing photon efficiency of metal-free stars peaks at ~120 M and declines at higher mass, an IMF with an approximate lower bound at M ~ 10-20 M and no VMS can maximize the ionizing photon budget and still be consistent with the nucleosynthetic evidence. An IMF devoid of low-mass stars is justified independently by models of the formation of primordial stars. Using a semianalytic model for H I and He II reionization, we find that such an IMF can reproduce te [?] 0.10-0.14, consistent with the range from WMAP, without extreme astrophysical assumptions, provided that metal-free star formation persists 107-108 yr after star formation begins. Because stars in the mass range 50-140 M are the most efficient sources of ionizing photons but are expected to collapse to black holes without releasing metals, this IMF effectively decouples early metal enrichment and early ionization. Such an IMF may allow the unique properties of the zero-metallicity IMF to persist longer than they would in the pure VMS case and to contribute significantly to the global ionizing photon budget before halo self-enrichment and/or interhalo metal transport truncates metal-free star formation. We conclude, on the basis of these results, that VMSs are not necessary to meet the existing constraints commonly taken to motivate them.

We provide an in-depth analysis of the He II and H I absorption in the intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using spectra from the Far Ultraviolet Spectroscopic Explorer and the Ultraviolet-Visual Echelle Spectrograph on the Very Large Telescope. Following up on our earlier study, we focus here on two major topics: (1) small-scale variability (Dz [?] 10-3) in the ratio e = N(He img1.gif)/N(H img2.gif) and (2) an observed correlation of high-e absorbers (soft radiation fields) with voids in the (H I) Lya distribution. These effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc, together with radiative transfer through a filamentary IGM whose opacity variations control the penetration of 1-5 ryd radiation over 30-40 Mpc distances. Given the photon statistics and backgrounds, we can measure optical depths over the ranges 0.1 < timg3.gif < 2.3 and 0.02 < timg4.gif < 3.9 and reliably determine values of e [?] 4timg3.gif/timg4.gif over the range 0.1-460. Values e = 20-200 are consistent with models of photoionization by quasars with observed spectral indices as = 0-3. Values e > 200 may require additional contributions from starburst galaxies, heavily filtered quasar radiation, or density variations. Regions with e < 30 may indicate the presence of local hard sources. We find that e is higher in "void" regions, where H I is weak or undetected and ~80% of the path length has e > 100. These voids may be ionized by local soft sources (dwarf starbursts) or by QSO radiation softened by escape from the active galactic nucleus cores or transfer through the "cosmic web." The apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag between the reionization epochs of H I (zimg4.gif ~ 6.2 +- 0.2) and He II (zimg3.gif ~ 2.8 +- 0.2).

The first stars hold intrinsic interest for their uniqueness and for their potentially important contributions to galaxy formation, chemical enrichment, and feedback on the intergalactic medium (IGM). Although the sources of cosmological reionization are unknown at present, the declining population of large bright quasars at redshifts z > 3 implies that stars are the leading candidates for the sources that reionized the hydrogen in the IGM by z ~ 6. The metal-free composition of the first stars restricts the stellar energy source to proton-proton burning rather than the more efficient CNO cycle. Consequently, they are hotter, smaller, and have harder spectra than their present-day counterparts of finite metallicity. We present new results from a continuing study of metal-free stars from a cosmological point of view. We have calculated evolving spectra of Population III clusters, derived from a grid of zero-metallicity stellar evolutionary tracks. We find that H-ionizing photon production from metal-free stellar clusters takes twice as long as that of Population II to decline to 1/10 its peak value. In addition, metal-free stars produce substantially more photons than Population II in the He II (E > 4 ryd) continuum. We suggest that large Lya equivalent widths (WLya > 400 A) may provide a means of detecting metal-free stellar populations at high redshift and that He II recombination lines (ll1640, 4686) may confirm identifications of Population III. While Population III clusters are intrinsically bluer than their Population II counterparts, nebular continuum emission makes up this difference and may confuse attempts to discern Population III stars with broadband colors. In a companion paper, we explore the consequences of evolving spectra of Population III for the reionization of the IGM in both H and He.

We examine the significance of the first metal-free stars (Population III) for the cosmological reionization of H I and He II. These stars have unusually hard spectra, with the integrated ionizing photon rates from a Population III stellar cluster for H I and He II being 1.6 and 105 times stronger, respectively, than those from a Population II cluster. For the currently favored cosmology, we find that Population III stars alone can reionize H I at redshifts of z [?] 9 and 4.7 and He II at z [?] 5.1 and 0.7 for continuous and instantaneous modes of star formation, respectively. More realistic scenarios involving combinations of Population III and Population II stellar spectra yield similar results for hydrogen. Helium never reionizes completely in these cases; the ionization fraction of He III reaches a maximum of about 60% at z ~ 5.6 if Population III star formation lasts for 109 yr. Future data on H I reionization can test the amount of small-scale power available to the formation of the first objects and provide a constraint on values of s8 [?] 0.7. Since current UV observations indicate an epoch of reionization for He II at z ~ 3, He II may reionize more than once. Measurements of the He II Gunn-Peterson effect in the intergalactic medium at redshifts z [?] 3 may reveal the significance of Population III stars for He II reionization, particularly in "void" regions that may contain "relic" ionization from early Population III stellar activity.

We have developed a code that models the formation, destruction, radiative transfer, and vibrational/rotational excitation of H2 in a detailed fashion. We discuss generally how such codes, together with Far Ultraviolet Spectrographic Explorer (FUSE) observations of H2 in diffuse and translucent lines of sight, may be used to infer various physical parameters. We illustrate the effects of changes in the major physical parameters (UV radiation field, gas density, metallicity), and we point out the extent to which changes in one parameter may be mirrored by changes in another. We provide an analytic formula for the molecular fraction, fimg1.gif, as a function of cloud column density, radiation fields, and grain formation rate of H2. Some diffuse and translucent lines of sight may be concatenations of multiple distinct clouds viewed together. Such situations can give rise to observables that agree with the data, complicating the problem of uniquely identifying one set of physical parameters with a line of sight. Finally, we illustrate the application of our code to an ensemble of data, such as our FUSE survey of H2 in the Large and Small Magellanic Clouds, in order to constrain the elevated UV radiation field intensity and reduced grain formation rate of H2 in those low-metallicity environments.

We report the first ensemble results from the Far Ultraviolet Spectroscopic Explorer survey of molecular hydrogen in lines of sight with AV [?] 1 mag. We have developed techniques for fitting computed profiles to the low-J lines of H2, and thus determining column densities for J = 0 and J = 1, which contain [?]99% of the total H2. From these column densities and ancillary data we have derived the total H2 column densities, hydrogen molecular fractions, and kinetic temperatures for 23 lines of sight. This is the first significant sample of molecular hydrogen column densities of ~1021 cm-2, measured through UV absorption bands. We have also compiled a set of extinction data for these lines of sight, which sample a wide range of environments. We have searched for correlations of our H2-related quantities with previously published column densities of other molecules and extinction parameters. We find strong correlations between H2 and molecules such as CH, CN, and CO, in general agreement with predictions of chemical models. We also find the expected correlations between hydrogen molecular fraction and various density indicators such as kinetic temperature, CN abundance, the steepness of the far-UV extinction rise, and the width of the 2175 A bump. Despite the relatively large molecular fractions, we do not see the values greater than 0.8 expected in translucent clouds. With the exception of a few lines of sight, we see little evidence for the presence of individual translucent clouds in our sample. We conclude that most of the lines of sight are actually composed of two or more diffuse clouds similar to those found toward targets like z Oph. We suggest a modification in terminology to distinguish between a "translucent line of sight" and a "translucent cloud."

We describe a moderate-resolution Far Ultraviolet Spectroscopic Explorer (FUSE) survey of H2 along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. Our survey is sensitive to N(H2) [?] 1014 cm-2; the highest column densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H2 abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractionsimg1.gif = 0.010img2.gif for the SMC andimg1.gif = 0.012img3.gif for the LMC, compared withimg1.gif = 0.095 for the Galactic disk over a similar range of reddening. The dominant uncertainty in this measurement results from the systematic differences between 21 cm radio emission and Lya in pencil beam sight lines as measures of N(H I). These results imply that the diffuse H2 masses of the LMC and SMC are 8 x 106 and 2 x 106 M, respectively, 2% and 0.5% of the H I masses derived from 21 cm emission measurements. The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 x 10-18 cm3 s-1, and incident radiation fields ranging from 10-100 times the Galactic mean value. We find that these high-radiation, low formation rate models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitation ratios in the Clouds. We use H2 column densities in low rotational states (J = 0 and 1) to derive kinetic and/or rotational temperatures of diffuse interstellar gas, and we find that the distribution of rotational temperatures is similar to Galactic gas, with [?]T01[?] = 82 +- 21 K for clouds with N(H2) [?] 1016.5 cm-2. There is only a weak correlation between detected H2 and far-infrared fluxes as determined by IRAS, perhaps as a result of differences in the survey techniques. We find that the surface density of H2 probed by our pencil beam sight lines is far lower than that predicted from the surface brightness of dust in IRAS maps. We discuss the implications of this work for theories of star formation in low-metallicity environments.

We use high-resolution UV spectra of the radio-quiet quasar (QSO) H1821+643 (zem = 0.297), obtained with the Space Telescope Imaging Spectrograph (STIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE), to study the ionization and metallicity of an intervening O VI absorption line system at zabs = 0.1212. This absorber has the following notable properties: (1) several galaxies are close to the sight line at the absorber redshift, including an actively star-forming galaxy at a projected distance of 144 himg1.gif kpc, (2) there is a complex cluster of H I Lya absorption lines near the O VI redshift, including at least five components spread over a velocity range of ~700 km s-1, (3) the strongest Lya line in the cluster appears to be composed of a mildly saturated component with a typical b-value blended with a remarkably broad component with b [?] 85 km s-1, (4) the O VI absorption is not aligned with the strongest (saturated) H I absorption, but instead is well aligned with the very broad component, and (5) the only detected species (at the 4 s level) are O VI and H I, despite coverage of strong transitions of abundant elements (e.g., C II, C III, and C IV). Based on these constraints, we find that the absorption line properties can be produced in collisionally ionized gas with 105.3 [?] T [?] 105.6 K and -1.8 [?] [O/H] [?] -0.6. However, we find that photoionization is also viable if the path length l through the absorbing gas is long enough; simple photoionization models require 85 [?] l [?] 1900 kpc and -1.1 [?] [O/H] [?] -0.3. We briefly discuss how observations of X-ray absorption lines due to O VII and O VIII could be used, in principle, to break the ionization mechanism degeneracy, and we conclude with some comments regarding the nature of O VI absorbers.

We report the second study from the FUSE survey of molecular hydrogen in translucent clouds for the line of sight toward HD 110432. This star lies beyond the Coalsack dark nebula, and, with E(B-V) = 0.40 and AV = 1.32, this line of sight bridges the gap between less-extinguished diffuse-cloud lines of sight with AV ~ 1, such as z Ophiuchi, and translucent-cloud lines of sight with AV [?] 2, such as HD 73882. Through profile fitting and a curve-of-growth analysis, we have derived rotational populations for H2 for J = 0-7. The line of sight has a total molecular hydrogen column density, log N(H2) = 20.68 +- 0.05 cm-2, nearly identical to that toward z Oph but a factor of 3 less than that toward HD 73882. The ratio of N(J = 1) to N(J = 0) yields a kinetic temperature Tkin = 63 +- 7 K, which is similar to other lines of sight with AV [?] 1. The high-J lines show considerable excitation above this temperature. However, the high-J lines cannot be well fitted to a single excitation temperature, and the even-J lines of para-hydrogen appear slightly enhanced relative to the odd-J lines of ortho-hydrogen. The high-J excitation is similar to that toward z Oph but is much smaller than that toward HD 73882. Chemical modeling indicates that the physical conditions in the cloud(s) are very similar to those in the cloud(s) toward z Oph. An analysis of IUE spectra of the Lya line gives N(H I) = 20.85 +- 0.15 cm-2. Combined with N(H2), we derive a hydrogen molecular fraction, fimg1.gif = 0.58 +- 0.12, statistically identical to that found for the lines of sight toward z Oph and HD 73882. From the FUSE data and a curve-of-growth analysis using the same component structure as H2, we find log N(HD) = 15.2img2.gif cm-2, which is between the values found for z Oph and HD 73882. Profile fitting suggests smaller b values for HD than for H2, and log N(HD) = 16.0img3.gif cm-2. From FUSE and IUE data, we derive log N(CO) [?] 14.3 cm-2, assuming the same component structure as CH or log N(CO) [?] 14.8 cm-2, if all observed CO is cospatial with the strongest CH component. From the combined measurements of hydrogen- and carbon-containing molecules, the line of sight toward HD 110432 appears quite similar to the diffuse-cloud line of sight toward z Oph and quite dissimilar to the translucent-cloud line of sight toward HD 73882. Upcoming FUSE observations will further explore the transition between diffuse and translucent clouds.

We present new spectra obtained with the Faint Object Spectrograph aboard the Hubble Space Telescope of the HH 47A bow shock and Mach disk that cover the entire spectral range between ll2220 and 6810. In addition to emission lines seen previously from HH objects, we uncover over a dozen weak Fe IItransitions in the ultraviolet. The flux ratios between these permitted lines can only be understood if transitions to the ground state are resonantly scattered within HH 47A. The expected column density of Fe II within HH 47A suffices to scatter these lines, although the scattering optical depths imply that the Fe II line broadening must exceed that expected from thermal motions. Excitation of ultraviolet Fe II occurs locally within HH 47A, probably from collisions within the hot postshock gas and not from UV pumping from some nearby O stars. The data show no evidence for significant depletion of Fe within HH 47A. The emission line's fluxes and ratios indicate that jet material currently enters the Mach disk with a density of ~350 cm-3 and a velocity of ~40 km s-1. The mass-loss rate of the exciting star, as measured by the mass flux through the Mach disk, is 1.6x10$\mathstrut{^{-8}}$ M yr-1. This mass-loss rate is considerably lower than that closer to the star where the jet is brighter, probably because the density along the jet is highly nonuniform. A single-shock velocity does not match the bow shock spectrum well. We propose that secondary shocks reheat the gas within the cooling zone of the HH 47A bow shock. Compression from the first shock will cause these secondary shocks to be strongly magnetized, and the secondary shocks should emit strongly in low-excitation lines such as Mg II, C II], and [S II]. The weak blue continua seen at optical wavelengths in spectra of the Mach disk and bow shock extend into the ultraviolet and have spectral energy distributions and total fluxes consistent with those expected from two-photon emission.

We present the science cases and technological discussions that came from the workshop titled "Finding the ultraviolet (UV)-Visible Path Forward" held at NASA GSFC 2015 June 25-26. The material presented outlines the compelling science that can be enabled by a next generation space-based observatory dedicated for UV-visible science, the technologies that are available to include in that observatory design, and the range of possible alternative launch approaches that could also enable some of the science. The recommendations to the Cosmic Origins Program Analysis Group from the workshop attendees on possible future development directions are outlined.