According to Pei, Fall, & Hauser (1999), the global metallicity evolution of the Universe can be represented by the ratio of the total metal content to the total gas content measured in damped Lyman–α (DLA) systems (the “column density weighted metallicity” à la Pettini). To minimize dust obscuration effects, a DLA sample with negligible dust content is considered, namely, 50 DLAs with log NHI < 20.8. The global metallicity found shows clear evidence of redshift evolution that goes from ~ 1/30 solar at z ~ 4.1 to solar at z ~ 0.4. More generally, DLAs with measured heavy elements probe the ISM of high redshift galaxies. The whole sample collected from the literature contains 75 DLAs. The metallicity is calculated adopting for the dust correction the most general method used so far, based on models of the ISM dust depletions in the Galaxy. The intrinsic metallicity evolution of DLA galaxies is d log ZDLA/dz = −0.33 ± 0.06.

We still know relatively little about the local metal density outside of galaxies and clusters, but at high redshifts (z ~ 3) the advent of highly efficient high-resolution spectrographs on the new 8–10m telescopes has revolutionized the study of quasar absorption lines. This allows us to track the bulk of the baryons in the Universe, which are in highly observable form at this redshift, and to determine the metal density of the gas that contains most of the baryonic material. One of the more surprising results that has emerged is how uniformly the intergalactic gas at redshifts 2–4 is pervaded by metals, and we discuss current limits on such metal contamination of the low density gas. These measurements will be useful in understanding and calibrating feedback processes in early generations of small galaxies. Future work will track the metal distribution at higher redshift using bright z = 5 SDSS quasars.

Elemental abundance patterns in very metal-poor halo field stars and globular cluster stars play a crucial role both in guiding theoretical models of nucleosynthesis and in providing constraints upon the early star formation and concomitant nucleosynthesis history of our Galaxy. The abundance patterns characterizing the oldest and most metal deficient stars ([Fe/H] ≤ −3) are entirely consistent with their being products of metal-poor massive stars of lifetimes τ ≤ 108 years. This includes both the elevated abundances of the alpha-elements (O, Mg, Si, S, Ca, and Ti) relative to iron-peak elements and the dominance of r-process elements over s-process elements. The nucleosynthetic contributions of lower mass AGB stars of longer lifetimes (τ ≈ 109 years) begin to appear at metallicities [Fe/H] ≈ −2.5, while clear evidence for iron-peak nuclei produced in supernovae Ia (τ ≥ 1-2x109 years?) does not appear until metallicities approaching [Fe/H] ~ −1. Similar trends are also suggested by abundances determined for gas clouds at high redshifts. We review the manner in which a knowledge of the abundances of the stellar and gas components of early populations, as a function of [Fe/H], time, and/or redshift, can be used to set constraints on their star formation and nucleosynthesis histories.

I review the recent Chandra results on the sources of the X-ray background and the X-ray properties of the SCUBA sources. We conclude that 10–20% of the IR background is produced by active galaxies and a similar fraction of the SCUBA sources harbor luminous AGN. Many of the Chandra sources are apparently luminous infrared galaxies themselves, but factors of 2–10 below the SCUBA limits. We summarize the X-ray evidence for metal production in groups and clusters and point out that these data require considerably more star formation than inferred from optical stellar data. The abundance ratios of Fe and Si indicate that much of the metals in groups and clusters was produced by massive stars, and the lack of evolution in Fe out to z ~ 0.5 argues for quite an early origin for the metals. This same process also seems to have injected considerable energy into the gas in groups and clusters, which may have dominated the mechanism of star formation and produced a metal-enriched intergalactic medium

Metcalfe et al. (1995, 1996) have shown that galaxy counts from the UV to the near-IR are well-fitted by simple evolutionary models where the space density of galaxies remains constant with look-back time while the star-formation rate rises exponentially. We now extend these results, first by using data from the Herschel Deep Field to show that these same models give detailed fits to the faint galaxy r - i : b - r colour-colour diagram. We then use these models to predict the number counts of high redshift galaxies detected by the Lyman break technique. At z ≈ 3 there is almost exact agreement between our prediction and the data, suggesting that the space density of galaxies at z ≈ 3 may be close to its local value. At z ≈ 4 the space density of bright galaxies remains unchanged; however, the space density of dwarf galaxies is significantly lower than it is locally, suggesting that we have detected an epoch of dwarf galaxy formation at z ≈ 4. Finally, significant numbers of Lyman-break galaxy candidates are also detected at z ≈ 6 in the Hubble and Herschel Deep Fields; taking this observation together with a number of recent detections of spectroscopically confirmed z ≈ 6 galaxies suggests that the space density of bright galaxies at z ≈ 6 remains comparable to the local space density, and thus that the epoch of formation of bright galaxies may lie at yet higher redshift.

The optical/far–IR extragalactic background light (EBL) from both resolved and unresolved extragalactic sources is an indicator of the total luminosity of cosmic structures, as the cumulative emission from young and evolved galactic systems, as well as from active galactic nuclei (AGNs), is recorded in this radiation. This is a brief review of some of the implications of the observed brightness of the night sky for the stellar mass density and average metallicity of the universe today, and of the possible contribution of MACHO progenitors and QSOs to the EBL. Assuming a Salpeter initial mass function with a cutoff below 0.6 M⊙, a lower limit of Ωg+Sh2 > 0.0015 I60 can be derived to the visible (recycled gas + stars) mass density required to generate an EBL at a level of IEBL = 60 I60 nW m−2 sr−1. Our latest, ‘best–guess’ estimate is Ωg+sh2 ≈ 0.0023 I60, which implies a mean metallicity at the present–epoch of yZΩg+s/Ωb ≈ 0.2 Z⊙. If massive dark halos around spiral galaxies are partially composed of faint, old white dwarfs, i.e., if a non–negligible fraction (~ a few percent) of the nucleosynthetic baryons is locked in the remnants (MACHOs) of intermediate–mass stars forming at very high redshifts, then the bright early phases of such halos should contribute significantly to the observed EBL. Assuming a standard black hole accretion model for quasar activity and using recent observations of the quasar population and new synthesis models for the cosmic X-ray background, we estimate a present mass density of QSO remnants of ρBH ≈ 3 x 105 M⊙ Mpc−3 for a 10% efficiency of accreted mass–to–radiation conversion. The quasar contribution to the brightness of the night sky is IQSO ≈ 2 nW m−2 sr−1.

Recent advances have led to the discoveries of both galaxies and quasars out to z ~ 6. The reionization epoch itself, thought to lie somewhere in the range 6 < zi < 15, may be within reach over the next decade. Prospects for the detection of the first luminous objects in the Universe are reviewed - the objects that reionized the Universe and brought an end to the “Dark Ages”. Alternative possibilities for detecting the reionization epoch are also discussed, including direct detection of the intergalactic medium itself at that epoch.

The number density and clustering properties of Lyman-break galaxies (LBGs) observed at redshift z ~ 3 are best explained by assuming that they are associated with the most massive haloes at z ~ 3 predicted in hierarchical models of structure formation. In this paper we study, under the same assumption, how star formation and chemical enrichment may have proceeded in the LBG population. A consistent model, in which the amount of cold gas available for star formation must be regulated, is suggested. It is found that gas cooling in dark haloes provides a natural regulation process. In this model, the star formation rate in an LBG host halo is roughly constant over about 1 Gyr. The predicted star formation rates and effective radii are consistent with observations. The metallicity of the gas associated with an LBG is roughly equal to the chemical yield, or about the order of 1Z⊙ for a Salpeter IMF. The contribution to the total metals of LBGs is roughly consistent with that obtained from the observed cosmic star formation history. The model predicts a marked radial metallicity gradient in a galaxy, with the gas in the outer region having much lower metallicity. As a result, the metallicities for the damped Lyman-alpha absorption systems expected from the LBG population are low. Since LBG halos are filled with hot gas in this model, their contributions to the soft X-ray background and to the UV ionization background are calculated and discussed.

A detailed analysis of the oxygen abundance profile has been carried out on a sample of spiral galaxies from which very good data was available. The early-type galaxies of our sample display gradients that are flatter, and overall levels of O/H abundances that are higher, than those of normal late-type galaxies. Early-type galaxies show an identical trend in the behavior of extrapolated central abundance versus morphological type to that shown by late-type galaxies with strong bars, even in the absence of a bar. On a diagram showing extrapolated central abundances versus morphological types, two clearly separated sequences appear: late-type barred galaxies and early-type (barred or unbarred) galaxies clearly fall on a sequence 0.5 dex in abundance below that of normal late-type galaxies. This behavior is consistent with theoretical models of morphological evolution of disk galaxies by the formation and dissolution of a bar over a period of a few 109 yr, where later type galaxies (Sd, Sc, SBc) evolve into earlier-type disk galaxies (Sb, Sa) through transitory SBc and SBb phases.

The extragalactic background light (EBL), exclusive of the cosmic microwave background, consists of the cumulative radiative output from all energy sources in the universe since the epoch of recombination. Most of this energy is released at ultraviolet and optical wavelengths. However, observations show that a significant fraction of the EBL falls in the 10 to 1000 μm wavelength regime. This provides conclusive evidence that we live in a dusty universe, since only dust can efficiently absorbs a significant fraction of the background energy and reemit it at infrared wavelengths. The general role of dust in forming the cosmic infrared background (CIB) is therefore obvious. However, its role in determining the exact spectral shape of the CIB is quite complex. The CIB spectrum depends on the microscopic physical properties of the dust, its composition, abundance, and spatial distribution relative to the emitting sources, and its response to evolutionary processes that can modify all the factors listed above. This paper will present a brief summary of the many ways dust affects the intensity and spectral shape of the cosmic infrared background. In an Appendix we present new limits on the mid-infrared intensity of the CIB using TeV γ-ray observations of Mrk 501.

This article outlines a method to interpret the extragalactic background light in terms of the large-scale, average properties of galaxies, including the comoving densities of stars and interstellar gas, metals, and dust. These quantities are related by a series of coupled conservation-type equations analogous to the equations of galactic chemical evolution. This approach enables us to combine observations of the emission and absorption in galaxies and thus to relate their average stellar and interstellar contents. Applications of the method include predictions of the global history of star formation from absorption-line observations and corrections to the cosmic UV emissivity for absorption by dust.

By consistently connecting the chemical and spectrophotometric evolution of galaxies we are able to include the effect of dust absorption. The time and redshift evolution of the extinction is based on the evolution of the gas content and metallicity. We present our evolutionary synthesis models which include dust absorption to analyze the UV emission in various galaxy types. We are able to predict the extinction E(B-V) for different galaxy types as a function of redshift. We further use these models to explore the range of metallicities in normal and starbursting galaxies and the metallicity distribution of the stellar population. Comparing our model spectral energy distributions (SEDs) with templates from R. C. Kennicutt's (1992, ApJS 79, 255) and A. L. Kinney et al.'s (1996, ApJ, 467, 38) atlas we show the detailed agreement with integrated spectra of galaxies and point out the importance of aperture effects. Combined with a cosmological model we show the differences in the evolutionary and K-corrections by comparing models with and without dust.

Infrared (IR) emission from Galactic dust is frequently used as an indicator of the star formation rate (SFR). Recently, A. K. Inoue, H. Hirashita, & H. Kamaya derived a formula for conversion from the IR luminosity to the SFR based on a physical model of H II regions (PASJ, 52, 539, 2000). They expressed this as SFR/(M⊙ yr−1) = {1.1 x 10-10(1-η)/(0.4–0.22f+0.6∊)}(LIR/L⊙), where f is the fraction of ionizing photons absorbed by hydrogen, ∊ is the efficiency of dust absorption for non-ionizing photons, η is the cirrus fraction of the observed dust luminosity, and LIR is the observed luminosity of dust emission. Since f depends on the dust-to-gas ratio and the dust-to-gas ratio is related to metallicity, we present the dependence of the formula on metallicity.

Our metallicity-dependent conversion formula is applied to the cosmic star formation history. Based on a recent model of the cosmic star formation history and metal enrichment history, we find that the photons from OB stars are not efficiently reprocessed in the IR before z ~ 3 because of a low dust-to-gas ratio. This indicates that the star formation rate estimated from the submillimeter luminosity using an empirical formula is significantly underestimated (by at least a factor of 3).

In this paper, we investigate the star formation and chemical evolution of damped Lyman-α systems (DLAs) based on the disk galaxy formation model developed by H. J. Mo, S. Mao, & S. D. M. White (1998, MNRAS, 295, 319). We propose that the DLAs are the central galaxies of less massive dark haloes present at redshifts z ~ 3, and that they should inhabit haloes of moderately low circular velocity. We adopt the empirical Schmidt law of star formation rates, and a closed box model of chemical evolution in which an approximation known as instantaneous recycling is assumed. In calculating the predicted distribution of metallicities for DLAs in our models, two cases are considered. One is that, using the closed box model, empirical Schmidt law, and star formation epoch, the distribution of metallicity can be directly calculated. The other is that, when the simple gravitational instability of a thin isothermal gas disk first discussed by A. Toomre (1964, ApJ, 139, 1217) is considered, star formation occurs only in the region where the surface density of gas satisfies the critical value — rather than everywhere in the gas disk. We first obtain the region where star formation can occur by assuming that the disk has a flat rotation curve and that the rotational velocity is equal to the circular velocity of the surrounding dark matter halo. We then calculate the metallicity distribution for case one. We assume that star formation in each DLA lasts for a period of 1 Gyr from redshifts z = 3. There is only one output parameter in our model, i.e., the stellar yield, which relates to the epoch of star formation. It is obtained by normalizing the predicted distribution of metallicity to the mean value of 1/13 Z⊙ as presented by M. Pettini, L. J. Smith, D. L. Kind, & R. W. Hunstead (1997, ApJ, 486, 665). The predicted metallicity distribution is consistent with current (rather limited) observational data. A random distribution of galactic disks is taken into account.

A detailed investigation of the abundance gradients and their evolution along the Galactic disk has recently appeared (Hou, J. L., Prantzos, N., & Boissier, S. 2000, A&A, in press; astro-ph/0007164). A chemical evolution model of S. Boissier & N. Pranzos (1999, MNRAS, 307, 857) was quite successful in reproducing the main observational constraints both in the solar neighborhood and the entire Milky Way disk. Studied elements include He, C, N, O, Ne, Mg, Al, Si, S, Ar and Fe. We use metallicity dependent yields for massive stars with and without mass loss. We find that most observed abundance profiles are correctly reproduced by massive star yields, but C and N require supplementary sources. We argue that massive, mass losing stars can totally account for the abundance profile of C, while intermediate mass stars are the main source of N. We also find that the adopted “inside-out” formation scheme for the Milky Way disk produces abundance profiles steeper in the past. Using current data on planetary nebulae of type I, II, and III, on N, Ne, S, Ar as observational constraints for gradient evolution, we find that it is difficult to conclude whether the gradient steepens or flattens with time. However, for a given interval of Galactic age, our model predicts that the corresponding abundance scatter is smaller in the inner disk than in the outer regions.

The far infrared background is a sink for the hidden aspects of galaxy formation. At optical wavelengths, ellipticals and spheroids are old, even at z ~ 1. Neither the luminous formation phase nor their early evolution is seen in the visible. We infer that ellipticals and, more generally, most spheroids must have formed in dust-shrouded starbursts. In this article, we show how separate tracking of disk and spheroid star formation enables us to infer that disks dominate near the peak in the cosmic star formation rate at z ≲ 2 and in the diffuse ultraviolet/optical/infrared background, whereas spheroid formation dominates the submillimetre background.

The electronic catalog of infrared and optical photometry in the Hubble Deep Field South (NICMOS) identifies galaxies at redshifts ranging from z near 0 through z greater than 10. In this paper we try to investigate the formation and evolution of different structures in the Universe, using cosmological N-body simulations. By means of 2563/5123-point, 5–25 Mpc simulations, we traced the relation among the evolution of first order filamentary web structures, galactic and cluster structures, and second order filamentary web structures. These simulations have been made in order to derive the environmental effects (first/second order collapse, heating/cooling mergers) in the early Universe (10 > z > 2), closely related to galactic and cluster evolution.

IRAS, COBE, and ISO have demonstrated the unique importance of a cryogenic infrared telescope in space for studying diffuse infrared backgrounds and for teasing out the individual point sources which contribute to them. This importance results from the extremely high infrared sensitivity of such telescopes, particularly to diffuse radiation. The next cryogenic infrared telescope will be NASA's Space Infrared Telescope Facility (SIRTF), which is currently in the final stages of construction leading to launch in 2002. SIRTF will be the first infrared space observatory to make extensive use - both for imaging and spectroscopy - of large format infrared detector arrays. The sensitivity and spatial and spectral coverage of SIRTF's array-based instruments endow SIRTF with great power for the study of the cosmic infrared background (CIRB) and related scientific issues. This paper reviews the SIRTF mission design and measurement functionality and describes SIRTF's potential studies of the CIRB, drawing examples from the programs planned by the SIRTF Guaranteed Time Observers (GTO's). We also summarize the opportunities for community participation in SIRTF.

ASTRO-F (IRIS) is the first Japanese satellite dedicated to infrared astronomy. The primary purpose of this project is to investigate the birth and evolution of galaxies in the early universe through deep, wide-field surveys at wavelengths ranging from 2 to 200 microns. The spatial resolution and the point source sensitivity are nearly the same as those of the aperture diffraction limit and the natural background and/or confusion limit, respectively. In the far-infrared wavelength band, ASTRO-F will conduct an all-sky survey like the IRAS survey with several tens of times higher sensitivity and several times better spatial resolution. In the near- and mid-infrared, wide area sky-surveys will be conducted over pre-selected portions of the sky. In addition to these photometric surveys, low-resolution spectroscopic capabilities are available for all wavelength bands. The ASTRO-F mission will produce a fundamental database for the next generation of advanced observatories, for example FIRST, and NGST, and will complement the SIRTF mission by virtue of its wide sky coverage. The launch by an M-V rocket is scheduled for February or March of 2004.