Chemical Enrichment History Research Articles

Uvby-βphotometry of high-velocity and metal-poor stars

uvby(-β) photometry has been obtained for an additional 411 very metal-poor stars selected from the HK survey, and used to derive basic parameters such as interstellar reddenings, metallicities, photometric classifications, distances, and relative ages. Interstellar reddenings adopted from the Schlegel et al. ([CITE]) maps agree well with those from the intrinsic-color calibration of Schuster & Nissen ([CITE]). [Fe/H] values are obtained from the CaII K line index of the HK survey combined with the uvby and UBV photometry. The diagram is seen to be very useful for classifying these very metal-poor field stars into categories similar to those derived from globular cluster color-magnitude diagrams; the HK survey has detected metal-poor candidates extending from the red-giant to the blue-horizontal branch, and from the horizontal branch to subluminous stars. Distances derived from UBV photometry agree reasonably well with those from uvby, considering the paucity of good calibrating stars and the extrapolations required for the most metal-poor stars. These very metal-poor stars are compared to M 92 in the diagram, and evidence is seen for field stars 1-3 Gyrs younger than this globular cluster; uncertainties in the [Fe/H] scale for M 92 would only tend to increase this age difference. Significant reddening uncertainties for M 92 are unlikely but might decrease this difference. The significance of these younger very metal-poor stars is discussed in the context of Galactic evolution, mentioning such possibilities as hierarchical star-formation/mass-infall of very metal-poor material and/or accretion events whereby this material has been acquired from other (dwarf) galaxies with different formation and chemical-enrichment histories.

The Era of Massive Population III Stars: Cosmological Implications and Self‐Termination

The birth and death of the first generation of stars have important implications for the thermal state and chemical properties of the intergalactic medium (IGM) in the early universe. Sometime after recombination, the neutral chemically pristine gas was reionized by ultraviolet photons emitted from the first stars but was also enriched with heavy elements when these stars ended their lives as energetic supernovae. Using the results from previous high-resolution cosmological simulations of early structure formation that include radiative transfer, we show that a significant volume fraction of the IGM can be metal polluted, as well as ionized, by massive Population III stars formed in small-mass (~106-107 M?) halos early on. If most of the early generation stars die as pair-instability supernovae with energies up to ~1053 ergs, the volume-averaged mean metallicity will quickly reach Z ~ 10-4 Z? by a redshift of ~15-20, possibly causing a transition to the formation of a stellar population that is dominated by low-mass stars. In this scenario, the early chemical enrichment history should closely trace the reionization history of the IGM, and the end of the Population III era is marked by the completion of reionization and preenrichment by z ~ 15. We conclude that, while the preenrichment may partially account for the metallicity floor in high-redshift Ly? clouds, it does not significantly affect the elemental abundance in the intracluster medium.

Quantifying the Drivers of Star Formation on Galactic Scales. I. The Small Magellanic Cloud

We use the star formation history of the Small Magellanic Cloud (SMC) to place quantitative limits on the effect of tidal interactions and gas infall on the star formation and chemical enrichment history of the SMC. The coincident timing of two recent ( 10% and could be as large as 70%.

The Star Formation History of the Small Magellanic Cloud

We present the spatially resolved star formation and chemical enrichment history of the Small Magellanic Cloud (SMC) across the entire central 4° × 45 area of the main body, based on UBVI photometry from our Magellanic Clouds Photometric Survey. We find that (1) approximately 50% of the stars that ever formed in the SMC formed prior to 8.4 Gyr ago (z > 1.2 for WMAP cosmology); (2) the SMC formed relatively few stars between 8.4 and 3 Gyr ago; (3) there was a rise in the mean star formation rate during the most recent 3 Gyr punctuated by "bursts" at ages of 2.5, 0.4, and 0.06 Gyr; (4) the bursts at 2.5 and 0.4 Gyr are temporally coincident with past perigalactic passages of the SMC with the Milky Way; (5) there is preliminary evidence for a large-scale annular structure in the 2.5 Gyr burst; and (6) the chemical enrichment history derived from our analysis is in agreement with the age-metallicity relation of the SMC's star clusters. Consistent interpretation of the data required an ad hoc correction of 0.1–0.2 mag to the B-V colors of 25% of the stars; the cause of this anomaly is unknown, but we show that it does not strongly influence our results.

Iron abundance diagnostics in high-redshift QSOs

The abundance of α-process elements such as magnesium and carbon relative to iron, measured from the broad emission lines of QSOs, can serve as a diagnostic of the star formation and chemical enrichment histories of their host galaxies. We investigate the relationship between Fe/Mg and Fe/C abundance ratios and the resulting Fe II / Mg II λ2800 and Fe II / 1900Å-blend flux ratios, both of which have been measured in QSOs out to z≈6. Using a galactic chemical evolution model based on a starburst in a giant elliptical galaxy, we find that these flux ratios are good tracers of the chemical enrichment of the nuclei. However, the values of these ratios measured in objects at z≈6 suggest that iron enrichment has occurred more rapidly in these objects than predicted by the assumed elliptical starburst model, assuming currently favored cosmologies. This in turn suggests that refinements to the galactic chemical evolution models are needed.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

DeepHubble Space TelescopeImaging of IC 1613. II. The Star Formation History

We have taken deep images of an outlying field in the Local Group dwarf irregular galaxy IC 1613 with the WFPC2 aboard the Hubble Space Telescope in the standard broadband F555W (V, 8 orbits) and F814W (I,16 orbits) filters. The photometry reaches to V=27.7 (M-V=+3.4) and I=27.1 (M-I=+2.8) at the 50% completeness level, the deepest to date for an isolated dwarf irregular galaxy. We analyze the resulting color-magnitude diagram (CMD) and compare it with CMDs created from theoretical stellar models using three different methods to derive a star formation history (SFH) as well as constrain the chemical evolution for IC 1613. All three methods find an enhanced star formation rate (SFR), at roughly the same magnitude (factor of 3), over roughly the same period (from 3 to 6 Gyr ago). Additionally, all three methods were driven to similar age-metallicity relationships (AMR) that show an increase from [Fe/H]approximate to-1.3 at earliest times to [Fe/H]approximate to-0.7 at present. Good agreement is found between the AMR which is derived from the CMD analysis and that which can be inferred from the derived SFH at all but the earliest ages. The agreement between the three models and the self-consistency of the derived chemical enrichment history support the reality of the derived SFH of IC 1613 and, more generally, are supportive of the practice of constructing galaxy SFHs from CMDs. A comparison of the newly observed outer field with an earlier studied central field of IC 1613 shows that the SFR in the outer field has been significantly depressed during the last Gyr. This implies that the optical scale length of the galaxy has been decreasing with time and that comparison of galaxies at intermediate redshift with present-day galaxies should take this effect into account. Comparing the CMD of the outer field of IC 1613 with CMDs of Milky Way dSph companions, we find strong similarities between IC 1613 and the more distant dSph companions (Carina, Fornax, Leo I, and Leo II) in that all are dominated by star formation at intermediate ages. In particular, the SFH and AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr galaxies cannot be distinguished from dSphs by their intermediate-age stellar populations. This type of a SFH may also be evidence for slower or suppressed early star formation in dwarf galaxies due to photoionization after the reionization of the universe by background radiation. Assuming that IC 1613 is typical of a dIrr evolving in isolation, since most of the star formation occurs at intermediate ages, these dwarf systems cannot be responsible for the fast chemical enrichment of the intergalactic medium that is seen at high redshift. There is no evidence for any large-amplitude bursts of star formation in IC 1613, and we find it highly unlikely that analogs of IC 1613 have contributed to the excess of faint blue galaxies in existing galaxy redshift surveys.

Chemical Substructure in the Milky Way Halo: A New Population of Old Stars

We report the results of a coherent study of a new class of halo stars defined on the basis of the chemical compositions of three metal-poor objects (½Fe=H �’� 2) that exhibit unusually low abundances of � -element (Mg, Si, Ca) and neutron-capture (Sr, Y, Ba) material. Our analyses confirm and expand on earlier reports of atypical � - and neutron-capture abundances in BD +80 � 245, G4-36, and CS 22966-043. We also find that the latter two stars exhibit unusual relative abundance enhancements within the iron peak (Cr, Mn, Ni, Zn), along with what may be large abundances of Ga, an element not previously reported as being observed in any metal-poor star. These results provide further evidence that chemical enrichment and star formation histories varied from region to region within the Milky Way halo. Comparing the chemical abundances of the newly identified stellar population to supernova model yields, we derive supernova ratios of Type Ia versus Type II events in the range of 0:6dðNIa=NIIÞNew Popd1:3. For the Sun, we derive 0:18 � 0:01 < ðNIa=NIIÞ� < 0:25 � 0:06, supernova ratios in good agreement with values found in the literature. Given the relatively low metallicity and relatively high NIa=NII hi ratios of the low-� stars studied here, these objects may have been born from material produced in the yields of the earliest Type Ia supernova events. We also report the results of a preliminary attempt to employ the observed chemical abundances of low-metallicity stars in the identification, and possible cosmic evolution, of Type Ia supernova progenitors, and we discuss the limitations of current model yields. Subject headings: Galaxy: abundances — Galaxy: halo — nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: Population II — supernovae: general On-line material: color figures, machine-readable table

The ESI/Keck II Damped Lyα Abundance Database

This paper presents chemical abundance measurements for 37 damped Ly? systems at z > 2.5 observed with the Echellette Spectrograph and Imager on the Keck II telescope. We measure the H I column densities of these damped systems through Voigt profile fits to their Ly? profiles, and we implement the apparent optical depth method to determine ionic column densities. Figures and tables of all relevant data are presented. A full analysis of the chemical enrichment history described by these observations will be presented in a future paper. This data set is also valuable for efficiently planning future echelle observations and for rough abundance pattern analyses. We aim to make this entire data set public within 3 years of this publication.

DeepHubble Space TelescopeImaging of Sextans A. III. The Star Formation History

We present a measurement of the star formation history of Sextans A, based on WFPC2 photometry that is 50% complete to V = 27.5 (MV ~ +1.9) and I = 27.0. The star formation history and chemical enrichment history have been measured through modeling of the color-magnitude diagram (CMD). We find evidence for increased reddening in the youngest stellar populations and an intrinsic metallicity spread at all ages. Sextans A has been actively forming stars at a high rate for ~2.5 Gyr ago, with an increased rate beginning ~0.1 Gyr ago. We find a nonzero number of stars older than 2.5 Gyr, but because of the limited depth of the photometry, a detailed star formation history at intermediate and older ages has considerable uncertainties. The mean metallicity was found to be [M/H] ≈ -1.4 over the measured history of the galaxy, with most of the enrichment happening at ages of at least 10 Gyr. We also find that an rms metallicity spread of 0.15 dex at all ages allows the best fits to the observed CMD. We revisit our determination of the recent star formation history (age ≤ 0.7 Gyr) using blue helium-burning (BHeB) stars and find good agreement for all but the last 25 Myr, a discrepancy resulting primarily from different distances used in the two analyses and the differential extinction in the youngest populations. This indicates that star formation histories determined solely from BHeB stars should be confined to CMD regions where no contamination from reddened main-sequence stars is present.

Quasar Elemental Abundances at High Redshifts

We examine rest-frame ultraviolet spectra of 70 high-redshift quasars (z ≥ 3.5) to study the chemical enrichment history of the gas closely related to the quasars and thereby estimate the epoch of first star formation. The fluxes of several ultraviolet emission lines were investigated within the framework of the most recent photoionization models to estimate the metallicity of the gas associated with the high-redshift quasars. Standard photoionization parameters and the assumption of secondary nitrogen enrichment indicate an average abundance of Z/Z☉ 4-5 in the line-emitting gas. Assuming a timescale of τevol 0.5-0.8 Gyr for the chemical enrichment of the gas, the first major star formation for quasars with z 4 should have started at a redshift of zf 6-8, corresponding to an age of the universe of several times 108 yr (H0 = 65 km s-1 Mpc-1, ΩM = 0.3, ΩΛ = 0.7). We note that this also appears to be the era of reionization of the universe. Finally, there is some evidence for a positive luminosity-metallicity relation in this high-redshift quasar sample.

Differences in Carbon and Nitrogen Abundances between Field and Cluster Early-Type Galaxies

Central line-strength indices were measured in the blue spectral region for a sample of 98 early-type galaxies in different environments. For most indices (Mg b and Fe in particular), elliptical galaxies in rich clusters and in low-density regions follow the same index-σ relations. However, striking spectral differences between field elliptical galaxies and their counterparts in the central region of the Coma Cluster are found for the first time, with galaxies in the denser environment showing significantly lower C4668 and CN2 absorption strengths. The most convincing interpretation of these results is that they represent a difference in abundance ratios arising from distinct star formation and chemical-enrichment histories of galaxies in different environments. A scenario in which elliptical galaxies in clusters are fully assembled at earlier stages than their low-density counterparts is discussed.

The elemental abundance pattern in a galaxy at z = 2.626.

The discovery of metal-poor stars (where metal is any element more massive than helium) has enabled astronomers to probe the chemical enrichment history of the Milky Way. More recently, element abundances in gas inside high-redshift galaxies has been probed through the absorption lines imprinted on the spectra of background quasars, but these have typically yielded measurements of only a few elements. Furthermore, interpretation of these abundances is complicated by the fact that differential incorporation of metals into dust can produce an abundance pattern similar to that expected from nucleosynthesis by massive stars. Here we report the observation of over 25 elements in a galaxy at redshift z = 2.626. With these data, we can examine nucleosynthetic processes independent of the uncertainty arising from depletion. We find that the galaxy was enriched mainly by massive stars (M > 15 solar masses) and propose that it is the progenitor of a massive elliptical galaxy. The detailed abundance patterns suggest that boron is produced through processes that act independently of metallicity, and may require alternative mechanisms for the nucleosynthesis of germanium.

Spectroscopy of Low Surface Brightness Galaxies with the Hobby-Eberly Telescope

We have obtained low resolution spectra of nineteen red and blue low surface brightness galaxies, using the Marcario Low Resolution Spectrograph on the 9.2m Hobby-Eberly Telescope. These galaxies form a very heterogeneous class, whose spectra qualitatively resemble those of high surface brightness galaxies covering the full range of spectra seen in galaxies of Hubble types from E to Irr. We use a combination of emission line (EW(Halpha), NII/Halpha) and absorption line (Mgb, Hbeta, ) based diagnostics to investigate the star-formation and chemical enrichment histories of these galaxies. These are diverse, with some galaxies having low metallicity and very young mean stellar ages, and other galaxies showing old, super-solar metallicity stellar populations. In contrast with some previous studies which found a strong trend of decreasing metallicity with decreasing central surface brightness, we find a population of galaxies with low surface brightness and near-solar metallicity. Correlations between several of the gas phase and stellar population age and metallicity indicators are used to place contraints on plausible evolutionary scenarios for LSB galaxies. The redshift range spanned by these galaxies is broad, with radial velocities from 3400 km/s to more than 65000 km/s. A subset of the sample galaxies have published HI redshifts and gas masses based on observations with the Arecibo 305m single-dish radio telescope, which place these galaxies far off of the mean Tully-Fisher relation. Our new optical redshifts do not agree with the published HI redshifts for these galaxies. Most of the discrepancies can be explained by beam confusion in the Arecibo observations, causing erroneous HI detections for some of the galaxies.

Evidence of a Strong N [CSC]v[/CSC]/C [CSC]iv[/CSC] Correlation between Emission and Absorption Lines in Active Galactic Nuclei

The narrow absorption lines (NALs) that are seen in the rest-frame ultraviolet near the systemic redshift of active galactic nuclei (AGNs) are not always intrinsic to the near-nuclear region but may originate in the host galaxy or in neighboring galaxies intervening along the line of sight. A variety of criteria have been sought—and several identified—as evidence of an intrinsic origin. We have measured both emission and absorption lines in a Hubble Space Telescope Faint Object Spectrograph sample of objects with both C IV and N V NALs within ±5000 km s-1 of the systemic redshift. We find a strong (>99.5% confidence) linear correlation between the N V/C IV ratio in broad emission lines and that in NALs. A control sample of AGNs with NALs separated by larger velocities shows no such correlation. Our finding thus identifies an additional test for the intrinsic nature of NALs in any given object. The correlation shows that the chemical-enrichment histories and/or ionization parameters of the NAL clouds are closely related to those of clouds that produce the broad emission lines.

Extended star formation in dwarf spheroidal galaxies: The cases of Draco, Sextans, and Ursa Minor

Star formation and chemical enrichment histories of the dwarf spheroidal galaxies (dSphs) Draco, Sextans, and Ursa Minor are investigated by means of chemical evolution models and a simulation code for colour-magnitude diagrams (CMDs). The CMD simulation code is designed to fully consider effects of the chemical evolution on stellar evolution and photometric properties. Comparisons between the chemical evolution models and the observed abundance patterns reveal that the star formation rates were very low (1-5% of that of the solar neighbourhood disc) and that the initial star formation continued for a long duration (> 3.9-6.5 Gyr) in these dSphs. This star formation history can reproduce morphologies of the observed CMDs, such as narrow red giant branches and red horizontal branches. Hence, both of the abundance patterns and the morphologies of the CMDs can be explained by the star formation histories characterised by the low star formation rate and the long duration of the star formation period. Because of the low star formation rates, plenty of gas remains at the final epoch of star formation. We suggest that gas stripping by the Galaxy results in termination of star formation in the dSphs.

History of Star Formation and Chemical Enrichment in the Milky Way Disk

Based on a physical treatment of the star formation law similar to that given by Efstathiou, we have improved our two-component chemical evolution model for the Milky Way disk. Two gas infall rates are compared, one exponential, one Gaussian. It is shown that the star formation law adopted in this paper depends more strongly on the gas surface density than that in Chang et al. It has large effects on the history of star formation and gas evolution of the whole disk. In the solar neighborhood, the history of chemical evolution and star formation is not sensitive to whether the infall rate is Gaussian or exponential. For the same infall time scale, both forms predict the same behavior for the current properties of the Galactic disk. The model predictions do depend on whether or not the infall time scale varies with the radius, but current available observations cannot decide which case is the more realistic. Our results also show that it would be inadequate to describe the gradient evolution along the Galactic disk by only one word ``flatter'' or ``steeper'', as was suggested by Hou et al. and Chiapinni et al. We point out that both the absolute value and the evolution of the abundance gradient may be different in the inner and outer regions.

Dwarf elliptical galaxies: structure, star formation and colour-magnitude diagrams

The aim of this paper is to cast light on the formation and evolution of elliptical galaxies by means of N-body/hydro-dynamical simulations that include star formation, feed-back and chemical evolution. Particular attention is paid to the case of dwarf spheroidals of the Local Group which, thanks to their proximity and modern ground-based and space instrumentation, can be resolved into single stars so that independent determinations of their age and star formation history can be derived. Indeed, the analysis of the color-magnitude diagram of their stellar content allows us to infer the past history of star formation and chemical enrichment thus setting important constraints on galactic models. Dwarf galaxies are known to exhibit complicated histories of star formation ranging from a single very old episode to a series of bursts over most of the Hubble time. By understanding the physical process driving star formation in these objects, we might be able to infer the mechanism governing star formation in more massive elliptical galaxies. Given these premises, we start from virialized haloes of dark matter, and follow the infall of gas into the potential wells and the formation of stars. We find that in objects of the same total mass, different star formation histories are possible, if the collapse phase started at different initial densities. We predict the final structure of dwarf spheroidal galaxies, their kinematics, their large scale distribution of gas and stars, and their detailed histories of the star formation and metal enrichment. Using a population synthesis technique, star formation and metal enrichment rates are then adopted to generate the present color-magnitude diagrams of the stellar populations hosted by dwarf spheroidal galaxies. The simulations are made assuming the red-shift of galaxy formation zfor = 5 and varying the cosmological parameters H0 and q0. The resulting color-magnitude diagrams are then compared with the observational ones for some dwarf spheroidals of the Local Group.

The Galactic Thick Disk Stellar Abundances

We present first results from a program to measure the chemical abundances of a large (N > 30) sample of thick disk stars with the principal goal of investigating the formation history of the Galactic thick disk. We have obtained high-resolution, high signal-to-noise spectra of 10 thick disk stars with the HIRES spectrograph on the 10 m Keck I telescope. Our analysis confirms previous studies of O and Mg in the thick disk stars, which reported enhancements in excess of the thin disk population. Furthermore, the observations of Si, Ca, Ti, Mn, Co, V, Zn, Al, and Eu all argue that the thick disk population has a distinct chemical history from the thin disk. With the exception of V and Co, the thick disk abundance patterns match or tend toward the values observed for halo stars with [Fe/H] ≈ -1. This suggests that the thick disk stars had a chemical enrichment history similar to the metal-rich halo stars. With the possible exception of Si, the thick disk abundance patterns are in excellent agreement with the chemical abundances observed in the metal-poor bulge stars, suggesting the two populations formed from the same gas reservoir at a common epoch. The principal results of our analysis are as follows. (1) All 10 stars exhibit enhanced α/Fe ratios with O, Si, and Ca showing tentative trends of decreasing overabundances with increasing [Fe/H]. In contrast, the Mg and Ti enhancements are constant. (2) The light elements Na and Al are enhanced in these stars. (3) With the exception of Ni, Cr, and possibly Cu, the iron-peak elements show significant departures from the solar abundances. The stars are deficient in Mn, but overabundant in V, Co, Sc, and Zn. (4) The heavy elements Ba and Y are consistent with solar abundances, but Eu is significantly enhanced. If the trends of decreasing O, Si, and Ca with increasing [Fe/H] are explained by the onset of Type Ia SN, then the thick disk stars formed over the course of 1 Gyr. We argue that this formation time-scale would rule out most dissipational collapse scenarios for the formation of the thick disk. Models which consider the heating of an initial thin disk—either through gradual heating mechanisms or a sudden merger event—are favored. These observations provide new tests of theories of nucleosynthesis in the early universe. In particular, the enhancements of Sc, V, Co, and Zn may imply overproduction during an enhanced α-rich freeze out fueled by neutrino-driven winds. Meanwhile, the conflicting trends for Mg, Ti, Ca, Si, and O pose a difficult challenge to our current understanding of nucleosynthesis in Type Ia and Type II SN. The Ba/Eu ratios favor r-process dominated enrichment for the heavy elements, consistent with the ages (tage > 10 Gyr) expected for these stars. Finally, we discuss the impact of the thick disk abundances on interpretations of the abundance patterns of the damped Lyα systems. The observations of mildly enhanced Zn/Fe imply an interpretation for the damped systems which includes a dust depletion pattern on top of a Type II SN enrichment pattern. We also argue that the S/Zn ratio is not a good indicator of nucleosynthetic processes.

Metallicity Evolution in the Early Universe.

Observations of the damped Lyalpha systems provide direct measurements on the chemical enrichment history of neutral gas in the early universe. In this Letter, we present new measurements for four damped Lyalpha systems at high redshift. Combining these data with [Fe/H] values culled from the literature, we investigate the metallicity evolution of the universe from z approximately 1.5 to 4.5. Contrary to our expectations and the predictions of essentially every chemical evolution model, the N(H i)-weighted mean [Fe/H] metallicity exhibits minimal evolution over this epoch. For the individual systems, we report tentative evidence for an evolution in the unweighted [Fe/H] mean and the scatter in [Fe/H], with the higher redshift systems showing lower scatter and lower typical [Fe/H] values. We also note that no damped Lyalpha system has &sqbl0;Fe&solm0;H&sqbr0;<-2.7 dex. Finally, we discuss the potential impact of small number statistics and dust on our conclusions and consider the implications of these results on chemical evolution in the early universe.

Cosmic Histories of Stars, Gas, Heavy Elements, and Dust in Galaxies

We investigate a set of coupled equations that relates the stellar, gaseous, chemical, and radiation contents of the universe averaged over the whole population of galaxies. Using as input the available data from quasar absorption-line surveys, optical imaging and redshift surveys, and the COBE DIRBE and FIRAS extragalactic infrared background measurements, we obtain solutions for the cosmic histories of stars, interstellar gas, heavy elements, dust, and radiation from stars and dust in galaxies. Our solutions reproduce remarkably well a wide variety of observations that were not used as input. These include the integrated background light from galaxy counts from near-ultraviolet to near-infrared wavelengths, the rest-frame optical and near-infrared emissivities at various redshifts from surveys of galaxies, the mid-infrared and far-infrared emissivities of the local universe from the IRAS survey, the mean abundance of heavy elements at various epochs from surveys of damped Ly? systems, and the global star formation rates at several redshifts from H?, mid-infrared, and submillimeter observations. The chemical enrichment history of the intergalactic medium implied by our models is also consistent with the observed mean metal content of the Ly? forest at high redshifts. We infer that the dust associated with star-forming regions is highly inhomogeneous and absorbs a significant fraction of the starlight, with only 41%-46% of the total in the extragalactic optical background and the remaining 59%-54% reprocessed by dust into the infrared background. The solutions presented here provide an intriguing picture of the cosmic mean history of galaxies over much of the Hubble time. In particular, the process of galaxy formation appears to have undergone an early period of substantial inflow to assemble interstellar gas at z 3, a subsequent period of intense star formation and chemical enrichment at 1 z 3, and a recent period of decline in the gas content, star formation rate, optical stellar emissivity, and infrared dust emissivity at z 1.