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The Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer (COBE) spacecraft was designed primarily to conduct a systematic search for an isotropic cosmic infrared background (CIB) in ten photometric bands from 1.25 to 240 µm. The results of that search are presented here. Conservative limits on the CIB are obtained from the minimum observed brightness in all-sky maps at each wavelength, with the faintest limits in the DIRBE spectral range being at 3.5 µm (νI ν < 64 nW m −2 sr −1 , 95% CL) and at 240 µm (νI ν < 28 nW m −2 sr −1 , 95% CL). The bright foregrounds from interplanetary dust scattering and emission, stars, and interstellar dust emission are the principal impediments to the DIRBE measurements of the CIB. These foregrounds have been modeled and removed from the sky maps. Assessment of the random and systematic uncertainties in the residuals and tests for isotropy show that only the 140 and 240 µm data provide candidate detections of the CIB. The residuals and their uncertainties provide CIB upper limits more restrictive than the dark sky limits at wavelengths from 1.25 to 100 µm. No plausible solar system or Galactic source of the observed 140 and 240 µm residuals can be identified, leading to the conclusion that the CIB has been detected at levels of νI ν = 25 ± 7 and 14 ± 3 nW m −2 sr −1 at 140 and 240 µm respectively. The integrated energy from 140 to 240 µm, 10.3 nW m −2 sr −1 , is about twice the integrated optical light from the galaxies in the Hubble Deep Field, suggesting that star formation might have been heavily enshrouded by dust at high redshift. The detections and upper limits reported here provide new constraints on models of the history of energy-releasing processes and dust production since the decoupling of the cosmic microwave background from matter.

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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 midinfrared 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 Lya systems, and the global star formation rates at several redshifts from Ha, 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 Lya forest at high redshifts. We infer that the dust associated with starforming regions is highly inhomogeneous and absorbs a signiÐcant fraction of the starlight, with only 41%È46% of the total in the extragalactic optical background and the remaining 59%È54% 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 inÑow to assemble interstellar gas at a subsequent period of intense star formation and chemical enrichment at and a recent z Z 3, 1 [ z [ 3, period of decline in the gas content, star formation rate, optical stellar emissivity, and infrared dust emissivity at z [ 1.

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A direct measurement of the extragalactic background light (EBL) can provide important constraints on the integrated cosmological history of star formation, metal and dust production, and the conversion of starlight into infrared emission by dust. In this paper we examine the cosmological implications of the recent detection of the EBL in the 125 to 5000 km wavelength region by the Di †use Infrared Background Experiment (DIRBE) and Far Infrared Absolute Spectrophotometer (FIRAS) on board the Cosmic Background Explorer (COBE). We Ðrst show that the 140 and 240 km isotropic residual emission found in the DIRBE data cannot be produced by foreground emission sources in the solar system or the Galaxy. The DIRBE 140 and 240 km isotropic residuals, and by inference the FIRAS residuals as well, are therefore extragalactic. Assuming that most of the 140 and 240 km emission is from dust yields a 2 p lower limit of lI(l) B 5 nW m~2 sr~1 for the EBL at 100 km. The integrated EBL detected by the COBE between 140 and 5000 km is D16 nW m~2 sr~1, roughly 20%È50% of the integrated EBL intensity expected from energy release by nucleosynthesis throughout cosmic history. This also implies that at least D5%È15% of the baryonic mass density implied by big bang nucleosynthesis has been processed through stars. The COBE observations provide important constraints on the cosmic star formation rate, and we calculate the EBL spectrum for various star formation histories. The results show that the UV and optically determined cosmic star formation rates fall short in producing the observed 140 to 5000 km background. The COBE observations require the star formation rate at redshifts of z B 1.5 to be larger than that inferred from UV-optical observations by at least a factor of 2. This excess stellar energy must be mainly generated by massive stars, since it otherwise would result in a local K-band luminosity density that is larger than observed. The energy sources could either be yet undetected dust-enshrouded galaxies, or extremely dusty star-forming regions in observed galaxies, and they may be responsible for the observed iron enrichment in the intracluster medium. The exact star formation history or scenarios required to produce the EBL at far-IR wavelengths cannot be unambiguously resolved by the COBE observations and must await future observations.

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