The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background. IV. Cosmological Implications

Abstract

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 μm wavelength region by the Diffuse Infrared Background Experiment (DIRBE) and Far Infrared Absolute Spectrophotometer (FIRAS) on board the Cosmic Background Explorer ( COBE ). We first show that the 140 and 240 μm 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 μm isotropic residuals, and by inference the FIRAS residuals as well, are therefore extragalactic. Assuming that most of the 140 and 240 μm emission is from dust yields a 2 σ lower limit of ν I (ν) ≈ 5 nW m -2 sr -1 for the EBL at 100 μm. The integrated EBL detected by the COBE between 140 and 5000 μm is ~16 nW m -2 sr -1 , roughly 20%-50% of the integrated EBL intensity expected from energy release by nucleosynthesis throughout cosmic history. This also implies that at least ~5%-15% 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 μm background. The COBE observations require the star formation rate at redshifts of z ≈ 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.