The COBE Diffuse Infrared Background Experiment search for the cosmic infrared background. IV. Cosmological implications

A direct measurement of the extragalactic background light (EBL) can provid e important constraints on the integrated cosmological history of star form ation, metal and dust production, and the conversion of starlight into infr ared emission by dust. In this paper we examine the cosmological implicatio ns of the recent detection of the EEL in the 125 to 5000 mu m wavelength re gion 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 mu m isotropic residual emissio n found in the DIRBE data cannot be produced by foreground emission sources in the solar system or the Galaxy. The DIRBE 140 and 240 mu m isotropic re siduals, and by inference the FIRAS residuals as well, are therefore extrag alactic. Assuming that most of the 140 and 240 mu m emission is from dust y ields a 2 sigma lower limit of vI(v) approximate to 5 nW m(-2) sr(-1) for t he EBL at 100 mu m. The integrated EBL detected by the COBE between 140 and 5000 mu m is similar to 16 nW m(-2) sr(-1), roughly 20%-50% of the integra ted EEL intensity expected from energy release by nucleosynthesis throughou t cosmic history. This also implies that at least similar to 5%-15% of the baryonic mass density implied by big bang nucleosynthesis has been processe d through stars. The COBE observations provide important constraints on the cosmic star formation rate, and we calculate the EEL spectrum for various star formation histories. The results show that the UV and optically determ ined cosmic star formation rates fall short in producing the observed 140 t o 5000 mu m background. The COBE observations require the star formation ra te at redshifts of z approximate to 1.5 to be larger than that inferred fro m UV-optical observations by at least a factor of 2. This excess stellar en ergy must be mainly generated by massive stars, since it otherwise would re sult 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. T he exact star formation history or scenarios required to produce the EEL at far-IR wavelengths cannot be unambiguously resolved by the COBE observatio ns and must await future observations.