The Spectrum of the Extragalactic Far Infrared Background from the COBE Firas Observations

The COBE FIRAS data contain foreground emission from interplanetary, Galactic interstellar dust and extragalactic background emission. We use three different methods to separate the various emission components, and derive the spectrum of the extragalactic Far InfraRed Background (FIRB). Each method relies on a different set of assumptions, which affect the FIRB spectrum in different ways. Despite this, the FIRB spectra derived by these different methods are remarkably similar. The average spectrum that we derive in the \nu = 5 - 80 cm^{-1} (2000-125 \um) frequency interval is: I(\nu) = (1.3 \pm 0.4)\times10^{-5} (\nu /\nu_0)^{0.64 \pm 0.12} P_{\nu}(18.5 \pm 1.2 K), where \nu_0=100 cm^{-1} (\lambda_0=100 \um) and P is the Planck function. The derived FIRB spectrum is consistent with the DIRBE 140 and 240 \um detections. The total intensity received in the 5 - 80 cm^{-1} frequency interval is 14 nW m^{-2} sr^{-1}, and comprises about 20% of the total intensity expected from the energy release from nucleosynthesis throughout the history of the universe.