Detection of a far-infrared excess with DIRBE at 60 and 100 microns

From analysis of the DIRBE weekly averaged sky maps, we have detected substantial flux in the 60 and 100 mum channels in excess of expected zodiacal and Galactic emission. Two methods are used to separate zodiacal light from more distant emission. Method I makes use of the time dependence of the north-south annual variation observed at the ecliptic poles. This method is robust against errors in the interplanetary dust (IPD) model but does not demonstrate isotropy of the background. Method II measures the ecliptic latitude dependence of the dust over a range of ecliptic latitudes (/beta/ > 35 degrees) at solar elongation e = 90 degrees. This allows the excess to be determined in each week of the DIRBE mission for high redundancy, but the results depend weakly on the IPD model. Both methods give consistent results at 60 and 100 mum. The observed signal is consistent with an isotropic background at the level nuI(nu) = 28.1 +/- 1.8 +/- 7(syst) nW m(-2) sr(-1) at 60 mum and 24.6 +/- 2.5 +/- 8 nW m(-2) sr(-1) at 100 mum. The IR excess detected at 140 and 240 mum by these methods agrees with previous measurements, which are thought to be the cosmic infrared background (CIB). The detections at 60 and 100 mum are new. The integrated IR excess in the window 45-125 mum is 23 +/- 8 nW m(-2) sr(-1), to be added to the 18 +/- 4 nW m(-2) sr(-1) previously measured with the DIRBE and FIRAS instruments in the window 125-2 mm. While this new excess is not necessarily the GIB, we have ruled out all known sources of emission in the solar system and Galaxy. We therefore tentatively interpret this signal as the CIB and consider the implications of such energy production from the viewpoint of star formation efficiency and black hole accretion efficiency. However, the IR excess exceeds limits on the CIB derived from the inferred opacity of the intergalactic medium to observed TeV photons, thus casting doubt on this interpretation. There is currently no satisfactory explanation for the 60-100 Clm excess.