Detection of a Far-Infrared Excess with DIRBE at 60 and 100 Microns
D. P. Finkbeiner, M. Davis, D. J. Schlegel;
ApJ, 2000, 544, 81
ABSTRACT:From analysis of the DIRBE weekly averaged sky maps, we have detected substantial flux in the 60 and 100 μm 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
(|β|>35deg) at solar elongation e=90deg.
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 μm.
The observed signal is consistent with an isotropic
background at the level νIν=28.1+/-1.8+/-7(syst) nW
m-2 sr-1 at 60 μm and 24.6+/-2.5+/-8 nW m-2
sr-1 at 100 μm.
The IR excess detected at 140 and 240 μm by these
methods agrees with previous measurements, which are thought to be the cosmic
infrared background (CIB).
The detections at 60 and 100 μm are new.
The
integrated IR excess in the window 45-125 μm 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 CIB, 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 μm excess.
KEYWORDS: cosmology: observations, cosmology: diffuse radiation, galaxies: active, gamma rays: observations, infrared: ism: continuum, interplanetary medium
CODE: finkbeiner2000