Determination of the Far‐Infrared Cosmic Background UsingCOBEDIRBE and WHAM Data

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Abstract
Determination of the cosmic infrared background (CIB) at far-infrared wavelengths using COBE DIRBE data is limited by the accuracy to which foreground interplanetary and Galactic dust emission can be modeled and subtracted. Most previous determinations of the far-infrared CIB were based on the detection of residual isotropic emission in sky maps from which the emission from interplanetary dust and the neutral interstellar medium were removed. In this paper we use the Wisconsin Hα Mapper (WHAM) Northern Sky Survey as a tracer of the ionized medium to examine the effect of this foreground component on determination of the CIB. We decompose DIRBE far-infrared data for five high Galactic latitude regions into H I- and Hα-correlated components and a residual component. We find the Hα-correlated component to be consistent with zero for each region, yielding CIB results that are essentially the same as those previously derived by the COBE DIRBE team: νIν(nW m-2 sr-1) < 75, Spitzer 160 μm source counts is 13.7 ± 1.7 and 10.7 ± 1.4 nW m-2 sr-1 at 140 and 240 μm, respectively. We derive upper limits on the 100 μm emissivity of the ionized medium that are typically about 40% of the 100 μm emissivity of the neutral atomic medium. If Hα is not a reliable tracer of far-infrared emission, we show that our analysis would underestimate the emissivity of the ionized medium and could slightly overestimate the CIB.
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