8.2. Isotropic Component
Line emission and continuum processes from warm ionized gas in the Galaxy also contribute an isotropic component to the DGL. For |b| > 5°, Reynolds (1992) finds that H emission strength matches the prediction of a path-length through a slab model for the galaxy, I(H) 2.9 × 10-7 csc|b| ergs s-1 cm-2 sr-1. Fortunately, H emission, specifically, is irrelevant for us because the relative throughput of the F555W bandpasses at H (~ 6562Å) is only ~ 10% of the peak filter throughput. The strongest H emission expected in our field would contribute 0.01 × 10-9ergs s-1 cm-2 sr-1 Å-1, which corresponds to 0.01% of the total background, and 1% of the expected EBL. The next strongest line, [OIII] at 5007Å, is near the peak of the F555W sensitivity, but it is fainter than H by a factor of 20 (Reynolds 1985, Shields et al. 1981) and will contribute at most 0.05% of the expected EBL.
More important than line emission is the two-photon, free-free, and bound-free continua emitted by ionized gas with the density implied by the detected H emission. The combined spectrum of free-free, bound-free, and two-photon emission was calculated by Aller (1987) as a function of electron and ion densities, and has been expressed by Reynolds (1992) as a function of the observed H emission; it is a function of the temperature of the warm ionized medium. For our purposes, a conservative estimates of the isotropic continuum from gas with temperature T ~ 104 is given by Aller (1987) and Reynolds (1992) as I( 3700Å) < 0.3 × 10-9ergs s-1 cm-2 sr-1 Å-1 and I( 3700Å) < 0.01 × 10-9ergs s-1 cm-2 sr-1 Å-1 (see Aller 1987 and Reynolds 1992 for discussion). We include these contributions in our estimate of the DGL.