2013ApJ...767...80I

We have conducted B-, g-, V-, and R-band imaging in a 45'x40' field containing part of the high Galactic latitude translucent cloud MBM32, and correlated the intensity of diffuse optical light S_ν(λ) with that of 100 µm emission S_ν(100 µm). A χ² minimum analysis is applied to fit a linear function to the measured correlation and derive the slope parameter b(λ) = ΔS_ν(λ)/ΔS_ν(100 µm) of the best-fit linear function. Compiling a sample by combining our b(λ) and published ones, we show that the b(λ) strength varies from cloud to cloud by a factor of four. Finding that b(λ) decreases as S_ν(100 µm) increases in the sample, we suggest that a nonlinear correlation including a quadratic term of S_ν(100 µm)² should be fitted to the measured correlation. The variation of optical depth, which is A_V= 0.16-2.0 in the sample, can change b(λ) by a factor of 2-3. There would be some contribution to the large b(λ) variation from the forward-scattering characteristic of dust grains which is coupled to the non-isotropic interstellar radiation field (ISRF). Models of the scattering of diffuse Galactic light (DGL) underestimate the b(λ) values by a factor of two. This could be reconciled by deficiency in UV photons in the ISRF or by a moderate increase in dust albedo. Our b(λ) spectrum favors a contribution from extended red emission (ERE) to the diffuse optical light; b(λ) rises from B to V faster than the models, seems to peak around 6000 Å and decreases toward long wavelengths. Such a characteristic is expected from the models in which the DGL is combined with ERE.