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Register a new userDiffuse Galactic Light in the Field of the Translucent High Galactic Latitude Cloud MBM32
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We have conducted B, g, V, and R-band imaging in a 45x40 arcmin^2 field containing part of the high Galactic latitude translucent cloud MBM32, and correlated the intensity of diffuse optical light S_ u(lambda) with that of 100 micron emission S_ u(100um). A chi^2 minimum analysis is applied to fit a linear function to the measured correlation and derive the slope parameter b(lambda)= Delta S_ u(lambda) / Delta S_ u(100um) of the best-fit linear function. Compiling a sample by combining our b(lambda) and published ones, we show that the b(lambda) strength varies from cloud to cloud by a factor of 4. Finding that b(lambda) decreases as S_ u(100um) increases in the sample, we suggest that a non-linear correlation including a quadratic term of S_ u(100um)^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(lambda) by a factor of 2 - 3. There would be some contribution to the large b(lambda) 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(lambda) values by a factor of 2. This could be reconciled by deficiency in UV photons in the ISRF or by a moderate increase in dust albedo. Our b(lambda) spectrum favors a contribution from extended red emission (ERE) to the diffuse optical light; b(lambda) rises from B to V faster than the models, seems to peak around 6000 AA, and decreases towards long wavelengths. Such a characteristic is expected from the models in which the DGL is combined with ERE.
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Observational study on near-infrared (IR) scattering properties of interstellar dust grains has been limited due to its faintness. Using all-sky maps obtained from Diffuse Infrared Background Experiment (DIRBE), we investigate the scattering property from diffuse Galactic light (DGL) measurements at 1.25, 2.2, and 3.5 {mu}m in addition to our recent analyses of diffuse near-IR emission (Sano et al. 2015; Sano et al. 2016). As a result, we first find that the intensity ratios of near-IR DGL to 100 {mu}m emission increase toward low Galactic latitudes at 1.25 and 2.2 {mu}m. The derived latitude dependence can be reproduced by a scattered light model of interstellar dust with a large scattering asymmetry factor g = <cos{theta}> of $0.8^{+0.2}_{-0.3}$ at 1.25 and 2.2 {mu}m, assuming an infinite Galaxy disk as an illuminating source. The derived asymmetry factor is comparable to the values obtained in the optical, but several times larger than that expected from a recent dust model. Since possible latitude dependence of ultraviolet-excited dust emission at 1.25 and 2.2 {mu}m would reduce the large asymmetry factor to the reasonable value, our result may indicate the first detection of such an additional emission component in the diffuse interstellar medium.
Near-infrared (IR) diffuse Galactic light (DGL) consists of scattered light and thermal emission from interstellar dust grains illuminated by interstellar radiation field (ISRF). At 1.25 and 2.2um, recent observational study shows that intensity ratios of the DGL to interstellar 100um dust emission steeply decrease toward high Galactic latitudes (b). In this paper, we investigate origin(s) of the b-dependence on the basis of models of thermal emission and scattered light. Combining a thermal emission model with regional variation of the polycyclic aromatic hydrocarbon abundance observed with Planck, we show that contribution of the near-IR thermal emission component to the observed DGL is less than ~20%. We also examine the b-dependence of the scattered light, assuming a plane-parallel Galaxy with smooth distributions of the ISRF and dust density along vertical direction, and assuming a scattering phase function according to a recently developed model of interstellar dust. We normalize the scattered light intensity to the 100um intensity corrected for deviation from the cosecant-b law according to the Planck observation. As the result, the present model taking all the b-dependence of dust and ISRF properties can account for the observed b-dependence of the near-IR DGL. However, uncertainty of the correction for the 100um emission is large and other normalizing quantities may be appropriate for more robust analysis of the DGL.
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