Galactic Latitude Dependence of Near-infrared Diffuse Galactic Light: Thermal Emission or Scattered Light?

Abstract

Near-infrared (IR) diffuse Galactic light (DGL) consists of scattered light and thermal emission from interstellar dust grains illuminated by the interstellar radiation field (ISRF). At 1.25 and , a recent observational study shows that intensity ratios of the DGL to interstellar dust emission steeply decrease toward high Galactic latitudes ( b ). In this paper, we investigate the origin(s) of the b -dependence on the basis of models of thermal emission and scattered light. Combining a thermal emission model with the regional variation of the polycyclic aromatic hydrocarbon abundance observed with Planck , we show that the contribution of the near-IR thermal emission component to the observed DGL is lower than . 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 the 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 intensity corrected for deviation from the cosecant- b law according to the Planck observation. As the result, the present model that considers the b -dependence of dust and the ISRF properties can account for the observed b -dependence of the near-IR DGL. However, the uncertainty in the correction for the emission is large, and other normalizing quantities may be appropriate for a more robust analysis of the DGL.