First Detection of Galactic Latitude Dependence of Near-Infrared Diffuse Galactic Light from DIRBE Reanalysis

TL;DR

This study analyzes all-sky near-infrared data from DIRBE to detect Galactic latitude dependence in diffuse Galactic light, revealing a large dust scattering asymmetry factor and suggesting possible additional emission components in the interstellar medium.

Contribution

First detection of Galactic latitude dependence of near-IR diffuse Galactic light, indicating a large dust scattering asymmetry factor and potential additional emission components.

Findings

Intensity ratios increase toward low Galactic latitudes at 1.25 and 2.2 μm.

Derived asymmetry factor g ≈ 0.8, larger than recent dust model predictions.

Possible detection of ultraviolet-excited dust emission component.

Abstract

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.