FIRST DETECTION OF GALACTIC LATITUDE DEPENDENCE OF NEAR-INFRARED DIFFUSE GALACTIC LIGHT FROM DIRBE RENALYSIS

Abstract

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 the Diffuse Infrared Background Experiment, we investigate the scattering property from diffuse Galactic light (DGL) measurements at 1.25, 2.2, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>3.5</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>μ</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:math> , in addition to our recent analyses of diffuse near-IR emission. As a result, we first find that the intensity ratios of near-IR DGL to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>100</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>μ</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:math> emission increase toward low Galactic latitudes at 1.25 and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2.2</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>μ</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:math> . The derived latitude dependence can be reproduced by a scattered light model of interstellar dust with a large scattering asymmetry factor <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>g</mml:mi> <mml:mo>≡</mml:mo> <mml:mo stretchy="true">〈</mml:mo> <mml:mi>cos</mml:mi> <mml:mi>θ</mml:mi> <mml:mo stretchy="true">〉</mml:mo> </mml:math> of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.2</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> at 1.25 and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2.2</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>μ</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:math> , 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 a possible latitude dependence of ultraviolet-excited dust emission at 1.25 and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2.2</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>μ</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:math> 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.