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A new estimate of the Galactic interstellar radiation field between 0.1 microns and 1000 microns

T. A. Porter, A. W. Strong

TL;DR

The paper addresses the need for a modern, three-dimensional Galactic interstellar radiation field (ISRF) to improve cosmic-ray propagation and diffuse gamma-ray modeling via inverse Compton scattering. It describes a comprehensive forward-model of the ISRF that combines an updated stellar distribution (including an offset Galactic bar and a truncated inner disc) with a physically motivated dust model, producing self-consistent optical and infrared fields that include absorption, scattering, and transient as well as equilibrium heating. Initial results show generally good agreement with local ISRF observations, with some UV and mid-infrared discrepancies indicating possible missing stellar populations or emission components. The authors plan to couple the ISRF to the GALPROP propagation code to derive the Galactic diffuse gamma-ray spectrum and to release the ISRF data publicly for community use.

Abstract

Cosmic-ray electrons and positrons propagating in the Galaxy produce diffuse gamma-rays via the inverse Compton (IC) process. The low energy target photon populations with which the cosmic-rays interact during propagation are produced by stars, this stellar light being reprocessed by Galactic dust. Detailed modelling of the Galactic stellar distribution, dust distribution, and treatment of the absorption and scattering of light is therefore required to obtain accurate models for the low energy Galactic photon distribution and spectrum. Using a realistic Galactic stellar distribution model, and dust distribution, we calculate the diffuse radiation field from stars in the Galaxy (the `optical' radiation field), including absorption and scattering. Using a dust heating code, we self-consistently calculate the infra-red radiation field for the same dust model used for the optical calculation; both transient and equilibrium heating are included. We present the calculated radiation field spectra and distributions, and will use these to calculate the expected Galactic diffuse IC gamma-ray spectrum.

A new estimate of the Galactic interstellar radiation field between 0.1 microns and 1000 microns

TL;DR

The paper addresses the need for a modern, three-dimensional Galactic interstellar radiation field (ISRF) to improve cosmic-ray propagation and diffuse gamma-ray modeling via inverse Compton scattering. It describes a comprehensive forward-model of the ISRF that combines an updated stellar distribution (including an offset Galactic bar and a truncated inner disc) with a physically motivated dust model, producing self-consistent optical and infrared fields that include absorption, scattering, and transient as well as equilibrium heating. Initial results show generally good agreement with local ISRF observations, with some UV and mid-infrared discrepancies indicating possible missing stellar populations or emission components. The authors plan to couple the ISRF to the GALPROP propagation code to derive the Galactic diffuse gamma-ray spectrum and to release the ISRF data publicly for community use.

Abstract

Cosmic-ray electrons and positrons propagating in the Galaxy produce diffuse gamma-rays via the inverse Compton (IC) process. The low energy target photon populations with which the cosmic-rays interact during propagation are produced by stars, this stellar light being reprocessed by Galactic dust. Detailed modelling of the Galactic stellar distribution, dust distribution, and treatment of the absorption and scattering of light is therefore required to obtain accurate models for the low energy Galactic photon distribution and spectrum. Using a realistic Galactic stellar distribution model, and dust distribution, we calculate the diffuse radiation field from stars in the Galaxy (the `optical' radiation field), including absorption and scattering. Using a dust heating code, we self-consistently calculate the infra-red radiation field for the same dust model used for the optical calculation; both transient and equilibrium heating are included. We present the calculated radiation field spectra and distributions, and will use these to calculate the expected Galactic diffuse IC gamma-ray spectrum.

Paper Structure

This paper contains 3 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Modelling the Interstellar Radiation Field
  3. Diffuse Gamma-Rays

Figures (2)

  • Figure 1: Local radiation field modelled as described in the text. Black line : total radiation field, including CMBR. Blue solid line : total optical. Blue dashed line : total scattered light. Red line : total infra-red. Data : thick dot-dashed line, Apollo; thick solid line, DIRBE; thick dashed line, FIRAS.
  • Figure 2: Spatial variation of the total radiation field throughout the Galaxy. Black line : total radiation field for GC. Magenta line : total radiation field for $R = 0$ kpc, $z = 5$ kpc. Blue line : total radiation field for $R = 4$ kpc, $z = 0$ kpc. Red line : total radiation field for $R = 12$ kpc, $z = 0$ kpc. Green line : total radiation field for $R = 20$ kpc, $z = 0$ kpc.