The Three-Dimensional Spatial Distribution of Interstellar Gas in the Milky Way: Implications for Cosmic Rays and High-Energy Gamma-Ray Emissions

TL;DR

This paper develops detailed 3D models of interstellar gas distribution in the Milky Way, incorporating spiral arms and disk warping, to improve cosmic ray propagation and gamma-ray emission predictions.

Contribution

It introduces empirically fitted 3D gas density models that enhance the realism of cosmic ray and gamma-ray emission simulations over traditional 2D models.

Findings

3D gas models significantly alter cosmic ray propagation parameters.

New models reveal non-trivial features in gamma-ray intensity maps.

Propagation parameters differ markedly from those using 2D models.

Abstract

Direct measurements of cosmic ray (CR) species combined with observations of their associated gamma-ray emissions can be used to constrain models of CR propagation, trace the structure of the Galaxy, and search for signatures of new physics. The spatial density distribution of the interstellar gas is a vital element for all these studies. So far models have employed the 2D cylindrically symmetric geometry, but their accuracy is well behind that of the available data. In this paper, 3D spatial density models for the neutral and molecular hydrogen are constructed based on empirical model fitting to gas line-survey data. The developed density models incorporate spiral arms and account for the warping of the disk, and the increasing gas scale height with radial distance from the Galactic center. They are employed together with the GALPROP CR propagation code to investigate how the new 3D gas models affect calculations of CR propagation and high-energy gamma-ray intensity maps. The calculations made reveal non-trivial features that are directly related to the new gas models. The best-fit values for propagation model parameters employing 3D gas models are presented and they differ significantly from the values derived with the 2D gas density models that have been widely used. The combination of 3D CR and gas density models provide a more realistic basis for the interpretation of non-thermal emissions from the Galaxy.