A clumpy-cloud PDR model of the global far-infrared line emission of the Milky Way

TL;DR

This paper presents a model of the Milky Way's far-infrared line emission based on a clumpy, UV-penetrated cloud scenario, successfully matching COBE observations within a factor of two.

Contribution

It introduces a novel clumpy PDR model with a power-law clump spectrum that explains the Galactic FIR line emission without additional assumptions.

Findings

Model reproduces COBE FIR line intensities within a factor of two.

Clumpy cloud scenario explains the distribution of FIR emission across the Galaxy.

Power-law clump spectrum aligns with observed ISM structure.

Abstract

The fractal structure of the interstellar medium suggests that the interaction of UV radiation with the ISM as described in the context of photon-dominated regions (PDR) dominates most of the physical and chemical conditions, and hence the far-infrared and submm emission from the ISM in the Milky Way. We investigate to what extent the Galactic FIR line emission of the important species CO, C, C+, and O, as observed by the Cosmic Background Explorer (COBE) satellite can be modeled in the framework of a clumpy, UV-penetrated cloud scenario. The far-infrared line emission of the Milky Way is modeled as the emission from an ensemble of clumps with a power law clump mass spectrum and mass-size relation with power-law indices consistent with the observed ISM structure. The individual clump line intensities are calculated using the KOSMA-tau PDR-model for spherical clumps. The model parameters for the cylindrically symmetric Galactic distribution of the mass density and volume filling factor are determined by the observed radial distributions. A constant FUV intensity, in which the clumps are embedded, is assumed. We show that this scenario can explain, without any further assumptions and within a factor of about 2, the absolute FIR-line intensities and their distribution with Galactic longitude as observed by COBE.