The Far Ultraviolet Background

TL;DR

This paper investigates the sources of the diffuse far-ultraviolet background, concluding that known Galactic processes can account for most or all of the previously unexplained offset component.

Contribution

It identifies conventional Galactic sources that can explain the offset component of the FUV background, reducing the need for exotic explanations.

Findings

Two-thirds or more of the offset component can be explained by known processes.

Galactic Hot and Warm Ionized Medium contribute to FUV emission.

Lyman-beta fluorescence also plays a role in the FUV background.

Abstract

The diffuse far-ultraviolet (FUV) background has received considerable attention from astronomers since the 1970's. The initial impetus came from the hope of detecting UV radiation from the hot intergalactic medium. The central importance of the FUV background to the physics (heating and ionization) of the diffuse atomic phases motivated the next generation of experiments. The consensus view is that the diffuse FUV emission at high latitudes has three components: stellar FUV reflected by dust grains (diffuse Galactic light or DGL), FUV from other galaxies and the intergalactic medium (extra-galactic background light or EBL) and a component of unknown origin (and referred to as the "offset" component). During the1980's, there was some discussion that decaying dark matter particles produced FUV radiation. In this paper I investigate production of FUV photons by conventional sources: line emission from Galactic Hot Ionized Medium, two-photon emission from the Galactic Warm Ionized Medium and low-velocity shocks, and Lyman-beta fluorescence of hydrogen at several locales in the Solar System (the interplanetary medium, the exosphere and the thermosphere of Earth). I conclude that two thirds and arguably all of the offset component can be explained by the sum of the radiation from the processes listed above.