The Mystery of the Cosmic Diffuse Ultraviolet Background Radiation

Abstract

The diffuse cosmic background radiation in the Galaxy Evolution Explorer far-ultraviolet (FUV, 1300-1700 Å) is deduced to originate only partially in the dust-scattered radiation of FUV-emitting stars: the source of a substantial fraction of the FUV background radiation remains a mystery. The radiation is remarkably uniform at both far northern and far southern Galactic latitudes and increases toward lower Galactic latitudes at all Galactic longitudes. We examine speculation that this might be due to interaction of the dark matter with the nuclei of the interstellar medium, but we are unable to point to a plausible mechanism for an effective interaction. We also explore the possibility that we are seeing radiation from bright FUV-emitting stars scattering from a "second population" of interstellar grains-grains that are small compared with FUV wavelengths. Such grains are known to exist, and they scatter with very high albedo, with an isotropic scattering pattern. However, comparison with the observed distribution (deduced from their 100 μm emission) of grains at high Galactic latitudes shows no correlation between the grains' location and the observed FUV emission. Our modeling of the FUV scattering by small grains also shows that there must be remarkably few such "smaller" grains at high Galactic latitudes, both north and south; this likely means simply that there is very little interstellar dust of any kind at the Galactic poles, in agreement with Perry and Johnston. We also review our limited knowledge of the cosmic diffuse background at ultraviolet wavelengths shortward of Lyα-it could be that our "second component" of the diffuse FUV background persists shortward of the Lyman limit and is the cause of the reionization of the universe.

Dust scattering from the Taurus Molecular Cloud

“…We believe that the main source of scatter between our modelled fluxes and the data is the complex structure of the TMC. However, we do find offsets similar to the 700 -900 ph cm −2 s −1 sr −1Å−1 found by Murthy (2016) including 200 -300 ph cm −2 s −1 sr −1Å−1 from airglow Murthy (2014a), molecular hydrogen fluorescence in the FUV (Hurwitz 1994;Lim et al 2013) and a possibly new, unknown component (Henry et al 2015;Hamden et al 2013).…”

“…We find offsets of about 1000 ph cm −2 s −1 sr −1Å−1 in both bands of which some part may be due to foreground contributors such as airglow and some to an unknown component (Henry et al 2015).…”

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS)

“…The nature of this diffuse component emission is still unknown and the subject of several studies (see e.g. Henry et al 2015, who test the hypothesis of a dark-matter interaction with the interstellar medium, or of small dust grains; and references therein).…”

Diffuse Radiation from the Aquila Rift

“…Henry et al (2015) suggested that these were due to a different, unknown contributor to the diffuse UV which was associated with the Galaxy but not part of the dust which contributes to the IR emission. They also pointed out that separate analyses of the background in small areas might individually be able to fit the flux by dust scattering but may not be consistent across regions.…”