The Mystery of the Cosmic Diffuse Ultraviolet Background Radiation

The diffuse cosmic background radiation in the GALEX far ultraviolet (FUV, 1300 \AA\ - 1700 \AA) 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 it increases toward lower Galactic latitudes at all Galactic longitudes. We examine speculation that it 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 (Draine 2011) and they scatter with very high albedo, with an isotropic scattering pattern. However, comparison with the observed distribution (deduced from their $100\ \mu$m 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 & Johnston (1982). We also review our limited knowledge of the cosmic diffuse background at ultraviolet wavelengths shortward of Lyman $\alpha$---it could be that our "second component" of the diffuse far-ultraviolet background persists shortward of the Lyman limit, and is the cause of the re-ionization of the Universe (Kollmeier et al. 2014).Comment: 73 pages, 31 figures, ApJ accepte

Measurement of Dust Optical Properties in Coalsack

We have used FUSE and Voyager observations of dust scattered starlight in the neighborhood of the Coalsack Nebula to derive the optical constants of the dust grains. The albedo is consistent with a value of $0.28 \pm 0.04$ and the phase function asymmetry factor with a value of $0.61 \pm 0.07$ throughout the spectral range from 900 -- 1200 \AA, in agreement with previous determinations as well as theoretical predictions. We have now observed two regions (Ophiuchus and Coalsack) with intense diffuse background radiation and in both cases have found that the emission is due to light from nearby hot stars scattered by a relatively thin foreground cloud, with negligible contribution from the background molecular cloud.Comment: Total 19 pages, Figures 9, Accepted for publication in Astrophysical Journa

GALEX Observations of Diffuse UV Radiation at High Spatial Resolution from the Sandage Nebulosity

Using the GALEX ultraviolet imagers we have observed a region of nebulosity first identified as starlight scattered by interstellar dust by Sandage (1976). Apart from airglow and zodiacal emission, we have found a diffuse UV background of between 500 and 800 \phunit in both the \galex FUV (1350 -- 1750 \AA) and NUV (1750 -- 2850 \AA). Of this emission, up to 250 \phunit is due to \htwo fluorescent emission in the FUV band; the remainder is consistent with scattering from interstellar dust. We have estimated the optical constants to be $a = 0.3; g = 0.7$ in the FUV and $a = 0.5; g = 0.7$ in the NUV, implying highly forward scattering grains, plus an extragalactic contribution of as much as 150 \phunit. These are the highest spatial resolution observations of the diffuse UV background to date and show an intrinsic scatter beyond that expected from instrumental noise alone. Further modeling is required to understand the nature of this scatter and its implications for the ISM.Comment: Total 20 pages, Figures 9, Accepted for publication in Astrophysical Journa

Observations of Diffuse Ultraviolet Emission from Draco

We have studied small scale (2 arcmin) spatial variation of the diffuse UV radiation using a set of 11 GALEX deep observations in the constellation of Draco. We find a good correlation between the observed UV background and the IR 100 micron flux, indicating that the dominant contributor of the diffuse background in the field is the scattered starlight from the interstellar dust grains. We also find strong evidence of additional emission in the FUV band which is absent in the NUV band. This is most likely due to Lyman band emission from molecular hydrogen in a ridge of dust running through the field and to line emissions from species such as C IV (1550 A) and Si II (1533 A) in the rest of the field. A strong correlation exists between the FUV/NUV ratio and the FUV intensity in the excess emission regions in the FUV band irrespective of the optical depth of the region. The optical depth increases more rapidly in the UV than the IR and we find that the UV/IR ratio drops off exponentially with increasing IR due to saturation effects in the UV. Using the positional details of Spitzer extragalactic objects, we find that the contribution of extragalactic light in the diffuse NUV background is 49 +/- 13 photon units and is 30 +/- 10 photon units in the FUV band.Comment: 30 pages, 13 figures, Accepted for publication in The Astrophysical Journal (ApJ), November 2010, v723 issu

The Local Interstellar Ultraviolet Radiation Field

I have used the Hipparcos Input Catalog, together with Kurucz model stellar atmospheres, and information on the strength of the interstellar extinction, to create a model of the expected intensity and spectral distribution of the local interstellar ultraviolet radiation field, under various assumptions concerning the albedo a of the interstellar grains. (This ultraviolet radiation field is of particular interest because of the fact that ultraviolet radiation is capable of profoundly affecting the chemistry of the interstellar medium.) By comparing my models with the observations, I am able to conclude that the albedo a of the interstellar grains in the far ultraviolet is very low, perhaps a = 0.1. I also advance arguments that my present determination of this albedo is much more reliable than any of the many previous (and conflicting) ultraviolet interstellar grain albedo determinations. Beyond this, I show that the ultraviolet background radiation that is observed at high galactic latitudes must be extragalactic in origin, as it cannot be backscatter of the interstellar radiation field.Comment: Astrophysical Journal, in press; 9 figures + 16 text page