We have used GALEX observations of the North and South Galactic poles to study the diffuse ultraviolet background at locations where the Galactic light is expected to be at a minimum. We find offsets of 230 -290 photon units in the FUV (1531 Å) and 480 -580 photon units in the NUV (2361 Å). Of this, approximately 120 photon units can be ascribed to dust scattered light and another 110 (190 in the NUV) photon units to extragalactic radiation. The remaining radiation is, as yet, unidentified and amounts to 120 -180 photon units in the FUV and 300 -400 photon units in the NUV. We find that molecular hydrogen fluorescence contributes to the FUV when the 100 µm surface brightness is greater than 1.08 MJy sr −1 .
We have used data from the Galaxy Evolution Explorer to study the different components of the diffuse ultraviolet background in the region between the Galactic latitudes 70 • -80 • . We find an offset at zero dust column density (E(B -V) = 0) of 240 ± 18 photon units in the FUV (1539Å) and 394 ± 37 photon units in the NUV (2316 A). This is approximately half of the total observed radiation with the remainder divided between an extragalactic component of 114 ± 18 photon units in the FUV and 194 ± 37 photon units in the NUV and starlight scattered by Galactic dust at high latitudes. The optical constants of the dust grains were found to be a=0.4±0.1 and g=0.8±0.1 (FUV) and a=0.4±0.1 and g=0.5±0.1 (NUV). We cannot differentiate between a Galactic or extragalactic origin for the zero-offset but can affirm that it is not from any known source.
We study the alignment and rotational disruption of dust grains at the centre of our Galaxy using polarized thermal dust emission observed by SOFIA/HAWC+ and JCMT/SCUPOL at 53, 216, and 850 μm. We analyzed the relationship between the observed polarization degree with total emission intensity, dust temperature, gas column density, and polarization angle dispersion. Polarization degree from this region follows the predictions of the RAdiative Torque (RAT) alignment theory, except at high temperatures and long wavelengths where we found evidence for the rotational disruption of grains as predicted by the RAdiative Torque Disruption mechanism. The grain alignment and disruption sizes were found to be around 0.1 μm and 1 μm respectively. The maximum polarization degree observed was around p ∼ 13% at 216 μm and comes from a region of high dust temperature, low column density, and ordered magnetic field. Magnetically Enhanced RAT alignment (MRAT) was found to be important for grain alignment due to the presence of a strong magnetic field and can induce perfect alignment even when grains contain small iron clusters. We estimated the mass fraction of aligned grains using a parametric model for the fraction of the grains at high-J attractors and found it to correlate weakly with the observed polarization degree. We observe a change in the polarization ratio, from p216μm/p850μm < 1 to p216μm/p850μm > 1 at Td ≳ 35 K, which suggests a change in the grain model from a composite to a separate population of carbon and silicate grains as implied by previous numerical modeling.
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