128 research outputs found
Dust properties from GALEX observations of a UV halo around Spica
GALEX has detected ultraviolet halos extending as far as 5 around
four bright stars (Murthy et al. (2011)). These halos are produced by
scattering of starlight by dust grains in thin foreground clouds that are not
physically associated with the star. Assuming a simple model consisting of a
single layer of dust in front of the star, Murthy et al.(2011) have been able
to model these halo intensities and constrain the value of the phase function
asymmetry factor of the scattering grains in the FUV and NUV. However due
to the uncertainty in the dust geometry they could not constrain the albedo. In
this work we have tried to constrain the optical constants and dust geometry by
modeling the UV halo of Spica. Since the halo emission is not symmetric, we
have modeled the Northern and Southern parts of the halo separately. To the
North of Spica, the best-fit albedo is 0.260.1 and is 0.580.11 in
the FUV at the 90% confidence level. The corresponding limits on the distance
and optical depth () of the dust sheet is 3.651.05 pc and
0.0470.006 respectively. However, owing to a complicated dust distribution
to the South of Spica, we were unable to uniquely constrain the dust parameters
in that region. Nevertheless, by assuming the optical constants of the Northern
region and assuming a denser medium, we were able to constrain the distance of
the dust to 9.51.5 pc and the corresponding to 0.040.01.Comment: 4 pages, accepted for publication in Earth, Planets and Spac
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 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
---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
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 in the FUV and 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
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