55 research outputs found
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
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 and the phase
function asymmetry factor with a value of 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 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
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
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