7,361 research outputs found
The nature of the UV halo around the spiral galaxy NGC 3628
Thanks to deep UV observations with GALEX and Swift, diffuse UV haloes have
recently been discovered around galaxies. Based on UV-optical colours, it has
been advocated that the UV haloes around spiral galaxies are due to UV
radiation emitted from the disc and scattered off dust grains at high
latitudes. Detailed UV radiative transfer models that take into account
scattering and absorption can explain the morphology of the UV haloes, and they
require the presence of an additional thick dust disc next the to traditional
thin disc for half of the galaxies in their sample. We test whether such an
additional thick dust disc agrees with the observed infrared emission in NGC
3628, an edge-on galaxy with a clear signature of a thick dust disc. We extend
the far-ultraviolet radiative transfer models to full-scale panchromatic
models. Our model, which contains no fine-tuning, can almost perfectly
reproduce the observed spectral energy distribution from UV to mm wavelengths.
These results corroborate the interpretation of the extended UV emission in NGC
3628 as scattering off dust grains, and hence of the presence of a substantial
amount of diffuse extra-planar dust. A significant caveat, however, is the
geometrical simplicity and non-uniqueness of our model: other models with a
different geometrical setting could lead to a similar spectral energy
distribution. More detailed radiative transfer simulations that compare the
model results to images from UV to submm wavelengths are a way to break this
degeneracy, as are UV polarisation measurements.Comment: 6 pages, 2 figures, accepted for publication in Astronomy &
Astrophysic
Infinity in string cosmology: A review through open problems
We review recent developments in the field of string cosmology with
particular emphasis on open problems having to do mainly with geometric
asymptotics and singularities. We discuss outstanding issues in a variety of
currently popular themes, such as tree-level string cosmology asymptotics,
higher-order string correction effects, M-theory cosmology, braneworlds, and
finally ambient cosmology.Comment: 37 pages, to appear in the IJMPD, v2: matches published versio
Doughnut strikes sandwich: the geometry of hot medium in accreting black hole X-ray binaries
We study the effects of the mutual interaction of hot plasma and cold medium
in black hole binaries in their hard spectral state on the value of the
truncation radii of accretion discs. We consider a number of different
geometries. In contrast to previous theoretical studies, we use a modern
energy-conserving code for reflection and reprocessing from cold media. We show
that a static corona above a disc extending to the innermost stable circular
orbit produces spectra not compatible with those observed. They are either too
soft or require a much higher disc ionization than that observed. This
conclusion confirms a number of previous findings, but disproves a recent study
claiming an agreement of that model with observations. We show that the cold
disc has to be truncated in order to agree with the observed spectral hardness.
However, a cold disc truncated at a large radius and replaced by a hot flow
produces spectra which are too hard if the only source of seed photons for
Comptonization is the accretion disc. Our favourable geometry is a truncated
disc coexisting with a hot plasma either overlapping with the disc or
containing some cold matter within it, also including seed photons arising from
cyclo-synchrotron emission of hybrid electrons, i.e. containing both thermal
and non-thermal parts.Comment: 12 pages, 8 figures; A&A, in pres
Probing spatial homogeneity with LTB models: a detailed discussion
Do current observational data confirm the assumptions of the cosmological
principle, or is there statistical evidence for deviations from spatial
homogeneity on large scales? To address these questions, we developed a
flexible framework based on spherically symmetric, but radially inhomogeneous
Lemaitre-Tolman-Bondi (LTB) models with synchronous Big Bang. We expanded the
(local) matter density profile in terms of flexible interpolation schemes and
orthonormal polynomials. A Monte Carlo technique in combination with recent
observational data was used to systematically vary the shape of these profiles.
In the first part of this article, we reconsider giant LTB voids without dark
energy to investigate whether extremely fine-tuned mass profiles can reconcile
these models with current data. While the local Hubble rate and supernovae can
easily be fitted without dark energy, however, model-independent constraints
from the Planck 2013 data require an unrealistically low local Hubble rate,
which is strongly inconsistent with the observed value; this result agrees well
with previous studies. In the second part, we explain why it seems natural to
extend our framework by a non-zero cosmological constant, which then allows us
to perform general tests of the cosmological principle. Moreover, these
extended models facilitate explorating whether fluctuations in the local matter
density profile might potentially alleviate the tension between local and
global measurements of the Hubble rate, as derived from Cepheid-calibrated type
Ia supernovae and CMB experiments, respectively. We show that current data
provide no evidence for deviations from spatial homogeneity on large scales.
More accurate constraints are required to ultimately confirm the validity of
the cosmological principle, however.Comment: 18 pages, 12 figures, 2 tables; accepted for publication in A&
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