4,019 research outputs found
The dust distribution in edge-on galaxies. Radiative transfer fits of V and K'-band images
Aims: I have analyzed a sample of seven nearby edge-on galaxies observed in
the V and K'-band, in order to infer the properties of the dust distribution.
Methods: A radiative transfer model, including scattering, have been used to
decompose each image into a stellar disk, a bulge, and a dust disk. The
parameters describing the distributions have been obtained through standard X^2
minimization techniques.
Results: The dust disks fitted to the V-band images are consistent with
previous work in literature: the radial scalelength of dust is larger than that
for stars (h_d/h_s ~ 1.5); the dust disk has a smaller vertical scalelength
than the stellar (z_d/z_s ~ 1/3); the dust disk is almost transparent when seen
face-on (central, face-on, optical depth tau_0 =0.5-1.5). Faster radiative
transfer models which neglect scattering can produce equivalent fits, with
changes in the derived parameters within the accuracy of full fits including
scattering. In the K'-band, no trace is found of a second, massive, dust disk
which has been invoked to explain observations of dust emission in the
submillimeter. I discuss the model degeneracies and the effect of complex
structures on the fitted distributions. In particular, most bulges in the
sample show a box/peanuts morphology with large residuals; two
lower-inclination galaxies show a dust ring distribution, which could be the
cause for the large fitted dust scalelengths.Comment: A&A accepted, 22 pages, including 13 pages of "Online Material", 16
figure
Vindicating single-T modified blackbody fits to Herschel SEDs (Research Note)
I show here that the bulk of the dust mass in a galaxy can be equivalently
estimated from: i) the full spectral energy distribution of dust emission,
using the approach of Draine & Lee (2007) that includes a distribution of dust
grains and a range of interstellar radiation field intensities; ii) the
emission in the wavelength range 100um <= lambda <= 500um (covered by the
Herschel Space Observatory), by fitting to the data a simpler single
temperature modified blackbody. Recent claims on the contrary (Dale et al.
2012) should be interpreted as a caveat to use in the simpler fits an
absorption cross section which is consistent both in the normalization and in
the spectral index beta with that of the full dust model. I also show that the
dust mass does not depend significantly on the choice of beta, if both the dust
mass and the absorption cross section are derived with the same assumption on
beta.Comment: 5 pages, 2 figures, accepted for publication on A&
Monte Carlo simulations of dusty spiral galaxies: extinction and polarization properties
We present Monte Carlo simulations of dusty spiral galaxies, modelled as
bulge + disk systems, aimed to study their extinction and polarization
properties. The extinction parameters (absorption and scattering) of dust
grains are calculated from Mie's theory for a full distribution of sizes and
materials; the radiation transfer is carried on for the four Stokes parameters.
Photometric and polarimetric maps of galaxies of different optical depths,
inclinations and bulge-to-total ratios have been produced in the B and I
bandpasses. As expected, the effect of scattering is to reduce substantially
the extinction for a given optical depth, in particular for what concerns the
obscuration of bright bulge cores. For the same reason, scattering reduces also
the reddening, as evaluated from B-I maps. On the other hand the bluing
directly due to forward scattering is hardly appreciable. Radial color
gradients are often found. A comparison with ``sandwich'' models shows that
they fail dramatically to reproduce the extinction - optical depth relation.
The degree of linear polarization produced by scattering is usually of the
order of a few percent; it increases with optical depth, and with inclination
(less than 80 degrees). The polarization pattern is always perpendicular to the
major axis, unless the dust distribution is drastically modified. There is
little local correlation between extinction and polarization degree and there
is a trend of increasing polarization from the B to the I band. We discuss
implications and relevance of the results for studies of the structure and
morphology of spiral galaxies and of their interstellar medium.Comment: 43 pages, 13 Postscript figures, Latex AAS manuscript substyle, Ap.
J. Accepte
Where was Mean Solar Time first adopted?
It is usually stated in the literature that Geneva was the first city to
adopt mean solar time, in 1780, followed by London (or the whole of England) in
1792, Berlin in 1810 and Paris in 1816. In this short paper I will partially
revise this statement, using primary references when available, and provide
dates for a few other European cities. Although no exact date was found for the
first public use of mean time, the primacy seems to belong to England, followed
by Geneva in 1778-1779 (for horologists), Berlin in 1810, Geneva in 1821 (for
public clocks), Vienna in 1823, Paris in 1826, Rome in 1847, Turin in 1849, and
Milan, Bologna and Florence in 1860.Comment: 5 pages. Text changed to match the published versio
Dust Emissivity in the Far-Infrared
We have derived the dust emissivity in the Far-Infrared (FIR) using data
available in the literature. We use two wavelength dependences derived from
spectra of Galactic FIR emission (Reach et al. 1995). A value for the
emissivity, normalised to the extinction efficiency in the V band, has been
retrieved from maps of Galactic FIR emission, dust temperature and extinction
(Schlegel et al. 1998).
Our results are similar to other measurements in the Galaxy but only
marginally consistent with the widely quoted values of Hildebrand (1983)
derived on one reflection nebula. The discrepancy with measurements on other
reflection nebulae (Casey 1991) is higher and suggests a different grain
composition in these environments with respect to the diffuse interstellar
medium.
We measure dust masses for a sample of six spiral galaxies with FIR
observations and obtain gas-to-dust ratios close to the Galactic value.Comment: 5 pages, 1 ps file, A&A letter accepte
Effects of Clumping on the Observed Properties of Dusty Galaxies
We present Monte Carlo radiative transfer simulations for spiral galaxies
modelled as a stellar disk and a two-phase clumpy dust distribution. We divide
the volume occupied by the dust into a three-dimensional grid and assign each
cell a clump or smooth medium status. Cell dimension, clump dust mass and
spatial distribution are derived from the observed properties of Giant
Molecular Clouds and molecular gas in the Galaxy. We produce models for several
values of the optical depth and fraction of the interstellar medium residing in
clumps. As a general result, clumpy models are less opaque than the
corresponding homogeneous models. For the adopted parameters, the increase in
the fraction of energy that escapes the disk is moderate, resulting in surface
brightness profiles that are less than one magnitude brighter than those of the
homogeneous models. The effects of clumping are larger for edge-on views of the
disk. This is in contrast with previous preliminary results for clumping in the
literature. We show how differences arise from the different parametrisation
and clump distribution adopted. We also consider models in which a fraction of
the stellar radiation is emitted within the clumps. In this case, galaxies are
less transparent than in the case when only dust is clumped. The opacity can be
even higher than in the homogeneous case, depending on the fraction of embedded
stellar emission. We point out the implications of the results for the
determination of the opacity and dust mass of spiral galaxies.Comment: 11 pages, 8 figures, MNRAS accepted, also available at
http://www.arcetri.astro.it/~sbianchi/clumping.htm
ISO observations of spirals: modelling the FIR emission
ISO observations at 200 micron have modified our view of the dust component
in spiral galaxies. For a sample of seven resolved spirals we have retrieved a
mean temperature of 20K, about 10K lower than previous estimates based on IRAS
data at shorter wavelengths. Because of the steep dependence of far-infrared
emission on the dust temperature, the dust masses inferred from ISO fluxes are
a factor of 10 higher than those derived from IRAS data only, leading to
gas-to-dust ratios close to the value observed in the Galaxy. The scale-length
of the 200 micron emission is larger than for the IRAS 100 micron emission,
with colder dust at larger distances from the galactic centre, as expected if
the interstellar radiation field is the main source of dust heating. The 200
micron scale-length is also larger than the optical, for all the galaxies in
the sample. This suggests that the dust distribution is more extended than that
of the stars.A model of the dust heating is needed to derive the parameters of
the dust distribution from the FIR emission. Therefore, we have adapted an
existing radiative transfer code to deal with dust emission. Simulated maps of
the temperature distribution within the dust disk and of the dust emission at
any wavelength can be produced. The stellar spectral energy distribution is
derived from observations in the ultraviolet, optical and near infrared. The
parameters of the dust distribution (scale-lengths and optical depth) are
chosen to reproduce the observed characteristics of the FIR emission, i.e. the
shape of the spectrum, the flux and the spatial distribution. We describe the
application of the model to one of the galaxies in the sample, NGC 6946.Comment: 6 pages, 5 figures. Contribution to the proceedings of the workshop
"ISO Beyond Point Sources" held at VILSPA 14-17 September 199
Bidirectional deep-readout echo state networks
We propose a deep architecture for the classification of multivariate time
series. By means of a recurrent and untrained reservoir we generate a vectorial
representation that embeds temporal relationships in the data. To improve the
memorization capability, we implement a bidirectional reservoir, whose last
state captures also past dependencies in the input. We apply dimensionality
reduction to the final reservoir states to obtain compressed fixed size
representations of the time series. These are subsequently fed into a deep
feedforward network trained to perform the final classification. We test our
architecture on benchmark datasets and on a real-world use-case of blood
samples classification. Results show that our method performs better than a
standard echo state network and, at the same time, achieves results comparable
to a fully-trained recurrent network, but with a faster training
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