19,026 research outputs found
The spherical collapse model in a universe with cosmological constant
We generalize the spherical collapse model for the formation of dark matter
halos to apply in a universe with arbitrary positive cosmological constant. We
calculate the critical condition for collapse of an overdense region and give
exact values of the characteristic densities and redshifts of its evolution.Comment: 6 pages, 3 figures, contribution to the proceedings of the 3rd
International Workshop on the Identification of Dark Matter (IDM2000) in
York, in pres
Entropy of gas and dark matter in galaxy clusters
On the basis of a large scale 'adiabatic', namely non-radiative and
non-dissipative, cosmological smooth particle hydrodynamic simulation we
compare the entropy profiles of the gas and the dark matter (DM) in galaxy
clusters. The quantity K_g = T_g \rho_g^{-2/3} provides a measure for the
entropy of the intra-cluster gas. By analogy with the thermodynamic variables
of the gas the velocity dispersion of the DM is associated with a formal
temperature and thereby K_DM = \sigma_DM^2 \rho_DM^{-2/3} is defined. This DM
entropy is related to the DM phase space density by K_DM \propto Q_DM^{-2/3}.
In accord with other studies the DM phase space density follows a power law
behaviour, Q_DM \propto r^{-1.82}, which corresponds to K_DM \propto r^{1.21}.
The simulated intra-cluster gas has a flat entropy core within (0.8 \pm 0.4)
R_s, where R_s is the NFW scale radius. The outer profile follows the DM
behaviour, K_g \propto r^{1.21}, in close agreement with X-ray observations.
Upon scaling the DM and gas densities by their mean cosmological values we find
that outside the entropy core a constant ratio of K_g / K_{DM} = 0.71 \pm 0.18
prevails. By extending the definition of the gas temperature to include also
the bulk kinetic energy the ratio of the DM and gas extended entropy is found
to be unity for r > 0.8 R_s. The constant ratio of the gas thermal entropy to
that of the DM implies that observations of the intra-cluster gas can provide
an almost direct probe of the DM.Comment: 7 pages, 8 figures, accepted for publication in MNRAS, web page of
the The Marenostrum Numerical Cosmology Project :
http://astro.ft.uam.es/~marenostrum
Learning the Roots of Visual Domain Shift
In this paper we focus on the spatial nature of visual domain shift,
attempting to learn where domain adaptation originates in each given image of
the source and target set. We borrow concepts and techniques from the CNN
visualization literature, and learn domainnes maps able to localize the degree
of domain specificity in images. We derive from these maps features related to
different domainnes levels, and we show that by considering them as a
preprocessing step for a domain adaptation algorithm, the final classification
performance is strongly improved. Combined with the whole image representation,
these features provide state of the art results on the Office dataset.Comment: Extended Abstrac
The magnetoresistance tensor of La(0.8)Sr(0.2)MnO(3)
We measure the temperature dependence of the anisotropic magnetoresistance
(AMR) and the planar Hall effect (PHE) in c-axis oriented epitaxial thin films
of La(0.8)Sr(0.2)MnO(3), for different current directions relative to the
crystal axes, and show that both AMR and PHE depend strongly on current
orientation. We determine a magnetoresistance tensor, extracted to 4th order,
which reflects the crystal symmetry and provides a comprehensive description of
the data. We extend the applicability of the extracted tensor by determining
the bi-axial magnetocrystalline anisotropy in our samples
The shape of the LoTr 5 planetary nebula
We observed the large and faint planetary nebula (PN) around IN Com in H-alpha and [NII] light with a coronagraphic CCD device on the Wise Observatory reflector blocking the light from the central star. Our goal was to provide a second image of the object with which to confirm the features seen in the only published photograph from the paper reporting the discovery of this object. The nebula is extremely faint, but a combination of images totalling ~one and a half hours of exposure shows it fairly well. A novel image processing algorithm has been applied to the noisy image in order to reveal faint extended details of the images. The algorithm is based on a non-linear self-adaptive filter applied to the wavelet transform of the image. The nebula is not round or elliptical, but shows a two-lobed and possibly three-lobed morphology, as well as a peculiar hole-like feature East of the central star. There is definite East-West and slightly less definite North-South asymmetry
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