486 research outputs found
Udryddelse af endemisk ektromeli i en kommerciel museavl ved hjaelp af hygiejniske og organisatoriske foranstaltninger
Discovery of a New Quadruple Lens HST 1411+5211
Gravitational lensing is an important tool for probing the mass distribution
of galaxies. In this letter we report the discovery of a new quadruple lens HST
1411+5211 found in archived WFPC2 images of the galaxy cluster CL140933+5226.
If the galaxy is a cluster member then its redshift is . The images of
the source appear unresolved in the WFC implying that the source is a quasar.
We have modeled the lens as both a single galaxy and a galaxy plus a cluster.
The latter model yields excellent fits to the image positions along with
reasonable parameters for the galaxy and cluster making HST 1411+5211 a likely
gravitational lens. Determination of the source redshift and confirmation of
the lens redshift would allow us to put strong constraints on the mass
distribution of the lensing galaxy.Comment: 11 pages including 1 postscript figure, aastex. Accepted to the ApJL.
Also available from:
http://www.astro.lsa.umich.edu:80/users/philf/www/papers/list.htm
Rheology and reptation of linear polymers. Ultrahigh molecular weight chain dynamics in the melt
Microscopic theory for quantum mirages in quantum corrals
Scanning tunneling microscopy permits to image the Kondo resonance of a
single magnetic atom adsorbed on a metallic surface. When the magnetic impurity
is placed at the focus of an elliptical quantum corral, a Kondo resonance has
been recently observed both on top of the impurity and on top of the focus
where no magnetic impurity is present. This projection of the Kondo resonance
to a remote point on the surface is referred to as quantum mirage. We present a
quantum mechanical theory for the quantum mirage inside an ideal quantum corral
and predict that the mirage will occur in corrals with shapes other than
elliptical
Chandra study of an overdensity of X-ray sources around two distant (z~0.5) clusters
We present results from a Chandra X-ray Observatory study of the field X-ray
source populations in 4 different observations: two high-redshift (z~0.5)
clusters of galaxies 3C295 and RXJ003033.2+261819; and two non-cluster fields
with similar exposure time. Surprisingly, the 0.5-2 keV source surface
densities (~900-1200 sources deg**-2 at a flux limit of 1.5x10**-15 erg
cm**-2s**-1) measured in an ~8'x8' area surrounding each cluster exceed by a
factor of ~2 the value expected on the basis of the ROSAT and Chandra
logN-logS, with a significance of ~2 sigma each, or ~3.5 sigma when the 2
fields are combined (i.e. a probability to be a statistical fluctuation of <1%
and <0.04%, respectively). The same analysis performed on the non-cluster
fields and on the outer chips of the cluster fields does NOT show evidence of
such an excess. In both cluster fields, the summed 0.5-10 keV spectrum of the
detected objects is well fitted by a power-law with Gamma~1.7 similar to AGNs
and shows no sign of intrinsic absorption. The few (~10 out of 35) optical
identifications available to date confirm that most of them are, as expected,
AGNs but the number of redshifts available is too small to allow conclusions on
their nature. We discuss possible interpretations of the overdensity in terms
of: a statistical variation of Cosmic Background sources; a concentration of
AGNs and/or powerful starburst galaxies associated with the clusters; and g
ravitational lensing of background QSO's by the galaxy clusters. All
explanations are however difficult to reconcile with the large number of excess
sources detected. Deeper X-ray observations and more redshifts measurements are
clearly required to settle the issue.Comment: 22 LateX pages (including Tables and Figures), uses psfig.sty and
emulateapj.sty. Accepted for publication in Astrophysical Journa
3C 295, a cluster and its cooling flow at z=0.46
We present ROSAT HRI data of the distant and X-ray luminous (L_x(bol)=2.6^
{+0.4}_{-0.2} 10^{45}erg/sec) cluster of galaxies 3C 295. We fit both a
one-dimensional and a two-dimensional isothermal beta-model to the data, the
latter one taking into account the effects of the point spread function (PSF).
For the error analysis of the parameters of the two-dimensional model we
introduce a Monte-Carlo technique. Applying a substructure analysis, by
subtracting a cluster model from the data, we find no evidence for a merger,
but we see a decrement in emission South-East of the center of the cluster,
which might be due to absorption. We confirm previous results by Henry &
Henriksen(1986) that 3C 295 hosts a cooling flow. The equations for the simple
and idealized cooling flow analysis presented here are solely based on the
isothermal beta-model, which fits the data very well, including the center of
the cluster. We determine a cooling flow radius of 60-120kpc and mass accretion
rates of dot{M}=400-900 Msun/y, depending on the applied model and temperature
profile. We also investigate the effects of the ROSAT PSF on our estimate of
dot{M}, which tends to lead to a small overestimate of this quantity if not
taken into account. This increase of dot{M} (10-25%) can be explained by a
shallower gravitational potential inferred by the broader overall profile
caused by the PSF, which diminishes the efficiency of mass accretion. We also
determine the total mass of the cluster using the hydrostatic approach. At a
radius of 2.1 Mpc, we estimate the total mass of the cluster (M{tot}) to be
(9.2 +/- 2.7) 10^{14}Msun. For the gas to total mass ratio we get M{gas}/M{tot}
=0.17-0.31, in very good agreement with the results for other clusters of
galaxies, giving strong evidence for a low density universe.Comment: 26 pages, 7 figures, accepted for publication in Ap
Kondo screening cloud effects in mesoscopic devices
We study how finite size effects may appear when a quantum dot in the Kondo
Coulomb blockade regime is embedded into a mesoscopic device with finite wires.
These finite size effects appear when the size of the mesoscopic device
containing the quantum dot is of the order of the size of Kondo cloud and
affect all thermodynamic and transport properties of the Kondo quantum dot. We
also generalize our results to the experimentally relevant case where the wires
contain several transverse modes/channels. Our results are based on
perturbation theory, Fermi liquid theory and slave boson mean field theory.Comment: 19 pages, 9 figure
Spectroscopic analysis of finite size effects around a Kondo quantum dot
We consider a simple setup in which a small quantum dot is strongly connected
to a finite size box. This box can be either a metallic box or a finite size
quantum wire.The formation of the Kondo screening cloud in the box strongly
depends on the ratio between the Kondo temperature and the box level spacing.
By weakly connecting two metallic reservoirs to the quantum dot, a detailed
spectroscopic analysis can be performed. Since the transport channels and the
screening channels are almost decoupled, such a setup allows an easier access
to the measure of finite-size effects associated with the finite extension of
the Kondo cloud.Comment: contribution to Les Houches proceeding, ``Quantum magnetism'' 200
Discovering study-specific gene regulatory networks
This article has been made available through the Brunel Open Access Publishing Fund.Microarrays are commonly used in biology because of their ability to simultaneously measure thousands of genes under different conditions. Due to their structure, typically containing a high amount of variables but far fewer samples, scalable network analysis techniques are often employed. In particular, consensus approaches have been recently used that combine multiple microarray studies in order to find networks that are more robust. The purpose of this paper, however, is to combine multiple microarray studies to automatically identify subnetworks that are distinctive to specific experimental conditions rather than common to them all. To better understand key regulatory mechanisms and how they change under different conditions, we derive unique networks from multiple independent networks built using glasso which goes beyond standard correlations. This involves calculating cluster prediction accuracies to detect the most predictive genes for a specific set of conditions. We differentiate between accuracies calculated using cross-validation within a selected cluster of studies (the intra prediction accuracy) and those calculated on a set of independent studies belonging to different study clusters (inter prediction accuracy). Finally, we compare our method's results to related state-of-the art techniques. We explore how the proposed pipeline performs on both synthetic data and real data (wheat and Fusarium). Our results show that subnetworks can be identified reliably that are specific to subsets of studies and that these networks reflect key mechanisms that are fundamental to the experimental conditions in each of those subsets
Kondo effect in coupled quantum dots: a Non-crossing approximation study
The out-of-equilibrium transport properties of a double quantum dot system in
the Kondo regime are studied theoretically by means of a two-impurity Anderson
Hamiltonian with inter-impurity hopping. The Hamiltonian, formulated in
slave-boson language, is solved by means of a generalization of the
non-crossing approximation (NCA) to the present problem. We provide benchmark
calculations of the predictions of the NCA for the linear and nonlinear
transport properties of coupled quantum dots in the Kondo regime. We give a
series of predictions that can be observed experimentally in linear and
nonlinear transport measurements through coupled quantum dots. Importantly, it
is demonstrated that measurements of the differential conductance , for the appropriate values of voltages and inter-dot tunneling
couplings, can give a direct observation of the coherent superposition between
the many-body Kondo states of each dot. This coherence can be also detected in
the linear transport through the system: the curve linear conductance vs
temperature is non-monotonic, with a maximum at a temperature
characterizing quantum coherence between both Kondo states.Comment: 20 pages, 17 figure
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