980 research outputs found
Properties of Very Luminous Galaxies
Recent analysis of the SSRS2 data based on cell-counts and two-point
correlation function has shown that very luminous galaxies are much more
strongly clustered than fainter galaxies. In fact, the amplitude of the
correlation function of very luminous galaxies () asymptotically
approaches that of clusters. In this paper we investigate the
properties of the most luminous galaxies, with blue absolute magnitude . We find that: 1) the population mix is comparable to that in other ranges
of absolute magnitudes; 2) only a small fraction are located in bona fide
clusters; 3) the bright galaxy-cluster cross-correlation function is
significantly higher on large scales than that measured for fainter galaxies;
4) the correlation length of galaxies brighter than \MB ,
expressed as a function of the mean interparticle distance, appears to follow
the universal dimensionless correlation function found for clusters and radio
galaxies; 5) a large fraction of the bright galaxies are in interacting pairs,
others show evidence for tidal distortions, while some appear to be surrounded
by faint satellite galaxies. We conclude that very luminous optical galaxies
differ from the normal population of galaxies both in the clustering and other
respects. We speculate that this population is highly biased tracers of mass,
being associated to dark halos with masses more comparable to clusters than
typical loose groups.Comment: 29 pages (6 figures) + 2 tables; paper with all figures and images
available at http://boas5.bo.astro.it/~cappi/papers.html; The Astronomical
Journal, in pres
Biasing in the Galaxy Distribution
We investigate the variation of galaxy clustering with luminosity using the
recently completed SSRS2 sample. Clustering measurements based on the two-point
correlation function and the variance of counts in cells reveal the existence
of a strong dependence of the clustering amplitude on luminosity for galaxies
brighter than L*, while no significant variation is detected for fainter
galaxies. We derive a relative bias versus magnitude relation which can be
compared with theoretical predictions. Existing models of galaxy formation
cannot adequately reproduce the simultaneous steep rise of biasing at high
luminosities and the plateau at the low-luminosity end. Improved modeling of
the halo-galaxy relation and larger samples including low luminosity galaxies
are required to draw more definitive conclusions.Comment: 29 pages including 5 figures (tarred, gzipped & uuencoded LaTeX &
postscript files; postscript version including figures also available at
http://www.bo.astro.it/bap/BAPhome.html); ApJ, accepted for publicatio
The XMM-LSS survey: the Class 1 cluster sample over the extended 11 deg and its spatial distribution
This paper presents 52 X-ray bright galaxy clusters selected within the 11
deg XMM-LSS survey. 51 of them have spectroscopic redshifts
(), one is identified at , and all together make
the high-purity "Class 1" (C1) cluster sample of the XMM-LSS, the highest
density sample of X-ray selected clusters with a monitored selection function.
Their X-ray fluxes, averaged gas temperatures (median keV),
luminosities (median ergs/s) and total mass
estimates (median ) are measured, adapting to
the specific signal-to-noise regime of XMM-LSS observations. The redshift
distribution of clusters shows a deficit of sources when compared to the
cosmological expectations, regardless of whether WMAP-9 or Planck-2013 CMB
parameters are assumed. This lack of sources is particularly noticeable at . However, after quantifying uncertainties due to small
number statistics and sample variance we are not able to put firm (i.e. ) constraints on the presence of a large void in the cluster
distribution. We work out alternative hypotheses and demonstrate that a
negative redshift evolution in the normalization of the relation
(with respect to a self-similar evolution) is a plausible explanation for the
observed deficit. We confirm this evolutionary trend by directly studying how
C1 clusters populate the space, properly accounting for selection
biases. We point out that a systematically evolving, unresolved, central
component in clusters and groups (AGN contamination or cool core) can impact
the classification as extended sources and be partly responsible for the
observed redshift distribution.[abridged]Comment: 33 pages, 21 figures, 3 tables ; accepted for publication in MNRA
Extension and estimation of correlations in Cold Dark Matter models
We discuss the large scale properties of standard cold dark matter
cosmological models characterizing the main features of the power-spectrum, of
the two-point correlation function and of the mass variance. Both the
real-space statistics have a very well defined behavior on large enough scales,
where their amplitudes become smaller than unity. The correlation function, in
the range 0<\xi(r)<1, is characterized by a typical length-scale r_c, at which
\xi(r_c)=0, which is fixed by the physics of the early universe: beyond this
scale it becomes negative, going to zero with a tail proportional to -(r^{-4}).
These anti-correlations represent thus an important observational challenge to
verify models in real space. The same length scale r_c characterizes the
behavior of the mass variance which decays, for r>r_c, as r^{-4}, the fastest
decay for any mass distribution. The length-scale r_c defines the maximum
extension of (positively correlated) structures in these models. These are the
features expected for the dark matter field: galaxies, which represent a biased
field, however may have differences with respect to these behaviors, which we
analyze. We then discuss the detectability of these real space features by
considering several estimators of the two-point correlation function. By making
tests on numerical simulations we emphasize the important role of finite size
effects which should always be controlled for careful measurements.Comment: 18 pages, 27 figures, accepted for publication in Astronomy and
Astrophysic
Nanosized superconducting constrictions
Nanowires of lead between macroscopic electrodes are produced by means of an
STM. Magnetic fields may destroy the superconductivity in the electrodes, while
the wire remains in the superconducting state. The properties of the resulting
microscopic Josephson junctions are investigated.Comment: 3 pages,3 eps figures include
Certainty Closure: Reliable Constraint Reasoning with Incomplete or Erroneous Data
Constraint Programming (CP) has proved an effective paradigm to model and
solve difficult combinatorial satisfaction and optimisation problems from
disparate domains. Many such problems arising from the commercial world are
permeated by data uncertainty. Existing CP approaches that accommodate
uncertainty are less suited to uncertainty arising due to incomplete and
erroneous data, because they do not build reliable models and solutions
guaranteed to address the user's genuine problem as she perceives it. Other
fields such as reliable computation offer combinations of models and associated
methods to handle these types of uncertain data, but lack an expressive
framework characterising the resolution methodology independently of the model.
We present a unifying framework that extends the CP formalism in both model
and solutions, to tackle ill-defined combinatorial problems with incomplete or
erroneous data. The certainty closure framework brings together modelling and
solving methodologies from different fields into the CP paradigm to provide
reliable and efficient approches for uncertain constraint problems. We
demonstrate the applicability of the framework on a case study in network
diagnosis. We define resolution forms that give generic templates, and their
associated operational semantics, to derive practical solution methods for
reliable solutions.Comment: Revised versio
The XXL Survey VII: A supercluster of galaxies at z=0.43
The XXL Survey is the largest homogeneous and contiguous survey carried out
with XMM-Newton. Covering an area of 50 square degrees distributed over two
fields, it primarily investigates the large-scale structures of the Universe
using the distribution of galaxy clusters and active galactic nuclei as tracers
of the matter distribution. Given its depth and sky coverage, XXL is
particularly suited to systematically unveiling the clustering of X-ray
clusters and to identifying superstructures in a homogeneous X-ray sample down
to the typical mass scale of a local massive cluster. A friends-of-friends
algorithm in three-dimensional physical space was run to identify large-scale
structures. In this paper we report the discovery of the highest redshift
supercluster of galaxies found in the XXL Survey. We describe the X-ray
properties of the clusters members of the structure and the optical follow-up.
The newly discovered supercluster is composed of six clusters of galaxies at a
median redshift z around 0.43 and distributed across approximately 30 by 15 arc
minutes (10 by 5 Mpc on sky) on the sky. This structure is very compact with
all the clusters residing in one XMM pointing; for this reason this is the
first supercluster discovered with the XXL Survey. Spectroscopic follow-up with
WHT (William Herschel Telescope) and NTT (New Technology Telescope) confirmed a
median redshift of z = 0.43. An estimate of the X-ray mass and luminosity of
this supercluster and of its total gas mass put XLSSC-e at the average mass
range of superclusters; its appearance, with two members of equal size, is
quite unusual with respect to other superclusters and provides a unique view of
the formation process of a massive structure.Comment: A&A, accepted; special XXL issu
Ăvaluation des compĂ©tences pratiques en fin de deuxiĂšme cycle des Ă©tudes mĂ©dicales : exemple du drainage du bas appareil urinaire
IntroductionLe drainage vĂ©sical peut, mal pratiquĂ©, ĂȘtre iatrogĂšne en se compliquant notamment dâinfections et de traumatismes de lâappareil urinaire. Cette Ă©tude a pour objectif de dĂ©terminer la capacitĂ© des Ă©tudiants en mĂ©decine de fin de deuxiĂšme cycle Ă pratiquer les diffĂ©rents moyens de drainage des urines. MatĂ©riel et mĂ©thodes Entre janvier et mars 2007, un questionnaire dâautoĂ©valuation des compĂ©tences de drainage urinaire a Ă©tĂ© envoyĂ© par mail Ă un Ă©chantillon reprĂ©sentatif dâĂ©tudiants en mĂ©decine en derniĂšre annĂ©e dâexternat, soit deux mois avant lâĂ©preuve de lâexamen national classant. RĂ©sultats Deux cent soixante-dix-sept rĂ©ponses ont Ă©tĂ© reçues et analysĂ©es. Soixante-douze Ă©tudiants (26 %) jugeaient quâils maĂźtrisaient le cathĂ©tĂ©risme urĂ©trovĂ©sical chez lâhomme et 106 Ă©tudiants (38,3 %) chez la femme Ă la fin de leur externat. Soixante et onze Ă©tudiants sur les 277 (25,5 %) avaient effectuĂ© un stage en urologie au cours de leur cursus. Parmi eux, 53,5 % estimaient acquis le sondage Ă demeure (SAD) chez lâhomme (p < 0,001) et 39 (54,9 %) chez la femme (p < 0,001). Soixante-treize Ă©tudiants (26,4 %) estimaient quâils maĂźtrisaient le sondage minute dâune femme ou dâun homme et un seul considĂ©rait la pose de cathĂ©ter sus-pubien comme acquis. Conclusion Lâapprentissage des gestes de drainage urinaire est enseignĂ© de façon inappropriĂ©e au cours des Ă©tudes mĂ©dicales puisque de jeunes mĂ©decins se sentent incapables de les reproduire au terme de leur externat. Cela est critiquable, car le sondage doit pouvoir ĂȘtre rĂ©alisĂ© par tous mĂ©decins non urologues dans leur pratique quotidienne, notamment en terrain hospitalier. Cette Ă©tude doit mener Ă une rĂ©flexion sur lâamĂ©lioration de lâenseignement des gestes pratiques mĂ©dicaux essentiels pendant lâexternat
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