10,680 research outputs found
Automated Detection of Voids in Redshift Surveys
We present a new void search algorithm for automated detection of voids in
three-dimensional redshift surveys. Based on a model in which the main features
of the LSS of the Universe are voids and walls, we classify the galaxies into
wall galaxies and field galaxies and we define voids as continuous volumes that
are devoid of any wall galaxies. Field galaxies are allowed within the voids.
The algorithm makes no assumptions regarding the shapes of the voids and the
only constraint that is imposed is that the voids are always thicker than a
preset limit, thus eliminating connections between adjacent voids through smal
breaches in the walls. By appropriate scaling of the parameters with the
selection function this algorithm can be used to analyze flux-limited surveys.
We test the algorithm on Voronoi tessellations and apply it to the SSRS2
redshift survey to derive the spectrum of void sizes and other void properties.
We find that the average diameter of a void is 37\pm 8 \h Mpc. We suggest the
usage of this fully automated algorithm to derive a quantitative description of
the voids, as another tool in describing the large scale structure of the
Universe and for comparison with numerical simulations.Comment: Accepted for publication in ApJ Letters; 13 page AAS latex file
including 1 table, 3 PS figures. Complete uuencoded compressed PostScript
file is available at ftp://shemesh.fiz.huji.ac.il or at
http://shemesh.fiz.huji.ac.il/papers.htm
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viSNE and Wanderlust, two algorithms for the visualization and analysis of high-dimensional single-cell data
The immune system presents a unique opportunity for studying development in mammals. White blood cells undergo differentiation and proliferation, a never-ending process throughout the life of the organism. Hematopoiesis, the development of cells in the immune system, depends upon the interaction between many different cell types (some of which comprise less than a tenth of a percent of the population), transient regulatory decisions, genomic rearrangement events, cell proliferation, and death. To capture these events we employ mass cytometry, a novel technology that measures fifty proteins simultaneously in single cells. Mass cytometry results in large quantities of high-dimensional data which challenges existing computational techniques. To address these challenges, we developed two dimensionality reduction algorithms for analyzing mass cytometry and other single-cell data. The first, viSNE, transforms high-dimensional data into an intuitive two-dimensional map, making it accessible to visual exploration. The second algorithm, Wanderlust, receives as input a static snapshot (where cells occupy different stages of their development) and constructs their developmental ordering: the developmental trajectory. viSNE maps healthy bone marrow into a canonical shape that separates cell subtypes. In leukemia, however, the shape is malformed: the maps of cancer samples are distinct from the healthy map and from each other. The algorithm highlights structure in the heterogeneity of surface phenotype expression in cancer, traverses the progression from diagnosis to relapse, and identifies a rare leukemia population in minimal residual disease settings. Wanderlust was applied to healthy B lineage cells, where the trajectory follows known marker expression trends and genetic recombination events. Using the Wanderlust trajectory we identified CD24 as an early marker of B cell development. The trajectory captures the coordination between several regulatory mechanisms (surface marker expression, signaling, proliferation and apoptosis) during crucial development checkpoints. As new technologies raise the number of simultaneously measured parameters in each cell to the hundreds, viSNE and Wanderlust will become a mainstay in analyzing and interpreting such experiments
The Size and Shape of Local Voids
We study the size and shape of low density regions in the local universe
which we identify in the smoothed density field of the PSCz flux limited IRAS
galaxy catalogue. After quantifying the systematic biases that enter in the
detection of voids using our data set and method, we identify, using a
smoothing length of 5 Mpc, 14 voids within 80 Mpc and using a
smoothing length of 10 Mpc, 8 voids within 130 Mpc. We study
the void size distribution and morphologies and find that there is roughly an
equal number of prolate and oblate-like spheroidal voids. We compare the
measured PSCz void shape and size distributions with those expected in six
different CDM models and find that only the size distribution can discriminate
between models. The models preferred by the PSCz data are those with
intermediate values of , independent of cosmology.Comment: final version, Accepted in MNRA
Finding Galaxy Clusters using Voronoi Tessellations
We present an objective and automated procedure for detecting clusters of
galaxies in imaging galaxy surveys. Our Voronoi Galaxy Cluster Finder (VGCF)
uses galaxy positions and magnitudes to find clusters and determine their main
features: size, richness and contrast above the background. The VGCF uses the
Voronoi tessellation to evaluate the local density and to identify clusters as
significative density fluctuations above the background. The significance
threshold needs to be set by the user, but experimenting with different choices
is very easy since it does not require a whole new run of the algorithm. The
VGCF is non-parametric and does not smooth the data. As a consequence, clusters
are identified irrispective of their shape and their identification is only
slightly affected by border effects and by holes in the galaxy distribution on
the sky. The algorithm is fast, and automatically assigns members to
structures.Comment: 11 pages, 11 figures. It uses aa.cls (included). Accepted by A&
Ceftriaxone-induced toxic epidermal necrolysis mimicking burn injury: a case report
<p>Abstract</p> <p>Introduction</p> <p>Toxic epidermal necrolysis is a rare exfoliative disorder with a high mortality rate.</p> <p>Case presentation</p> <p>We present a 70-year-old woman of Iranian descent who presented with toxic epidermal necrolysis that was initially diagnosed as a scald burn. Further anamnesis prompted by spread of the lesions during hospitalization revealed that the patient had been receiving ceftriaxone for several days. To the best of our knowledge, this is the first case of ceftriaxone-induced toxic epidermal necrolysis in the English literature.</p> <p>Conclusion</p> <p>Toxic epidermal necrolysis is an acute, life-threatening, exfoliative disorder with a high mortality rate. High clinical suspicion, prompt recognition, and initiation of supportive care is mandatory. Thorough investigation of the pathogenetic mechanisms is fundamental. Optimal treatment guidelines are still unavailable.</p
High frequency diffraction of an electromagnetic plane wave by an imperfectly conducting rectangular cylinder
Copyright @ 2011 IEEEWe shall consider the the problem of determining the scattered far wave field produced when a plane E-polarized wave is incident on an imperfectly conducting rectangular cylinder. By using the the uniform asymptotic solution for the problem of the diffraction of a plane wave by a right-angled impedance wedge, in conjunction with Keller's method, the a high frequency far field solution to the problem is given
On an Analytical Framework for Voids: Their abundances, density profiles and local mass functions
We present a general analytical procedure for computing the number density of
voids with radius above a given value within the context of gravitational
formation of the large scale structure of the universe out of Gaussian initial
conditions. To this end we develop an accurate (under generally satisfied
conditions) extension of unconditional mass function to constrained
environments, which allowes us both to obtain the number density of collapsed
objects of certain mass at any distance from the center of the void, and to
derive the number density of voids defined by those collapsed objects. We have
made detailed calculations for the spherically averaged mass density and halo
number density profiles for individual voids. We also present a formal
expression for the number density of voids defined by galaxies of a given type
and luminosity. This expression contains the probability for a collapsed object
of certain mass to host a galaxy of that type and luminosity as a function of
the environmental density. We propose a procedure to infer this function, which
may provide useful clues as to the galaxy formation process, from the observed
void densities.Comment: 14 pages, 7 figures, MNRAS in pres
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