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

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 $h^{-1}$ Mpc, 14 voids within 80 $h^{-1}$ Mpc and using a smoothing length of 10 $h^{-1}$ Mpc, 8 voids within 130 $h^{-1}$ 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 $\sigma_{8} (\simeq 0.83)$, independent of cosmology.Comment: final version, Accepted in MNRA

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

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