STRATEGIC MANAGEMENT OF GENERAL MERCHANDISE

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A Parallel Solver for Graph Laplacians

Problems from graph drawing, spectral clustering, network flow and graph partitioning can all be expressed in terms of graph Laplacian matrices. There are a variety of practical approaches to solving these problems in serial. However, as problem sizes increase and single core speeds stagnate, parallelism is essential to solve such problems quickly. We present an unsmoothed aggregation multigrid method for solving graph Laplacians in a distributed memory setting. We introduce new parallel aggregation and low degree elimination algorithms targeted specifically at irregular degree graphs. These algorithms are expressed in terms of sparse matrix-vector products using generalized sum and product operations. This formulation is amenable to linear algebra using arbitrary distributions and allows us to operate on a 2D sparse matrix distribution, which is necessary for parallel scalability. Our solver outperforms the natural parallel extension of the current state of the art in an algorithmic comparison. We demonstrate scalability to 576 processes and graphs with up to 1.7 billion edges.Comment: PASC '18, Code: https://github.com/ligmg/ligm

FUSE Detection of Galactic OVI Emission in the Halo above the Perseus Arm

Background observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) toward l=95.4, b=36.1 show OVI 1032,1038 in emission. This sight line probes a region of stronger-than-average soft X-ray emission in the direction of high-velocity cloud Complex C above a part of the disk where Halpha filaments rise into the halo. The OVI intensities, 1600+/-300 ph/s/cm^2/sr (1032A) and 800+/-300 ph/s/cm^2/sr (1038A), are the lowest detected in emission in the Milky Way to date. A second sight line nearby (l=99.3, b=43.3) also shows OVI 1032 emission, but with too low a signal-to-noise ratio to obtain reliable measurements. The measured intensities, velocities, and FWHMs of the OVI doublet and the CII* line at 1037A are consistent with a model in which the observed emission is produced in the Galactic halo by hot gas ejected by supernovae in the Perseus arm. An association of the observed gas with Complex C appears unlikely.Comment: accepted for publication in ApJL, 11 pages including 3 figure

Model for Gravitational Interaction between Dark Matter and Baryons

We propose a phenomenological model where the gravitational interaction between dark matter and baryons is suppressed on small, subgalactic scales. We describe the gravitational force by adding a Yukawa contribution to the standard Newtonian potential and show that this interaction scheme is effectively suggested by the available observations of the inner rotation curves of small mass galaxies. Besides helping in interpreting the cuspy profile of dark matter halos observed in N-body simulations, this potential regulates the quantity of baryons within halos of different masses.Comment: 4 pages, 2 figures, final versio

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&

Highly-Ionized High-Velocity Gas in the Vicinity of the Galaxy

We report the results of an extensive FUSE study of high velocity OVI absorption along 102 complete sight lines through the Galactic halo. The high velocity OVI traces a variety of phenomena, including tidal interactions with the Magellanic Clouds, accretion of gas, outflow from the Galactic disk, warm/hot gas interactions in a highly extended Galactic corona, and intergalactic gas in the Local Group. We identify 85 high velocity OVI features at velocities of -500 < v(LSR) < +500 km/s along 59 of the 102 sight lines. Approximately 60% of the sky (and perhaps as much as 85%) is covered by high velocity H+ associated with the high velocity OVI. Some of the OVI is associated with known high velocity HI structures (e.g., the Magellanic Stream, Complexes A and C), while some OVI features have no counterpart in HI 21cm emission. The smaller dispersion in the OVI velocities in the GSR and LGSR reference frames compared to the LSR is necessary (but not conclusive) evidence that some of the clouds are extragalactic. Most of the OVI cannot be produced by photoionization, even if the gas is irradiated by extragalactic background radiation. Collisions in hot gas are the primary OVI ionization mechanism. We favor production of some of the OVI at the boundaries between warm clouds and a highly extended [R > 70 kpc], hot [T > 10^6 K], low-density [n < 10^-4 cm^-3] Galactic corona or Local Group medium. A hot Galactic corona or Local Group medium and the prevalence of high velocity OVI are consistent with predictions of galaxy formation scenarios. Distinguishing between the various phenomena producing high velocity OVI will require continuing studies of the distances, kinematics, elemental abundances, and physical states of the different types of high velocity OVI features found in this study. (abbreviated)Comment: 78 pages of text/tables + 31 figures, AASTeX preprint format. All figures are in PNG format due to astro-ph space restrictions. Bound copies of manuscript and two accompanying articles are available upon request. Submitted to ApJ

Detection of X-ray Clusters of Galaxies by Matching RASS Photons and SDSS Galaxies within GAVO

A new method for a simultaneous search for clusters of galaxies in X-ray photon maps and optical galaxy maps is described. The merging of X-ray and optical data improves the source identification so that a large amount of telescope time for spectroscopic follow-up can be saved. The method appears thus ideally suited for the analysis of the recently proposed wide-angle X-ray missions like DUO and ROSITA. As a first application, clusters are extracted from the 3rd version of the ROSAT All-Sky Survey and the Early Date Release of the Sloan Digital Sky Survey (SDSS). The time-consuming computations are performed within the German Astrophysical Virtual Observatory (GAVO). On a test area of 140 square degrees, 75 X-ray clusters are detected down to an X-ray flux limit of $3-5\times 10^{-13} {\rm erg} {\rm s}^{-1} {\rm cm}^{-2}$ in the ROSAT energy band 0.1-2.4 keV. The clusters have redshifts $z\le 0.5$. The survey thus fills the gap between traditional large-area X-ray surveys and serendipitous X-ray cluster searches based on pointed observations, and has the potential to yield about 4,000 X-ray clusters after completion of SDSS.Comment: 19 pages, low-resolution figures, accepted for publication in Astronomy and Astrophysic