160 research outputs found
The Color Distributions of Globular Clusters in Virgo Elliptical Galaxies
This Letter presents the color distributions of the globular cluster (GC)
systems of 12 Virgo elliptical galaxies, measured using data from the Hubble
Space Telescope. Bright galaxies with large numbers of detected GC's show two
distinct cluster populations with mean V-I colors near 1.01 and 1.26. The GC
population of M86 is a clear exception; its color distribution shows a single
sharp peak near V-I=1.03. The absence of the red population in this galaxy, and
the consistency of the peak colors in the others, may be indications of the
origins of the two populations found in most bright elliptical galaxies.Comment: 5 pages, 1 figure, to be published in ApJ Letters Corrections to
introductio
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The SDSS data archive server
The Sloan Digital Sky Survey (SDSS) Data Archive Server (DAS) provides public access to data files produced by the SDSS data reduction pipeline. This article discusses challenges in public distribution of data of this volume and complexity, and how the project addressed them. The Sloan Digital Sky Survey (SDSS)1 is an astronomical survey of covering roughly one quarter of the night sky. It contains images of this area, a catalog of almost 300 million objects detected in those images, and spectra of more than a million of these objects. The catalog of objects includes a variety of data on each object. These data include not only basic information but also fit parameters for a variety of models, classifications by sophisticated object classification algorithms, statistical parameters, and more. If the survey contains the spectrum of an object, the catalog includes a variety of other parameters derived from its spectrum. Data processing and catalog generation, described more completely in the SDSS Early Data Release2 paper, consists of several stages: collection of imaging data, processing of imaging data, selection of spectroscopic targets from catalogs generated from the imaging data, collection of spectroscopic data, processing of spectroscopic data, and loading of processed data into a database. Each of these stages is itself a complex process. For example, the software that processes the imaging data determines and removes some instrumental signatures in the raw images to create 'corrected frames', models the point spread function, models and removes the sky background, detects objects, measures object positions, measures the radial profile and other morphological parameters for each object, measures the brightness of each object using a variety of methods, classifies the objects, calibrates the brightness measurements against survey standards, and produces a variety of quality assurance plots and diagnostic tables. The complexity of the spectroscopic data reduction pipeline is similar. Each pipeline deposits the results in a collection of files on disk. The Catalog Archive Server (CAS) provides an interface to a database of objects detected through the SDSS along with their properties and observational metadata. This serves the needs of most users, but some users require access to files produced by the pipelines. Some data, including the corrected frames (the pixel data itself corrected for instrumental signatures), the models for the point spread function, and an assortment of quality assurance plots, are not included in the database at all. Sometimes it is simply more convenient for a user to read data from existing files than to retrieve it using database queries. This is often the case, for example, when a user wants to download data a significant fraction of objects in the database. Users might need to perform analysis that requires more computing power than the CAS database servers can reasonably provide, and so need to download the data so that it can be analyzed with local resources. Users can derive observational parameters not measured by the standard SDSS pipeline from the corrected frames, metadata, and other data products, or simply use the output of tools with which they're familiar. The challenge in distributing these data is lies not in the distribution method itself, but in providing tools and support that allow users to find the data they need and interpret it properly. After introducing the data itself, this article describes how the DAS uses ubiquitous and well understood technologies to manage and distribute the data. It then discusses how it addresses the more difficult problem of helping the public find and use the data it contains, despite its complexity of its content and organization
Massively parallel simulations of binary black holes with Dendro-GR
We present results from the new Dendro-GR code. These include simulations of
binary black hole mergers for mass ratios up to q=16. Dendro-GR uses Wavelet
Adaptive Multi-Resolution (WAMR) to generate an unstructured grid adapted to
the spacetime geometry together with an octree based data structure. We
demonstrate good scaling, improved convergence properties and efficient use of
computational resources. We validate the code with comparisons to LazEv
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Running the Sloan Digital Sky Survey data archive server
The Sloan Digital Sky Survey (SDSS) Data Archive Server (DAS) provides public access to over 12Tb of data in 17 million files produced by the SDSS data reduction pipeline. Many tasks which seem trivial when serving smaller, less complex data sets present challenges when serving data of this volume and technical complexity. The included output files should be chosen to support as much science as possible from publicly released data, and only publicly released data. Users must have the resources needed to read and interpret the data correctly. Server administrators must generate new data releases at regular intervals, monitor usage, quickly recover from hardware failures, and monitor the data served by the DAS both for contents and corruption. We discuss these challenges, describe tools we use to administer and support the DAS, and discuss future development plans
The Surface Brightness Fluctuations and Globular Cluster Populations of M87 and its Companions
Using the surface brightness fluctuations in HST WFPC-2 images, we determine
that M87, NGC 4486B, and NGC 4478 are all at a distance of ~16 Mpc, while NGC
4476 lies in the background at ~21 Mpc. We also examine the globular clusters
of M87 using archived HST fields. We detect the bimodal color distribution, and
find that the amplitude of the red peak relative to the blue peak is greatest
near the center. This feature is in good agreement with the merger model of
elliptical galaxy formation, where some of the clusters originated in
progenitor galaxies while other formed during mergers.Comment: 5 pages, 2 figure
Relativistic MHD with Adaptive Mesh Refinement
This paper presents a new computer code to solve the general relativistic
magnetohydrodynamics (GRMHD) equations using distributed parallel adaptive mesh
refinement (AMR). The fluid equations are solved using a finite difference
Convex ENO method (CENO) in 3+1 dimensions, and the AMR is Berger-Oliger.
Hyperbolic divergence cleaning is used to control the
constraint. We present results from three flat space tests, and examine the
accretion of a fluid onto a Schwarzschild black hole, reproducing the Michel
solution. The AMR simulations substantially improve performance while
reproducing the resolution equivalent unigrid simulation results. Finally, we
discuss strong scaling results for parallel unigrid and AMR runs.Comment: 24 pages, 14 figures, 3 table
Perturbed disks get shocked. Binary black hole merger effects on accretion disks
The merger process of a binary black hole system can have a strong impact on
a circumbinary disk. In the present work we study the effect of both central
mass reduction (due to the energy loss through gravitational waves) and a
possible black hole recoil (due to asymmetric emission of gravitational
radiation). For the mass reduction case and recoil directed along the disk's
angular momentum, oscillations are induced in the disk which then modulate the
internal energy and bremsstrahlung luminosities. On the other hand, when the
recoil direction has a component orthogonal to the disk's angular momentum, the
disk's dynamics are strongly impacted, giving rise to relativistic shocks. The
shock heating leaves its signature in our proxies for radiation, the total
internal energy and bremsstrahlung luminosity. Interestingly, for cases where
the kick velocity is below the smallest orbital velocity in the disk (a likely
scenario in real AGN), we observe a common, characteristic pattern in the
internal energy of the disk. Variations in kick velocity simply provide a phase
offset in the characteristic pattern implying that observations of such a
signature could yield a measure of the kick velocity through electromagnetic
signals alone.Comment: 10 pages, 13 figures. v2: Minor changes, version to be published in
PR
High-Fidelity Multidisciplinary Design Optimization Methodology with Application to Rotor Blades
A multidisciplinary design optimization procedure has been developed and applied to rotorcraft simulations involving tightly-coupled, high-fidelity computational fluid dynamics and comprehensive analysis. A discretely-consistent, adjoint-based sensitivity analysis available in the fluid dynamics solver provides sensitivities arising from unsteady turbulent flows on unstructured, dynamic, overset meshes, while a complex-variable approach is used to compute structural sensitivities with respect to aerodynamic loads. The multidisciplinary sensitivity analysis is conducted through integrating the sensitivity components from each discipline of the coupled system. Accuracy of the coupled system for high-fidelity rotorcraft analysis is verified; simulation results exhibit good agreement with established solutions. A constrained gradient-based design optimization for a HART-II rotorcraft configuration is demonstrated. The computational cost for individual components of the multidisciplinary sensitivity analysis is assessed and improved
Simulating binary neutron stars: dynamics and gravitational waves
We model two mergers of orbiting binary neutron stars, the first forming a
black hole and the second a differentially rotating neutron star. We extract
gravitational waveforms in the wave zone. Comparisons to a post-Newtonian
analysis allow us to compute the orbital kinematics, including trajectories and
orbital eccentricities. We verify our code by evolving single stars and
extracting radial perturbative modes, which compare very well to results from
perturbation theory. The Einstein equations are solved in a first order
reduction of the generalized harmonic formulation, and the fluid equations are
solved using a modified convex essentially non-oscillatory method. All
calculations are done in three spatial dimensions without symmetry assumptions.
We use the \had computational infrastructure for distributed adaptive mesh
refinement.Comment: 14 pages, 16 figures. Added one figure from previous version;
corrected typo
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