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

Ubiquitous equatorial accretion disc winds in black hole soft states

High resolution spectra of Galactic Black Holes (GBH) reveal the presence of highly ionised absorbers. In one GBH, accreting close to the Eddington limit for more than a decade, a powerful accretion disc wind is observed to be present in softer X-ray states and it has been suggested that it can carry away enough mass and energy to quench the radio jet. Here we report that these winds, which may have mass outflow rates of the order of the inner accretion rate or higher, are an ubiquitous component of the jet-free soft states of all GBH. We furthermore demonstrate that these winds have an equatorial geometry with opening angles of few tens of degrees, and so are only observed in sources in which the disc is inclined at a large angle to the line of sight. The decrease in Fe XXV / Fe XXVI line ratio with Compton temperature, observed in the soft state, suggests a link between higher wind ionisation and harder spectral shapes. Although the physical interaction between the wind, accretion flow and jet is still not fully understood, the mass flux and power of these winds, and their presence ubiquitously during the soft X-ray states suggests they are fundamental components of the accretion phenomenon.Comment: Accepted for publication in MNRAS Letter

Numerical stability of a new conformal-traceless 3+1 formulation of the Einstein equation

There is strong evidence indicating that the particular form used to recast the Einstein equation as a 3+1 set of evolution equations has a fundamental impact on the stability properties of numerical evolutions involving black holes and/or neutron stars. Presently, the longest lived evolutions have been obtained using a parametrized hyperbolic system developed by Kidder, Scheel and Teukolsky or a conformal-traceless system introduced by Baumgarte, Shapiro, Shibata and Nakamura. We present a new conformal-traceless system. While this new system has some elements in common with the Baumgarte-Shapiro-Shibata-Nakamura system, it differs in both the type of conformal transformations and how the non-linear terms involving the extrinsic curvature are handled. We show results from 3D numerical evolutions of a single, non-rotating black hole in which we demonstrate that this new system yields a significant improvement in the life-time of the simulations.Comment: 7 pages, 2 figure

3D simulations of Einstein's equations: symmetric hyperbolicity, live gauges and dynamic control of the constraints

We present three-dimensional simulations of Einstein equations implementing a symmetric hyperbolic system of equations with dynamical lapse. The numerical implementation makes use of techniques that guarantee linear numerical stability for the associated initial-boundary value problem. The code is first tested with a gauge wave solution, where rather larger amplitudes and for significantly longer times are obtained with respect to other state of the art implementations. Additionally, by minimizing a suitably defined energy for the constraints in terms of free constraint-functions in the formulation one can dynamically single out preferred values of these functions for the problem at hand. We apply the technique to fully three-dimensional simulations of a stationary black hole spacetime with excision of the singularity, considerably extending the lifetime of the simulations.Comment: 21 pages. To appear in PR

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

Coherent Ro-vibrational Revivals in a Thermal Molecular Ensemble

We report an experimental and theoretical study of the evolution of vibrational coherence in a thermal ensemble of nitrogen molecules. Rotational dephasing and rephasing of the vibrational coherence is detected by coherent anti-Stokes Raman scattering. The existence of ro-vibrational coupling and the discrete energy spectrum of the rotational bath lead to a whole new class of full and fractional ro-vibrational revivals. Following the rich ro-vibrational dynamics on a nanosecond time scale with sub-picosecond time resolution enables us to determine the second-order ro-vibrational constant $gamma_e$ and assess new possibilities of controlling decoherence.Comment: submitted at Physical Review

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 $\nabla\cdot {\bf B}=0$ 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

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