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

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

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

A Survey of Open Clusters in the u'g'r'i'z' Filter System: I. Results for NGC2548 (M48)

We present initial results of a photometric survey of open star clusters, primarily in the southern hemisphere, taken in the u'g'r'i'z' filter system. While our entire observed sample covers more than 100 clusters, here we present data for NGC2548 (M48) which is a cluster characterized in the UBV and DDO photometric systems. We compare our results to the published values from other observers and to the Padova theoretical isochrones and metallicity curves. These observations demonstrate that the u'g'r'i'z' filters can play an important role in determining the metallicity of stars and clusters. We begin this series of papers with a study of NGC2548 because we have obtained data of this cluster not only with our main program telescope, the CTIO Curtis-Schmidt, but also with the US Naval Observatory (USNO) 1.0m telescope (the telescope used to define the u'g'r'i'z' system), and the Sloan Digital Sky Survey (SDSS) 0.5m Photometric Telescope (the photometric monitoring telescope used to calibrate the SDSS 2.5m telescope imaging data). We have used the data from this study to validate our ability to transform measurements obtained on other telescopes to the standard USNO 1.0m u'g'r'i'z' system. This validation is particularly important for very red stars, for which the original u'g'r'i'z' standard star network is poorly constrained.Comment: 32 pages, 8 figures. Complete, machine-readable versions of Tables 4-6 available at http://home.fnal.gov/~dtucker/OpenClusters/NGC2548/ . Accepted for publication in the Astronomical Journa

Discovery of New Ultracool White Dwarfs in the Sloan Digital Sky Survey

We report the discovery of five very cool white dwarfs in the Sloan Digital Sky Survey (SDSS). Four are ultracool, exhibiting strong collision induced absorption (CIA) from molecular hydrogen and are similar in color to the three previously known coolest white dwarfs, SDSS J1337+00, LHS 3250 and LHS 1402. The fifth, an ultracool white dwarf candidate, shows milder CIA flux suppression and has a color and spectral shape similar to WD 0346+246. All five new white dwarfs are faint (g > 18.9) and have significant proper motions. One of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system with a slightly warmer (T_{eff} ~ 5000K) white dwarf companion.Comment: 15 pages, 3 figures, submitted to ApJL. Higher resolution versions of finding charts are available at http://astro.uchicago.edu/~gates/findingchart

A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4

We present a catalog of 9316 spectroscopically confirmed white dwarfs from the Sloan Digital Sky Survey Data Release 4. We have selected the stars through photometric cuts and spectroscopic modeling, backed up by a set of visual inspections. Roughly 6000 of the stars are new discoveries, roughly doubling the number of spectroscopically confirmed white dwarfs. We analyze the stars by performing temperature and surface gravity fits to grids of pure hydrogen and helium atmospheres. Among the rare outliers are a set of presumed helium-core DA white dwarfs with estimated masses below 0.3 Msun, including two candidates that may be the lowest masses yet found. We also present a list of 928 hot subdwarfs.Comment: Accepted by the Astrophysical Journal Supplements, 25 pages, 24 figures, LaTeX. The electronic catalog, as well as diagnostic figures and links to the spectra, is available at http://das.sdss.org/wdcat/dr4

SDSS J092455.87+021924.9: an Interesting Gravitationally Lensed Quasar from the Sloan Digital Sky Survey

We report the discovery of a new gravitationally lensed quasar from the Sloan Digital Sky Survey, SDSS J092455.87+021924.9 (SDSS J0924+0219). This object was selected from among known SDSS quasars by an algorithm that was designed to select another known SDSS lensed quasar (SDSS 1226-0006A,B). Five separate components, three of which are unresolved, are identified in photometric follow-up observations obtained with the Magellan Consortium's 6.5m Walter Baade telescope at Las Campanas Observatory. Two of the unresolved components (designated A and B) are confirmed to be quasars with z=1.524; the velocity difference is less than 100 km sec^{-1} according to spectra taken with the W. M. Keck Observatory's Keck II telescope on Mauna Kea. A third stellar component, designated C, has the colors of a quasar with redshift similar to components A and B. The maximum separation of the point sources is 1.78". The other two sources, designated G and D, are resolved. Component G appears to be the best candidate for the lensing galaxy. Although component D is near the expected position of the fourth lensed component in a four image lens system, its properties are not consistent with being the image of a quasar at z~1.5. Nevertheless, the identical redshifts of components A and B and the presence of component C strongly suggest that this object is a gravitational lens. Our observations support the idea that a foreground object reddens the fourth lensed component and that another unmodeled effect (such as micro- or milli-lensing) demagnificates it, but we cannot rule out the possibility that SDSS0924+0219 is an example of the relatively rare class of ``three component'' lens systems.Comment: 24 pages, 6 figures, accepted by A

A New Milky Way Dwarf Galaxy in Ursa Major

In this Letter, we report the discovery of a new dwarf satellite to the Milky Way, located at ($\alpha_{2000}, \delta_{2000}$) $=$ (158.72,51.92) in the constellation of Ursa Major. This object was detected as an overdensity of red, resolved stars in Sloan Digital Sky Survey data. The color-magnitude diagram of the Ursa Major dwarf looks remarkably similar to that of Sextans, the lowest surface brightness Milky Way companion known, but with approximately an order of magnitude fewer stars. Deeper follow-up imaging confirms this object has an old and metal-poor stellar population and is $\sim$ 100 kpc away. We roughly estimate M$_V =$ -6.75 and $r_{1/2} =$ 250 pc for this dwarf. Its luminosity is several times fainter than the faintest known Milky Way dwarf. However, its physical size is typical for dSphs. Even though its absolute magnitude and size are presently quite uncertain, Ursa Major is likely the lowest luminosity and lowest surface brightness galaxy yet known.Comment: Replaced with ApJL accepted version. Includes some additional details, corrected references, and minor changes to Figure