44 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
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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
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 () (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 100 kpc away. We roughly
estimate M -6.75 and 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