1,286 research outputs found
A Census of Object Types and Redshift Estimates in the SDSS Photometric Catalog from a Trained Decision-Tree Classifier
We have applied ClassX, an oblique decision tree classifier optimized for
astronomical analysis, to the homogeneous multicolor imaging data base of the
Sloan Digital Sky Survey (SDSS), training the software on subsets of SDSS
objects whose nature is precisely known via spectroscopy. We find that the
software, using photometric data only, correctly classifies a very large
fraction of the objects with existing SDSS spectra, both stellar and
extragalactic. ClassX also accurately predicts the redshifts of both normal and
active galaxies in SDSS. To illustrate ClassX applications in SDSS research, we
(a) derive the object content of the SDSS DR2 photometric catalog and (b)
provide a sample catalog of resolved SDSS objects that contains a large number
of candidate AGN galaxies, 27,000, along with 63,000 candidate normal galaxies
at magnitudes substantially fainter than typical magnitudes of SDSS
spectroscopic objects. The surface density of AGN selected by ClassX to i~19 is
in agreement with that quoted by SDSS. When ClassX is applied to the
photometric data fainter than the SDSS spectroscopic limit, the inferred
surface density of AGN rises sharply, as expected. The ability of the
classifier to accurately constrain the redshifts of huge numbers (ultimately ~
10^7) of active galaxies in the photometric data base promises new insights
into fundamental issues of AGN research, such as the evolution of the AGN
luminosity function with cosmic time, the starburst--AGN connection, and
AGN--galactic morphology relationships.Comment: Accepted for publication in The Astronomical Journal, Vol. 130, 2005;
33 pages, 10 figures, 5 tables, AASTeX v5.0. Table 5 will be electronic in
the published journal, but available now at
http://www-int.stsci.edu/~margon/table5.ascii and
http://www-int.stsci.edu/~margon/table5.ascii.g
Extreme Ultraviolet Emission from Clusters of Galaxies: Inverse Compton Radiation from a Relic Population of Cosmic Ray Electrons?
We suggest that the luminous extreme ultraviolet (EUV) emission which has
been detected recently from clusters of galaxies is Inverse Compton (IC)
scattering of Cosmic Microwave Background (CMB) radiation by low energy cosmic
ray electrons in the intracluster medium. The cosmic ray electrons would have
Lorentz factors of gamma ~ 300, and would lose energy primarily by emitting EUV
radiation. These particles have lifetimes comparable to the Hubble time; thus,
the electrons might represent a relic population of cosmic rays produced by
nonthermal activity over the history of the cluster. The IC model naturally
explains the observed increase in the ratio of EUV to X-ray emission with
radius in clusters. The required energy in cosmic ray electrons is typically
1--10% of the thermal energy content of the intracluster gas. We suggest that
the cosmic ray electrons might have been produced by supernovae in galaxies, by
radio galaxies, or by particle acceleration in intracluster shocks.Comment: ApJ Letters, in press, 4 pages with 1 embedded figure, Latex in
emulateapj styl
Running a distributed virtual observatory: US Virtual Astronomical Observatory operations
Operation of the US Virtual Astronomical Observatory shares some issues with
modern physical observatories, e.g., intimidating data volumes and rapid
technological change, and must also address unique concerns like the lack of
direct control of the underlying and scattered data resources, and the
distributed nature of the observatory itself. In this paper we discuss how the
VAO has addressed these challenges to provide the astronomical community with a
coherent set of science-enabling tools and services. The distributed nature of
our virtual observatory-with data and personnel spanning geographic,
institutional and regime boundaries-is simultaneously a major operational
headache and the primary science motivation for the VAO. Most astronomy today
uses data from many resources. Facilitation of matching heterogeneous datasets
is a fundamental reason for the virtual observatory. Key aspects of our
approach include continuous monitoring and validation of VAO and VO services
and the datasets provided by the community, monitoring of user requests to
optimize access, caching for large datasets, and providing distributed storage
services that allow user to collect results near large data repositories. Some
elements are now fully implemented, while others are planned for subsequent
years. The distributed nature of the VAO requires careful attention to what can
be a straightforward operation at a conventional observatory, e.g., the
organization of the web site or the collection and combined analysis of logs.
Many of these strategies use and extend protocols developed by the
international virtual observatory community.Comment: 7 pages with 2 figures included within PD
HST image restoration: A comparison of pre- and post-servicing mission results
A variety of image restoration techniques (e.g., Wiener filter, Lucy-Richardson, MEM) have been applied quite successfully to the aberrated HST images. The HST servicing mission (scheduled for late 1993 or early 1994) will install a corrective optics system (COSTAR) for the Faint Object Camera and spectrographs and replace the Wide Field/Planetary Camera with a second generation instrument (WF/PC-II) having its own corrective elements. The image quality is expected to be improved substantially with these new instruments. What then is the role of image restoration for the HST in the long term? Through a series of numerical experiments using model point-spread functions for both aberrated and unaberrated optics, we find that substantial improvements in image resolution can be obtained for post-servicing mission data using the same or similar algorithms as being employed now to correct aberrated images. Included in our investigations are studies of the photometric integrity of the restoration algorithms and explicit models for HST pointing errors (spacecraft jitter)
Ground state and finite temperature signatures of quantum phase transitions in the half-filled Hubbard model on a honeycomb lattice
We investigate ground state and finite temperature properties of the
half-filled Hubbard model on a honeycomb lattice using quantum monte carlo and
series expansion techniques. Unlike the square lattice, for which magnetic
order exists at T=0 for any non-zero , the honeycomb lattice is known to
have a semi-metal phase at small and an antiferromagnetic one at large .
We investigate the phase transition at T=0 by studying the magnetic
structureU_c/tC(T)U>U_cU <
U_cUC(T)U \approx U_c$.Comment: 11 pages, 19 figure
Network Resources for Astronomers
The amount of data produced by large observational facilities and space
missions has led to the archiving and on-line accessibility of much of this
data, available to the entire astronomical community. This allows a much wider
multi-frequency approach to astronomical research than previously possible.
Here we provide an overview of these services, and give a basic description of
their contents and possibilities for accessing them. Apart from services
providing observational data, many of those providing general information, e.g.
on addresses, bibliographies, software etc. are also described. The field is
rapidly growing with improved network technology, and our attempt to keep the
report as complete and up-to-date as possible will inevitably be outdated
shortly. We will endeavor to maintain an updated version of this document
on-line.Comment: 53 pages; uuencoded compressed PostScript; includes one table, no
figures; Lyon-41 (Aug'94) and ESO-1033 (Sept'94), to appear in PASP, November
1994 issu
Astronomical Software Directory Service
This is the final report on the development of the Astronomical Software Directory Service (ASDS), a distributable, searchable, WWW-based database of software packages and their related documentation. ASDS provides integrated access to 56 astronomical software packages, with more than 16,000 URL's indexed for full-text searching
SDSS AGNs with X-ray Emission from ROSAT PSPC Pointed Observations
We present a sample of 1744 type 1 active galactic nuclei (AGNs) from the
Sloan Digital Sky Survey (SDSS DR4) spectroscopic catalog with X-ray
counterparts in the White-Giommi-Angelini Catalog (WGACAT) of ROSAT PSPC
pointed observations. Of 1744 X-ray sources, 1410 (80.9%) are new AGN
identifications. Of 4574 SDSS DR4 AGNs for which we found radio matches in the
catalog of radio sources from the FIRST catalog, 224 turned up in our sample of
SDSS X-ray AGNs. The sample objects are given in a catalog that contains
optical and X-ray parameters along with radio emission parameters where
available. We illustrate the content of our catalog and its potential for AGN
science by providing statistical relationships for the catalog data. The
potential of the morphological information is emphasized by confronting the
statistics of optically resolved and unresolved AGNs. The immediate properties
of the catalog objects include significant correlation of X-ray and optical
fluxes, which is consistent with expectations. Also expected is the decrease of
X-ray flux toward higher redshifts. The X-ray to optical flux ratio for the
unresolved AGNs exhibits a decline toward higher redshifts, in agreement with
previous results. The resolved AGNs, however, display the opposite trend. At a
given optical brightness, X-ray fluxes of radio-quiet AGNs by a factor of 2. We
caution, however, that because of the variety of selection effects present in
both the WGACAT and the SDSS, the interpretation of any relationships based on
our sample of X-ray AGNs requires a careful analysis of these effects.Comment: 34 pages, 18 figure
The Energy Spectrum of Primary Cosmic Ray Electrons in Clusters of Galaxies and Inverse Compton Emission
Models for the evolution of the integrated energy spectrum of primary cosmic
ray electrons in clusters of galaxies have been calculated, including the
effects of losses due to inverse Compton (IC), synchrotron, and bremsstrahlung
emission, and Coulomb losses to the intracluster medium (ICM). The combined
time scale for these losses reaches a maximum of ~3e9 yr for electrons with a
Lorentz factor ~300. Only clusters in which there has been a substantial
injection of relativistic electrons since z <~ 1 will have any significant
population of primary cosmic ray electrons at present. In typical models, there
is a broad peak in the electron energy distribution extending to gamma~300, and
a steep drop in the electron population beyond this. In clusters with current
particle injection, there is a power-law tail of higher energy electrons with
an abundance determined by the current rate of injection. A significant
population of electrons with gamma~300, associated with the peak in the
particle loss time, is a generic feature of the models. The IC and synchrotron
emission from these models was calculated. In the models, EUV and soft X-ray
emission are nearly ubiquitous. This emission is produced by electrons with
gamma~300. The spectra are predicted to drop rapidly in going from the EUV to
the X-ray band. The IC emission also extends down the UV, optical, and IR bands
with a fairly flat spectrum. Hard X-ray (HXR) and diffuse radio emission due to
high energy electrons (gamma~10e4) is present only in clusters which have
current particle acceleration. Assuming that the electrons are accelerated in
ICM shocks, one would only expect diffuse HXR/radio emission in clusters which
are currently undergoing a large merger.Comment: Accepted for publication in the Astrophysical Journal, with minor
revisons to wording for clarity and one additional reference. 19 pages with
16 embedded Postscript figures in emulateapj.sty. Abbreviated abstract belo
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