7,201 research outputs found
Long Term Variability of SDSS Quasars
We use a sample of 3791 quasars from the Sloan Digital Sky Survey (SDSS)
Early Data Release (EDR), and compare their photometry to historic plate
material for the same set of quasars in order to study their variability
properties. The time base-line we attain this way ranges from a few months to
up to 50 years. In contrast to monitoring programs, where relatively few
quasars are photometrically measured over shorter time periods, we utilize
existing databases to extend this base-line as much as possible, at the cost of
sampling per quasar. Our method, however, can easily be extended to much larger
samples. We construct variability Structure Functions and compare these to the
literature and model functions. From our modeling we conclude that 1) quasars
are more variable toward shorter wavelengths, 2) their variability is
consistent with an exponentially decaying light-curve with a typical time-scale
of ~2 years, 3) these outbursts occur on typical time-scales of ~200 years.
With the upcoming first data release of the SDSS, a much larger quasar sample
can be used to put these conclusions on a more secure footing.Comment: 16 pages, accepted for publication in AJ, Sept issu
Kinematics of Metal-Poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Non-Kinematically Selected Stars
(Abbreviated) We present a detailed analysis of the space motions of 1203
solar-neighborhood stars with metal abundances [Fe/H] <= -0.6, on the basis of
a recently revised and supplemented catalog of metal-poor stars selected
without kinematic bias (Beers et al. 2000). This sample, having available
proper motions, radial velocities, and distance estimates for stars with a wide
range of metal abundances, is by far the largest such catalog to be assembled
to date. Unlike essentially all previous kinematically selected catalogs, the
metal-poor stars in our sample exhibit a diverse distribution of orbital
eccentricities, e, with no apparent correlation between [Fe/H] and e. This
demonstrates, clearly and convincingly, that the evidence offered by Eggen,
Lynden-Bell, and Sandage (1962) for a rapid collapse of the Galaxy, an apparent
correlation between the orbital eccentricity of halo stars with metallicity, is
basically the result of their proper-motion selection bias. However, even in
our non-kinematically selected sample, we have identified a small concentration
of high-e stars at [Fe/H] = -1.7, which may originate, in part, from infalling
gas during the early formation of the Galaxy. The implications of our results
for the formation of the Galaxy are also discussed, in particular in the
context of the currently favored CDM theory of hierarchical galaxy formation.Comment: 51 pages, including 17 figures, to appear in AJ (June 2000), full
paper with all figures embedded available at
http://pluto.mtk.nao.ac.jp/people/chiba/preprint/halo5
A discrete, unitary, causal theory of quantum gravity
A discrete model of Lorentzian quantum gravity is proposed. The theory is
completely background free, containing no reference to absolute space, time, or
simultaneity. The states at one slice of time are networks in which each vertex
is labelled with two arrows, which point along an adjacent edge, or to the
vertex itself. The dynamics is specified by a set of unitary replacement rules,
which causally propagate the local degrees of freedom. The inner product
between any two states is given by a sum over histories. Assuming it converges
(or can be Abel resummed), this inner product is proven to be hermitian and
fully gauge-degenerate under spacetime diffeomorphisms. At least for states
with a finite past, the inner product is also positive. This allows a Hilbert
space of physical states to be constructed.Comment: 38 pages, 9 figures, v3 added to exposition and references, v4
expanded prospects sectio
Application of a Self-Similar Pressure Profile to Sunyaev-Zel'dovich Effect Data from Galaxy Clusters
We investigate the utility of a new, self-similar pressure profile for
fitting Sunyaev-Zel'dovich (SZ) effect observations of galaxy clusters. Current
SZ imaging instruments - such as the Sunyaev-Zel'dovich Array (SZA) - are
capable of probing clusters over a large range in physical scale. A model is
therefore required that can accurately describe a cluster's pressure profile
over a broad range of radii, from the core of the cluster out to a significant
fraction of the virial radius. In the analysis presented here, we fit a radial
pressure profile derived from simulations and detailed X-ray analysis of
relaxed clusters to SZA observations of three clusters with exceptionally high
quality X-ray data: A1835, A1914, and CL J1226.9+3332. From the joint analysis
of the SZ and X-ray data, we derive physical properties such as gas mass, total
mass, gas fraction and the intrinsic, integrated Compton y-parameter. We find
that parameters derived from the joint fit to the SZ and X-ray data agree well
with a detailed, independent X-ray-only analysis of the same clusters. In
particular, we find that, when combined with X-ray imaging data, this new
pressure profile yields an independent electron radial temperature profile that
is in good agreement with spectroscopic X-ray measurements.Comment: 28 pages, 6 figures, accepted by ApJ for publication (probably April
2009
Environmental Effect on the Associations of Background Quasars with Foreground Objects: II. Numerical Simulations
Using numerical simulations of cluster formation in the standard CDM model
(SCDM) and in a low-density, flat CDM model with a cosmological constant
(LCDM), we investigate the gravitational lensing explanation for the reported
associations between background quasars and foreground clusters. Under the
thin-lens approximation and the unaffected background hypothesis , we show that
the recently detected quasar overdensity around clusters of galaxies on scales
of arcminutes cannot be interpreted as a result of the gravitational
lensing by cluster matter and/or by their environmental and projected matter
along the line of sight, which is consistent with the analytical result based
on the observed cluster and galaxy correlations (Wu, et al. 1996). It appears
very unlikely that uncertainties in the modeling of the gravitational lensing
can account for the disagreement between the theoretical predictions and the
observations. We conclude that either the detected signal of the quasar-cluster
associations is a statistical fluke or the associations are are generated by
mechanisms other than the magnification bias.Comment: 15 pages, 5 figures, accepted for publication in Ap
A systematic review of community participation measures for people with intellectual disabilities
Background: Community participation is considered a fundamental aspect of quality of life and one of the essential goals of services for people with intellectual disabilities (ID), yet there is no agreed way of measuring community participation.
Method: Two systematic searches were performed across eight electronic databases to identify measures of community participation and identify validation studies for each measure. Measures were included if they were developed for adults with ID, measured extent of participation and had published information regarding content and psychometric properties. Each measure was evaluated on the basis of psychometric properties and in relation to coverage of nine domains of community participation from the International Classification of Functioning, Disability and Health (ICF).
Results: Eleven measures were selected with the quality rating scores varying substantially ranging from 2-11 of a possible 16.
Conclusions: The majority of measures were not sufficiently psychometrically tested. Findings suggest a need for the development of a psychometrically robust instrument
Evidence for Distinct Components of the Galactic Stellar Halo from 838 RR Lyrae Stars Discovered in the LONEOS-I Survey
We present 838 ab-type RR Lyrae stars from the Lowell Observatory Near Earth
Objects Survey Phase I (LONEOS-I). These objects cover 1430 deg^2 and span
distances ranging from 3-30 kpc from the Galactic Center. Object selection is
based on phased, photometric data with 28-50 epochs. We use this large sample
to explore the bulk properties of the stellar halo, including the spatial
distribution. The period-amplitude distribution of this sample shows that the
majority of these RR Lyrae stars resemble Oosterhoff type I, but there is a
significant fraction (26 %) which have longer periods and appear to be
Oosterhoff type II. We find that the radial distributions of these two
populations have significantly different profiles (rho_{OoI} ~ R^(-2.26 +-
0.07) and rho_{OoII} ~ R^(-2.88 +- 0.11). This suggests that the stellar halo
was formed by at least two distinct accretion processes and supports dual-halo
models.Comment: 18 pages, 28 figures, apjemulated, minor corrections and
clarifications. Accepted to ApJ on Jan 21, 200
Inflation for Bianchi IX model
The influence of Inflation on initial (i.e. at Planck's epoch) large
anisotropy of the Universe is studied, considering a more general metric than
the isotropic one: the locally rotationally symmetric (L.R.S.) Bianchi IX
metric. We find, then, a large set of initial conditions of intrinsic curvature
and shear allowing an inflationary epoch that make the anisotropy negligible.
These are not trivial because of the non-linearity of the Einstein's equations.Comment: 10 pages, Latex. To be published in Phisical Review
Evolution in Quantum Causal Histories
We provide a precise definition and analysis of quantum causal histories
(QCH). A QCH consists of a discrete, locally finite, causal pre-spacetime with
matrix algebras encoding the quantum structure at each event. The evolution of
quantum states and observables is described by completely positive maps between
the algebras at causally related events. We show that this local description of
evolution is sufficient and that unitary evolution can be recovered wherever it
should actually be expected. This formalism may describe a quantum cosmology
without an assumption of global hyperbolicity; it is thus more general than the
Wheeler-DeWitt approach. The structure of a QCH is also closely related to
quantum information theory and algebraic quantum field theory on a causal set.Comment: 20 pages. 8 figures. (v3: minor corrections, additional references
[2,3]) to appear in CQ
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