613 research outputs found
Galaxy Clustering Topology in the Sloan Digital Sky Survey Main Galaxy Sample: a Test for Galaxy Formation Models
We measure the topology of the main galaxy distribution using the Seventh
Data Release of the Sloan Digital Sky Survey, examining the dependence of
galaxy clustering topology on galaxy properties. The observational results are
used to test galaxy formation models. A volume-limited sample defined by
enables us to measure the genus curve with amplitude of at
Mpc smoothing scale, with 4.8\% uncertainty including all systematics
and cosmic variance. The clustering topology over the smoothing length interval
from 6 to Mpc reveals a mild scale-dependence for the shift
() and void abundance () parameters of the genus curve. We find
substantial bias in the topology of galaxy clustering with respect to the
predicted topology of the matter distribution, which varies with luminosity,
morphology, color, and the smoothing scale of the density field. The
distribution of relatively brighter galaxies shows a greater prevalence of
isolated clusters and more percolated voids. Even though early (late)-type
galaxies show topology similar to that of red (blue) galaxies, the morphology
dependence of topology is not identical to the color dependence. In particular,
the void abundance parameter depends on morphology more strongly than on
color. We test five galaxy assignment schemes applied to cosmological N-body
simulations of a CDM universe to generate mock galaxies: the
Halo-Galaxy one-to-one Correspondence model, the Halo Occupation Distribution
model, and three implementations of Semi-Analytic Models (SAMs). None of the
models reproduces all aspects of the observed clustering topology; the
deviations vary from one model to another but include statistically significant
discrepancies in the abundance of isolated voids or isolated clusters and the
amplitude and overall shift of the genus curve. (Abridged)Comment: 24 pages, 19 figures, 10 tables, submitted to ApJS. Version with full
resolution images is available at
http://astro.kias.re.kr/~cbp/doc/dr7Topo.pd
Transitions to palliative care in acute hospitals in England: qualitative study
Objective To explore how transitions to a palliative care approach are perceived to be managed in acute hospital settings in England
Correspondence - Extended Library Hours
Correspondence regarding increasing library weekend hours. Includes a reports entitled Cost Estimate in Response to ASG Resolution 89-4-F outlining costs of guards, students, and library staff; Friday & Saturday Evenings Turnstile Count for the period October 27 to November 12; Finals Week Patrons Between 5:00 - 10:00 PM for the 1988-89 school year; Circulation Weekend Statistics for the period October 27 to November 12, 1989; and Reference Weekend Statistics for the period October 27 to November 12, 1989
Three-Dimensional Genus Statistics of Galaxies in the SDSS Early Data Release
We present the first analysis of three-dimensional genus statistics for the
SDSS EDR galaxy sample. Due to the complicated survey volume and the selection
function, analytic predictions of the genus statistics for this sample are not
feasible, therefore we construct extensive mock catalogs from N-body
simulations in order to compare the observed data with model predictions. This
comparison allows us to evaluate the effects of a variety of observational
systematics on the estimated genus for the SDSS sample, including the shape of
the survey volume, the redshift distortion effect, and the radial selection
function due to the magnitude limit. The observed genus for the SDSS EDR galaxy
sample is consistent with that predicted by simulations of a
-dominated spatially-flat cold dark matter model. Standard
() cold dark matter model predictions do not match the
observations. We discuss how future SDSS galaxy samples will yield improved
estimates of the genus.Comment: 20 pages, 10 figures, accepted for publication in PASJ (Vol.54, No.5,
2002
Virtual reality applications in robotic simulations
Virtual reality (VR) provides a means to practice integrated extravehicular activities (EVA)/remote manipulator system (RMS) operations in the on-orbit configuration with no discomfort or risk to crewmembers. VR afforded the STS-61 crew the luxury of practicing the integrated EVA/RMS operations in an on-orbit configuration prior to the actual flight. The VR simulation was developed by the Automation and Robotics Division's Telepresence/Virtual Reality Lab and Integrated Graphics, Operations, and Analysis Lab (IGOAL) at JSC. The RMS Part Task Trainer (PTT) was developed by the IGOAL for RMS training in 1988 as a fully functional, kinematic simulation of the shuttle RMS and served as the RMS portion of the integrated VR simulation. Because the EVA crewmember could get a realistic view of the shuttle and payload bay in the VR simulation, he/she could explore different positions and views to determine the best method for performing a specific task, thus greatly increasing the efficiency of use of the neutral buoyancy facilities
Minkowski Functionals of SDSS galaxies I : Analysis of Excursion Sets
We present a first morphometric investigation of a preliminary sample from
the SDSS of 154287 galaxies with apparent magnitude 14.5<m_r<17.5 and redshift
0.001<z<0.4. We measure the Minkowski Functionals, which are a complete set of
morphological descriptors. To account for the complicated wedge--like geometry
of the present survey data, we construct isodensity contour surfaces from the
galaxy positions in redshift space and employ two complementary methods of
computing the Minkowski Functionals. We find that the observed Minkowski
Functionals for SDSS galaxies are consistent with the prediction of a
Lambda--dominated spatially--flat Cold Dark Matter model with random--Gaussian
initial conditions, within the cosmic variance estimated from the corresponding
mock catalogue. We expect that future releases of the SDSS survey will allow us
to distinguish morphological differences in the galaxy distribution with regard
to different morphological type and luminosity ranges.Comment: 35 pages, 13 figures, accepted for publication in PASJ. For preprint
with higher-resolution PS files, see
http://www.a.phys.nagoya-u.ac.jp/~hikage/MFs/mf_sdss.ps.g
Median Statistics, H_0, and the Accelerating Universe
(Abridged) We develop median statistics that provide powerful alternatives to
chi-squared likelihood methods and require fewer assumptions about the data.
Applying median statistics to Huchra's compilation of nearly all estimates of
the Hubble constant, we find a median value H_0=67 km/s/Mpc. Median statistics
assume only that the measurements are independent and free of systematic
errors. This estimate is arguably the best summary of current knowledge because
it uses all available data and, unlike other estimates, makes no assumption
about the distribution of measurement errors. The 95% range of purely
statistical errors is +/- 2 km/s/Mpc. The statistical precision of this result
leads us to analyze the range of possible systematic errors in the median,
which we estimate to be roughly +/- 5 km/s/Mpc (95% limits), dominating over
the statistical errors. A Bayesian median statistics treatment of high-redshift
Type Ia supernovae (SNe Ia) apparent magnitude versus redshift data from Riess
et al. yields a posterior probability that the cosmological constant Lambda > 0
of 70 or 89%, depending on the prior information used. The posterior
probability of an open universe is about 47%. Analysis of the Perlmutter et al.
high-redshift SNe Ia data show the best-fit flat-Lambda model favored over the
best-fit Lambda = 0 open model by odds of 366:1; corresponding Riess et al.
odds are 3:1 (assuming prior odds of 1:1).Median statistics analyses of the SNe
Ia data do not rule out a time-variable Lambda model, and may even favor it
over a time-independent Lambda and a Lambda = 0 open model.Comment: Significant revisions include discussion of systematic errors in the
median of H_0. Accepted for publication in The Astrophysical Journal, v548,
February 20, 2001 issue. 47 pages incl. figures and table
Two-Dimensional Topology of the 2dF Galaxy Redshift Survey
We study the topology of the publicly available data released by the 2dFGRS.
The 2dFGRS data contains over 100,000 galaxy redshifts with a magnitude limit
of b_J=19.45 and is the largest such survey to date. The data lie over a wide
range of right ascension (75 degree strips) but only within a narrow range of
declination (10 degree and 15 degree strips). This allows measurements of the
two-dimensional genus to be made.
The NGP displays a slight meatball shift topology, whereas the SGP displays a
bubble like topology. The current SGP data also have a slightly higher genus
amplitude. In both cases, a slight excess of overdense regions are found over
underdense regions. We assess the significance of these features using mock
catalogs drawn from the Virgo Consortium's Hubble Volume LCDM z=0 simulation.
We find that differences between the NGP and SGP genus curves are only
significant at the 1 sigma level. The average genus curve of the 2dFGRS agrees
well with that extracted from the LCDM mock catalogs.
We compare the amplitude of the 2dFGRS genus curve to the amplitude of a
Gaussian random field with the same power spectrum as the 2dFGRS and find,
contradictory to results for the 3D genus of other samples, that the amplitude
of the GRF genus curve is slightly lower than that of the 2dFGRS. This could be
due to a a feature in the current data set or the 2D genus may not be as
sensitive as the 3D genus to non-linear clustering due to the averaging over
the thickness of the slice in 2D. (Abridged)Comment: Submitted to ApJ A version with Figure 1 in higher resolution can be
obtained from http://www.physics.drexel.edu/~hoyle
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