64,087 research outputs found
Supernova Constraints and Systematic Uncertainties from the First Three Years of the Supernova Legacy Survey
We combine high-redshift Type Ia supernovae from the first three years of the Supernova Legacy Survey (SNLS) with other supernova (SN) samples, primarily at lower redshifts, to form a high-quality joint sample of 472 SNe (123 low-z, 93 SDSS, 242 SNLS, and 14 Hubble Space Telescope). SN data alone require cosmic acceleration at >99.999% confidence, including systematic effects. For the dark energy equation of state parameter (assumed constant out to at least z = 1.4) in a flat universe, we find w = â0.91^(+0.16)_(â0.20)(stat)^(+0.07)_(â0.14)(sys) from SNe only, consistent with a cosmological constant. Our fits include a correction for the recently discovered relationship between host-galaxy mass and SN absolute brightness. We pay particular attention to systematic uncertainties, characterizing them using a systematic covariance matrix that incorporates the redshift dependence of these effects, as well as the shape-luminosity and color-luminosity relationships. Unlike previous work, we include the effects of systematic terms on the empirical light-curve models. The total systematic uncertainty is dominated by calibration terms. We describe how the systematic uncertainties can be reduced with soon to be available improved nearby and intermediate-redshift samples, particularly those calibrated onto USNO/SDSS-like systems
Cross-correlation Weak Lensing of SDSS Galaxy Clusters I: Measurements
This is the first in a series of papers on the weak lensing effect caused by
clusters of galaxies in Sloan Digital Sky Survey. The photometrically selected
cluster sample, known as MaxBCG, includes ~130,000 objects between redshift 0.1
and 0.3, ranging in size from small groups to massive clusters. We split the
clusters into bins of richness and luminosity and stack the surface density
contrast to produce mean radial profiles. The mean profiles are detected over a
range of scales, from the inner halo (25 kpc/h) well into the surrounding large
scale structure (30 Mpc/h), with a significance of 15 to 20 in each bin. The
signal over this large range of scales is best interpreted in terms of the
cluster-mass cross-correlation function. We pay careful attention to sources of
systematic error, correcting for them where possible. The resulting signals are
calibrated to the ~10% level, with the dominant remaining uncertainty being the
redshift distribution of the background sources. We find that the profiles
scale strongly with richness and luminosity. We find the signal within a given
richness bin depends upon luminosity, suggesting that luminosity is more
closely correlated with mass than galaxy counts. We split the samples by
redshift but detect no significant evolution. The profiles are not well
described by power laws. In a subsequent series of papers we invert the
profiles to three-dimensional mass profiles, show that they are well fit by a
halo model description, measure mass-to-light ratios and provide a cosmological
interpretation.Comment: Paper I in a series; v2.0 includes ApJ referee's suggestion
GREAT3 results I: systematic errors in shear estimation and the impact of real galaxy morphology
We present first results from the third GRavitational lEnsing Accuracy
Testing (GREAT3) challenge, the third in a sequence of challenges for testing
methods of inferring weak gravitational lensing shear distortions from
simulated galaxy images. GREAT3 was divided into experiments to test three
specific questions, and included simulated space- and ground-based data with
constant or cosmologically-varying shear fields. The simplest (control)
experiment included parametric galaxies with a realistic distribution of
signal-to-noise, size, and ellipticity, and a complex point spread function
(PSF). The other experiments tested the additional impact of realistic galaxy
morphology, multiple exposure imaging, and the uncertainty about a
spatially-varying PSF; the last two questions will be explored in Paper II. The
24 participating teams competed to estimate lensing shears to within systematic
error tolerances for upcoming Stage-IV dark energy surveys, making 1525
submissions overall. GREAT3 saw considerable variety and innovation in the
types of methods applied. Several teams now meet or exceed the targets in many
of the tests conducted (to within the statistical errors). We conclude that the
presence of realistic galaxy morphology in simulations changes shear
calibration biases by per cent for a wide range of methods. Other
effects such as truncation biases due to finite galaxy postage stamps, and the
impact of galaxy type as measured by the S\'{e}rsic index, are quantified for
the first time. Our results generalize previous studies regarding sensitivities
to galaxy size and signal-to-noise, and to PSF properties such as seeing and
defocus. Almost all methods' results support the simple model in which additive
shear biases depend linearly on PSF ellipticity.Comment: 32 pages + 15 pages of technical appendices; 28 figures; submitted to
MNRAS; latest version has minor updates in presentation of 4 figures, no
changes in content or conclusion
The Mean and Scatter of the Velocity Dispersion-Optical Richness Relation for maxBCG Galaxy Clusters
The distribution of galaxies in position and velocity around the centers of
galaxy clusters encodes important information about cluster mass and structure.
Using the maxBCG galaxy cluster catalog identified from imaging data obtained
in the Sloan Digital Sky Survey, we study the BCG-galaxy velocity correlation
function. By modeling its non-Gaussianity, we measure the mean and scatter in
velocity dispersion at fixed richness. The mean velocity dispersion increases
from 202+/-10 km/s for small groups to more than 854+/-102 km/s for large
clusters. We show the scatter to be at most 40.5+/-3.5%, declining to
14.9+/-9.4% in the richest bins. We test our methods in the C4 cluster catalog,
a spectroscopic cluster catalog produced from the Sloan Digital Sky Survey DR2
spectroscopic sample, and in mock galaxy catalogs constructed from N-body
simulations. Our methods are robust, measuring the scatter to well within
one-sigma of the true value, and the mean to within 10%, in the mock catalogs.
By convolving the scatter in velocity dispersion at fixed richness with the
observed richness space density function, we measure the velocity dispersion
function of the maxBCG galaxy clusters. Although velocity dispersion and
richness do not form a true mass-observable relation, the relationship between
velocity dispersion and mass is theoretically well characterized and has low
scatter. Thus our results provide a key link between theory and observations up
to the velocity bias between dark matter and galaxies.Comment: 25 pages, 15 figures, 2 tables, published in Ap
QCD
We discuss QCD studies that will be possible at LEP2. We examine both
experimental and theoretical aspects of jets, fragmentation functions,
multiplicities and particle spectra.Comment: 44 pages, Latex, epsfig, 18 figures, to appear on the Report of the
Workshop on Physics at LEP2, CERN 96-01, vol. 1, 199
The binned bispectrum estimator: template-based and non-parametric CMB non-Gaussianity searches
We describe the details of the binned bispectrum estimator as used for the
official 2013 and 2015 analyses of the temperature and polarization CMB maps
from the ESA Planck satellite. The defining aspect of this estimator is the
determination of a map bispectrum (3-point correlator) that has been binned in
harmonic space. For a parametric determination of the non-Gaussianity in the
map (the so-called fNL parameters), one takes the inner product of this binned
bispectrum with theoretically motivated templates. However, as a complementary
approach one can also smooth the binned bispectrum using a variable smoothing
scale in order to suppress noise and make coherent features stand out above the
noise. This allows one to look in a model-independent way for any statistically
significant bispectral signal. This approach is useful for characterizing the
bispectral shape of the galactic foreground emission, for which a theoretical
prediction of the bispectral anisotropy is lacking, and for detecting a
serendipitous primordial signal, for which a theoretical template has not yet
been put forth. Both the template-based and the non-parametric approaches are
described in this paper.Comment: Latex 42 pages with 10 figures and JCAP macros. v2: corrected small
mistake in section 5.3, changed colour scale of slice figures, other minor
changes and additions, matches published versio
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