320,291 research outputs found
Mock weak lensing analysis of simulated galaxy clusters: bias and scatter in mass and concentration
(Abridged) We quantify the bias and scatter in galaxy cluster masses and
concentrations derived from an idealised mock weak gravitational lensing (WL)
survey, and their effect on the cluster mass-concentration relation. For this,
we simulate WL distortions on a population of background galaxies due to a
large (~3000) sample of galaxy cluster haloes extracted from the Millennium
Simulation at z~0.2. This study takes into account the influence of shape
noise, cluster substructure and asphericity as well as correlated large-scale
structure, but not uncorrelated large-scale structure along the line of sight
and observational effects. We find a small, but non-negligble, negative median
bias in both mass and concentration at a level of ~5%, the exact value
depending both on cluster mass and radial survey range. Both the mass and
concentration derived from WL show considerable scatter about their true
values. This scatter has, even for the highest mass clusters of M200 > 10^14.8
M_sun, a level of ~30% and ~20% for concentration and mass respectively and
increases strongly with decreasing cluster mass. For a typical survey analysing
30 galaxies per arcmin^2 over a radial range from 30" to 15' from the cluster
centre, the derived M200-c relation has a slope and normalisation too low
compared to the underlying true (3D) relation by ~40% and ~15% respectively.
The scatter and bias in mass are shown to reflect a departure at large radii of
the true WL shear/matter distribution of the simulated clusters from the NFW
profile adopted in modelling the mock observations. Orientation of the triaxial
cluster haloes dominates the concentration scatter (except at low masses, where
galaxy shape noise becomes dominant), while the bias in c is mostly due to
substructure within the virial radius.Comment: 18 pages, 12 figures. Minor changes to match final published paper.
High-resolution figures available at
http://www.ast.cam.ac.uk/~ybahe/Lensing.pd
Non-Gaussian Scatter in Cluster Scaling Relations
We investigate the impact of non-Gaussian scatter in the cluster
mass-observable scaling relation on the mass and redshift distribution of
clusters detected by wide area surveys. We parameterize non-Gaussian scatter by
incorporating the third and forth moments (skewness and kurtosis) into the
distribution P(Mobs|M). We demonstrate that for low scatter mass proxies the
higher order moments do not significantly affect the observed cluster mass and
redshift distributions. However, for high scatter mass indicators it is
necessary for the survey limiting mass threshold to be less than 10^14 h^-1
Msol to prevent the skewness from having a significant impact on the observed
number counts, particularly at high redshift. We also show that an unknown
level of non-Gaussianity in the scatter is equivalent to an additional
uncertainty on the variance in P(Mobs|M) and thus may limit the constraints
that can be placed on the dark energy equation of state parameter w.
Furthermore, positive skewness flattens the mass function at the high mass end,
and so one must also account for skewness in P(Mobs|M) when using the shape of
the mass function to constrain cluster scaling-relations.Comment: 6 Pages, 3 Figures, to be submitted to ApJ Letter
Tyler shape depth
In many problems from multivariate analysis, the parameter of interest is a
shape matrix, that is, a normalized version of the corresponding scatter or
dispersion matrix. In this paper, we propose a depth concept for shape matrices
that involves data points only through their directions from the center of the
distribution. We use the terminology Tyler shape depth since the resulting
estimator of shape, namely the deepest shape matrix, is the median-based
counterpart of the M-estimator of shape of Tyler (1987). Beyond estimation,
shape depth, like its Tyler antecedent, also allows hypothesis testing on
shape. Its main benefit, however, lies in the ranking of shape matrices it
provides, whose practical relevance is illustrated in principal component
analysis and in shape-based outlier detection. We study the invariance,
quasi-concavity and continuity properties of Tyler shape depth, the topological
and boundedness properties of the corresponding depth regions, existence of a
deepest shape matrix and prove Fisher consistency in the elliptical case.
Finally, we derive a Glivenko-Cantelli-type result and establish almost sure
consistency of the deepest shape matrix estimator.Comment: 28 pages, 5 figure
Application of Pade Approximants to Determination of alpha_s(M_Z^2) from Hadronic Event Shape Observables in e+e- Annihilation
We have applied Pade approximants to perturbative QCD calculations of event
shape observables in e+e- --> hadrons. We used the exact O(alpha_s^2)
prediction and the [0/1] Pade approximant to estimate the O(alpha_s^3) term for
15 observables, and in each case determined alpha_s(M_Z^2) from comparison with
hadronic Z^0 decay data from the SLD experiment. We found the scatter among the
alpha_s(M_Z^2) values to be significantly reduced compared with the standard
O(alpha_s^2) determination, implying that the Pade method provides at least a
partial approximation of higher-order perturbative contributions to event shape
observables.Comment: 15 pages, 1 EPS figure, Submitted to Physics Letters
A low energy rare event search with the MAJORANA DEMONSTRATOR
Abstract
The MAJORANA DEMONSTRATOR is sensitive to rare events near its energy threshold, including bosonic dark matter, solar axions, and lightly ionizing particles. In this analysis, a novel training set of low energy small-angle Compton scatter events is used to determine the efficiency of pulse shape analysis cuts, and we present updated bosonic dark matter and solar axion results from an 11.17 kg-y dataset using a 5 keV analysis threshold
The Fundamental Plane for cluster E and S0 galaxies
We have analyzed the Fundamental Plane (FP) for a sample of 226 E and S0
galaxies in ten clusters of galaxies. For photometry in Gunn r the best fitting
plane is log r_e=1.24 log sigma - 0.82 log _e + cst. The scatter is 0.084 in
log r_e. The slope of the FP is not significantly different from cluster to
cluster. The residuals of the FP correlate weakly with the velocity dispersion
and the surface brightness. Thus, to avoid biases of derived distances the
galaxies need to be selected in a homogeneous way. The FP has significant
intrinsic scatter. No other structural parameters like ellipticity or isophotal
shape can reduce the scatter significantly. The Mg_2-sigma relation differs
slightly from cluster to cluster. Galaxies in clusters with lower velocity
dispersions have systematically lower Mg_2. With the current stellar population
models, it is in best agreement with our results regarding the FP if the
offsets are mainly caused by differences in metallicity. Most of the distances
that we derive from the FP imply small peculiar motions, <1000km/s. The zero
point of the FP must therefore be quite stable. The residuals from the
Mg_2-sigma relation may be used to flag galaxies with deviant populations, and
possibly to correct the distance determinations for the deviations.Comment: 20 pages, gzipped PostScript, 14 figures included. Accepted for
publication in MNRA
SZE Observables, Pressure Profiles and Center Offsets in Magneticum Simulation Galaxy Clusters
We present a detailed study of the galaxy cluster thermal \ac{sze} signal
and pressure profiles using {\it Magneticum} Pathfinder hydrodynamical
simulations. With a sample of 50,000 galaxy clusters () out to , we find significant
variations in the shape of the pressure profile with mass and redshift and
present a new generalized NFW model that follows these trends. We show that the
thermal pressure at accounts for only 80~percent of the pressure
required to maintain hydrostatic equilibrium, and therefore even idealized
hydrostatic mass estimates would be biased at the 20~percent level. We compare
the cluster \ac{sze} signal extracted from a sphere with different virial-like
radii, a virial cylinder within a narrow redshift slice and the full light
cone, confirming small scatter () in the sphere and
showing that structure immediately surrounding clusters increases the scatter
and strengthens non self-similar redshift evolution in the cylinder.
Uncorrelated large scale structure along the line of sight leads to an increase
in the \ac{sze} signal and scatter that is more pronounced for low mass
clusters, resulting in non self-similar trends in both mass and redshift and a
mass dependent scatter that is at low masses. The scatter
distribution is consistent with log-normal in all cases. We present a model of
the offsets between the center of the gravitational potential and the \ac{sze}
center that follows the variations with cluster mass and redshift.Comment: 20 pages, 15 figures, submitted to MNRA
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