26 research outputs found

### Effects of the complex mass distribution of dark matter halos on weak lensing cluster surveys

Gravitational lensing effects arise from the light ray deflection by all of
the mass distribution along the line of sight. It is then expected that weak
lensing cluster surveys can provide us true mass-selected cluster samples. With
numerical simulations, we analyze the correspondence between peaks in the
lensing convergence $\kappa$-map and dark matter halos. Particularly we
emphasize the difference between the peak $\kappa$ value expected from a dark
matter halo modeled as an isolated and spherical one, which exhibits a
one-to-one correspondence with the halo mass at a given redshift, and that of
the associated $\kappa$-peak from simulations. For halos with the same expected
$\kappa$, their corresponding peak signals in the $\kappa$-map present a wide
dispersion. At an angular smoothing scale of $\theta_G=1\hbox{arcmin}$, our
study shows that for relatively large clusters, the complex mass distribution
of individual clusters is the main reason for the dispersion. The projection
effect of uncorrelated structures does not play significant roles. The
triaxiality of dark matter halos accounts for a large part of the dispersion,
especially for the tail at high $\kappa$ side. Thus lensing-selected clusters
are not really mass-selected. (abridged)Comment: ApJ accepte

### Reconstructing Three-dimensional Structure of Underlying Triaxial Dark Halos From Xray and Sunyaev-Zel'dovich Effect Observations of Galaxy Clusters

While the use of galaxy clusters as {\it tools} to probe cosmology is
established, their conventional description still relies on the spherical
and/or isothermal models that were proposed more than 20 years ago. We present,
instead, a deprojection method to extract their intrinsic properties from X-ray
and Sunyaev--Zel'dovich effect observations in order to improve our
understanding of cluster physics. First we develop a theoretical model for the
intra-cluster gas in hydrostatic equilibrium in a triaxial dark matter halo
with a constant axis ratio. In this theoretical model, the gas density profiles
are expressed in terms of the intrinsic properties of the dark matter halos.
Then, we incorporate the projection effect into the gas profiles, and show that
the gas surface brightness profiles are expressed in terms of the
eccentricities and the orientation angles of the dark halos. For the practical
purpose of our theoretical model, we provide several empirical fitting formulae
for the gas density and temperature profiles, and also for the surface
brightness profiles relevant to X-ray and Sunyaev--Zel'dovich effect
observations. Finally, we construct a numerical algorithm to determine the halo
eccentricities and orientation angles using our model, and demonstrate that it
is possible in principle to reconstruct the 3D structures of the dark halos
from the X-ray and/or Sunyaev-Zel'dovich effect cluster data alone without
requiring priors such as weak lensing informations and without relying on such
restrictive assumptions as the halo axial symmetry about the line-of-sight.Comment: Accepted version, new discussions added, typos and minor mistakes
corrected, ApJ in press (2004, Feb. 1 scheduled, Vol. 601, No. 2 issue),26
pages, 7 postscript figure

### Completeness in Photometric and Spectroscopic Searches for Clusters

We investigate, using simulated galaxy catalogues, the completeness of
searches for massive clusters of galaxies in redshift surveys or imaging
surveys with photometric redshift estimates, i.e. what fraction of clusters
(M>10^14/h Msun) are found in such surveys. We demonstrate that the matched
filter method provides an efficient and reliable means of identifying massive
clusters even when the redshift estimates are crude. In true redshift surveys
the method works extremely well. We demonstrate that it is possible to
construct catalogues with high completeness, low contamination and both varying
little with redshift.Comment: ApJ in press, 15 pages, 10 figure

### Measuring Angular Diameter Distances through Halo Clustering

Current and upcoming wide-field surveys for weak gravitational lensing and
the Sunyaev-Zel'dovich effect will generate mass-selected catalogues of dark
matter halos with internal or followup photometric redshift information. The
angular power spectrum of these halos provides additional information that
complements the previously-studied number counts. In particular, using the
shape of the linear power spectrum as a standard ruler that is calibrated by
cosmic microwave background measurements, we find that a survey of 4000 sqr.
deg. and a mass threshold of 10^14 M_sun can be used to determine the comoving
angular diameter distance as a function of redshift. In principle, this test
also allows an absolute calibration of the distance scale and measurement of
the Hubble constant. This test is largely insensitive to the details of halo
mass measurements, mass function, and halo bias. Determination of these
quantities would further allow a measurement of the linear growth rate of
fluctuations.Comment: 4 pages, 4 figures; final version published in ApJ Letter

### Measuring the Three-Dimensional Structure of Galaxy Clusters. I. Application to a Sample of 25 Clusters

We discuss a method to constrain the intrinsic three-dimensionale shapes of
galaxy clusters by combining X-Ray and Sunyaev-Zeldovich observations. The
method is applied to a sample of 25 X-Ray selected clusters, with measured
Sunyaev-Zeldovich temperature decrements. The sample turns out to be slightly
biased, with strongly elongated clusters preferentially aligned along the line
of sight. This result demonstrates that X-Ray selected cluster samples may be
affected by morphological and orientation effects even if a relatively high
threshold signal-to-noise ratio is used to select the sample. A large majority
of the clusters in our sample exhibit a marked triaxial structure, with
prolate-like shapes being slightly more likely than oblate-like ones; the
spherical hypothesis is strongly rejected for most sample members. Cooling flow
clusters do not show preferentially regular morphologies.Comment: 13 pages, 9 figures. Accepted by Ap

### Weak Lensing by High-Redshift Clusters of Galaxies - I: Cluster Mass Reconstruction

We present the results of a weak lensing survey of six high-redshift (z >
0.5), X-ray selected clusters of galaxies. We have obtained ultra-deep R-band
images of each cluster with the Keck Telescope, and have measured a weak
lensing signal from each cluster. From the background galaxy ellipticities we
create two-dimensional maps of the surface mass density of each cluster. We
find that the substructure seen in the mass reconstructions typically agree
well with substructure in both the cluster galaxy distributions and X-ray
images of the clusters. We also measure the one-dimensional radial profiles of
the lensing signals and fit these with both isothermal spheres and "universal"
CDM profiles. We find that the more massive clusters are less compact and not
as well fit by isothermal spheres as the less massive clusters, possibly
indicating that they are still in the process of collapse.Comment: 43 pages, 15 figures, uses aastex, submitted to ApJ 4 color plates
produced here as jpg's, larger versions of the jpgs can be found at
http://www.mpa-garching.mpg.de/~clow

### The Effect of the Cosmic Web on Cluster Weak Lensing Mass Estimates

In modern hierarchical theories of structure formation, rich clusters of
galaxies form at the vertices of a weblike distribution of matter, with
filaments emanating from them to large distances and with smaller objects
forming and draining in along these filaments. The amount of mass contained in
structure near the cluster can be comparable to the collapsed mass of the
cluster itself. As the lensing kernel is quite broad along the line of sight
around cluster lenses with typical redshifts near z=0.5, structures many Mpc
away from the cluster are essentially at the same location as the cluster
itself, when considering their effect on the cluster's weak lensing signal. We
use large-scale numerical simulations of structure formation in a
Lambda-dominated cold dark matter model to quantify the effect that large-scale
structure near clusters has upon the cluster masses deduced from weak lensing
analysis. A correction for the scatter in possible observed lensing masses
should be included when interpreting mass functions from weak lensing surveys.Comment: 14 pages, 11 figures. LaTeX2e, uses emulateapj.sty and
onecolfloat.st

### Measuring the Three-Dimensional Structure of Galaxy Clusters. II. Are clusters of galaxies oblate or prolate?

The intrinsic shape of galaxy clusters can be obtained through a combination
of X-ray and Sunyaev-Zeldovich effect observations once cosmological parameters
are assumed to be known. In this paper we discuss the feasibility of modelling
galaxy clusters as either prolate or oblate ellipsoids. We analyze the
intra-cluster medium distribution for a sample of 25 X-ray selected clusters,
with measured Sunyaev-Zeldovich temperature decrements. A mixed population of
prolate and oblate ellipsoids of revolution fits the data well, with prolate
shapes preferred on a 60-76% basis. We observe an excess of clusters nearly
aligned along the line of sight, with respect to what is expected from a
randomly oriented cluster population, which might imply the presence of a
selection bias in our sample. We also find signs that a more general triaxial
morphology might better describe the morphology of galaxy clusters. Additional
constraints from gravitational lensing could disentangle the degeneracy between
an ellipsoidal and a triaxial morphology, and could also allow an unbiased
determination of the Hubble constant.Comment: 9 pages, 8 figures, accepted for publication in Astrophys.

### The non-Gaussian tail of cosmic-shear statistics

Due to gravitational instability, an initially Gaussian density field
develops non-Gaussian features as the Universe evolves. The most prominent
non-Gaussian features are massive haloes, visible as clusters of galaxies. The
distortion of high-redshift galaxy images due to the tidal gravitational field
of the large-scale matter distribution, called cosmic shear, can be used to
investigate the statistical properties of the LSS. In particular, non-Gaussian
properties of the LSS will lead to a non-Gaussian distribution of cosmic-shear
statistics. The aperture mass ($M_{\rm ap}$) statistics, recently introduced as
a measure for cosmic shear, is particularly well suited for measuring these
non-Gaussian properties. In this paper we calculate the highly non-Gaussian
tail of the aperture mass probability distribution, assuming Press-Schechter
theory for the halo abundance and the `universal' density profile of haloes as
obtained from numerical simulations. We find that for values of $M_{\rm ap}$
much larger than its dispersion, this probability distribution is closely
approximated by an exponential, rather than a Gaussian. We determine the
amplitude and shape of this exponential for various cosmological models and
aperture sizes, and show that wide-field imaging surveys can be used to
distinguish between some of the currently most popular cosmogonies. Our study
here is complementary to earlier cosmic-shear investigations which focussed
more on two-point statistical properties.Comment: 9 pages, 5 figures, submitted to MNRA

### Constraining the Matter Power Spectrum Normalization using the SDSS/RASS and REFLEX Cluster surveys

We describe a new approach to constrain the amplitude of the power spectrum
of matter perturbations in the Universe, parametrized by sigma_8 as a function
of the matter density Omega_0. We compare the galaxy cluster X-ray luminosity
function of the REFLEX survey with the theoretical mass function of Jenkins et
al. (2001), using the mass-luminosity relationship obtained from weak lensing
data for a sample of galaxy clusters identified in Sloan Digital Sky Survey
commissioning data and confirmed through cross-correlation with the ROSAT
all-sky survey. We find sigma_8 = 0.38 Omega_0^(-0.48+0.27 Omega_ 0), which is
significantly different from most previous results derived from comparable
calculations that used the X-ray temperature function. We discuss possible
sources of systematic error that may cause such a discrepancy, and in the
process uncover a possible inconsistency between the REFLEX luminosity function
and the relation between cluster X-ray luminosity and mass obtained by Reiprich
& Bohringer (2001).Comment: Accepted to ApJ Letters. 4 pages using emulateapj.st