1,228 research outputs found
Probing the evolution of the substructure frequency in galaxy clusters up to z~1
Context. Galaxy clusters are the last and largest objects to form in the
standard hierarchical structure formation scenario through merging of smaller
systems. The substructure frequency in the past and present epoch provides
excellent means for studying the underlying cosmological model. Aims. Using
X-ray observations, we study the substructure frequency as a function of
redshift by quantifying and comparing the fraction of dynamically young
clusters at different redshifts up to z=1.08. We are especially interested in
possible biases due to the inconsistent data quality of the low-z and high-z
samples. Methods. Two well-studied morphology estimators, power ratio P3/P0 and
center shift w, were used to quantify the dynamical state of 129 galaxy
clusters, taking into account the different observational depth and noise
levels of the observations. Results. Owing to the sensitivity of P3/P0 to
Poisson noise, it is essential to use datasets with similar photon statistics
when studying the P3/P0-z relation. We degraded the high-quality data of the
low-redshift sample to the low data quality of the high-z observations and
found a shallow positive slope that is, however, not significant, indicating a
slightly larger fraction of dynamically young objects at higher redshift. The
w-z relation shows no significant dependence on the data quality and gives a
similar result. Conclusions. We find a similar trend for P3/P0 and w, namely a
very mild increase of the disturbed cluster fraction with increasing redshifts.
Within the significance limits, our findings are also consistent with no
evolution.Comment: A&A in pres
From the core to the outskirts: structure analysis of three massive galaxy clusters
The hierarchical model of structure formation is a key prediction of the
Lambda cold dark matter model, which can be tested by studying the large-scale
environment and the substructure content of massive galaxy clusters. We present
here a detailed analysis of the clusters RXCJ0225.9-4154, RXCJ0528.9-3927, and
RXCJ2308.3-0211, as part of a sample of massive X-ray luminous clusters located
at intermediate redshifts. We used a multiwavelength analysis, combining WFI
photometric observations, VIMOS spectroscopy, and the X-ray surface brightness
maps. We investigated the optical morphology of the clusters, we looked for
significant counterparts in the residual X-ray emission, and we ran several
tests to assess their dynamical state. We correlated the results to define
various substructure features, to study their properties, and to quantify their
influence on simple dynamical mass estimators. RXCJ0225 has a bimodal core, and
two massive galaxy groups are located in its immediate surroundings; they are
aligned in an elongated structure that is also detected in X-rays. RXCJ0528 is
located in a poor environment; an X-ray centroid shift and the presence of two
central BCGs provide mild evidence for a recent and active dynamical history.
RXCJ2308 has complex central dynamics, and it is found at the core of a
superstes-cluster. The complexity of the cluster's central dynamics reflects
the richness of its large-scale environment: RXCJ0225 and RXCJ2308 present a
mass fraction in substructures larger than the typical 0.05-0.15, whereas the
isolated cluster RXCJ0528 does not have any major substructures within its
virial radius. The largest substructures are found in the cluster outskirts.
The optical morphology of the clusters correlates with the orientation of their
BCG, and with the position of the main axes of accretion
The ROSAT-ESO Flux Limited X-ray Galaxy Cluster Survey (REFLEX II) I. Newly identified X-ray luminous clusters at z>=0.2
We report 19 intermediate redshift clusters newly detected in the ROSAT
All-Sky survey that are spectroscopically confirmed. They form a part of 911
objects in the REFLEX II cluster catalogue with a limiting flux of
1.8\times10^12 erg/s/cm2 in the 0.1-2.4 keV ROSAT band at redshift z >= 0.2. In
addition we report three clusters from the REFLEX III supplementary catalogue,
which contains objects below the REFLEX II flux limit but satisfies the
redshift constraint above. These clusters are spectroscopically followed-up by
our ESO NTT-EFOSC2 campaigns for the redshift measurement. We describe our
observing and data reduction methods. We show how X-ray properties such as
spectral hardness ratio and source extent can be used as important diagnostics
in selecting galaxy cluster candidates. Physical properties of the clusters are
subsequently calculated from the X-ray observations. This sample contains the
high mass and intermediate-redshift galaxy clusters for astrophysical and
cosmological applications.Comment: Astronomy and Astrophysics (in press
The extended ROSAT-ESO Flux Limited X-ray Galaxy Cluster Survey (REFLEX II) IV. X-ray Luminosity Function and First Constraints on Cosmological Parameters
The X-ray luminosity function is an important statistic of the census of
galaxy clusters and an important means to probe the cosmological model of our
Universe. Based on our recently completed REFLEX II cluster sample we construct
the X-ray luminosity function of galaxy clusters for several redshift slices
from to and discuss its implications. We find no significant
signature of redshift evolution of the luminosity function in the redshift
interval. We provide the results of fits of a parameterized Schechter function
and extensions of it which provide a reasonable characterization of the data.
Using a model for structure formation and galaxy cluster evolution we compare
the observed X-ray luminosity function with predictions for different
cosmological models. For the most interesting constraints for the cosmological
parameters and we obatain
and based on the statistical uncertainty alone.
Marginalizing over the most important uncertainties, the normalisation and
slope of the scaling relation, we find
and ( confidence limits). We compare our
results with those of the SZ-cluster survey provided by the PLANCK mission and
we find very good agreement with the results using PLANCK clusters as
cosmological probes, but we have some tension with PLANCK cosmological results
from the microwave background anisotropies. We also make a comparison with
other cluster surveys. We find good agreement with these previous results and
show that the REFLEX II survey provides a significant reduction in the
uncertainties compared to earlier measurements.Comment: Submitted for publication to Astronomy and Astrophysics, 15 pages, 17
figure
RASS-SDSS Galaxy Cluster Survey. VII. On the Cluster Mass to Light ratio and the Halo Occupation Distribution
We explore the mass-to-light ratio in galaxy clusters and its relation to the
cluster mass. We study the relations among the optical luminosity (),
the cluster mass () and the number of cluster galaxies within
() in a sample of 217 galaxy clusters with confirmed 3D
overdensity. We correct for projection effects, by determining the galaxy
surface number density profile in our cluster sample. This is best fitted by a
cored King profile in low and intermediate mass systems. The core radius
decreases with cluster mass, and, for the highest mass clusters, the profile is
better represented by a generalized King profile or a cuspy Navarro, Frenk &
White profile. We find a very tight proportionality between and
, which, in turn, links the cluster mass-to-light ratio to the Halo
Occupation Distribution vs. . After correcting for
projection effects, the slope of the and
relations is found to be , close, but still significantly less
than unity. We show that the non-linearity of these relations cannot be
explained by variations of the galaxy luminosity distributions and of the
galaxy M/L with the cluster mass. We suggest that the nonlinear relation
between number of galaxies and cluster mass reflects an underlying nonlinear
relation between number of subhaloes and halo mass.Comment: 15 pages, 15 figures, accepted for publication in A&
Studying the properties of galaxy cluster morphology estimators
X-ray observations of galaxy clusters reveal a large range of morphologies
with various degrees of disturbance, showing that the assumptions of
hydrostatic equilibrium and spherical shape which are used to determine the
cluster mass from X-ray data are not always satisfied. It is therefore
important for the understanding of cluster properties as well as for
cosmological applications to detect and quantify substructure in X-ray images
of galaxy clusters. Two promising methods to do so are power ratios and center
shifts. Since these estimators can be heavily affected by Poisson noise and
X-ray background, we performed an extensive analysis of their statistical
properties using a large sample of simulated X-ray observations of clusters
from hydrodynamical simulations. We quantify the measurement bias and error in
detail and give ranges where morphological analysis is feasible. A new,
computationally fast method to correct for the Poisson bias and the X-ray
background contribution in power ratio and center shift measurements is
presented and tested for typical XMM-Newton observational data sets. We studied
the morphology of 121 simulated cluster images and establish structure
boundaries to divide samples into relaxed, mildly disturbed and disturbed
clusters. In addition, we present a new morphology estimator - the peak of the
0.3-1 r500 P3/P0 profile to better identify merging clusters. The analysis
methods were applied to a sample of 80 galaxy clusters observed with
XMM-Newton. We give structure parameters (P3/P0 in r500, w and P3/P0_max) for
all 80 observed clusters. Using our definition of the P3/P0 (w) substructure
boundary, we find 41% (47%) of our observed clusters to be disturbed.Comment: Replaced to match version published in A&A, Eq. 1 correcte
Detection of X-ray Clusters of Galaxies by Matching RASS Photons and SDSS Galaxies within GAVO
A new method for a simultaneous search for clusters of galaxies in X-ray
photon maps and optical galaxy maps is described. The merging of X-ray and
optical data improves the source identification so that a large amount of
telescope time for spectroscopic follow-up can be saved. The method appears
thus ideally suited for the analysis of the recently proposed wide-angle X-ray
missions like DUO and ROSITA. As a first application, clusters are extracted
from the 3rd version of the ROSAT All-Sky Survey and the Early Date Release of
the Sloan Digital Sky Survey (SDSS). The time-consuming computations are
performed within the German Astrophysical Virtual Observatory (GAVO). On a test
area of 140 square degrees, 75 X-ray clusters are detected down to an X-ray
flux limit of in the
ROSAT energy band 0.1-2.4 keV. The clusters have redshifts . The
survey thus fills the gap between traditional large-area X-ray surveys and
serendipitous X-ray cluster searches based on pointed observations, and has the
potential to yield about 4,000 X-ray clusters after completion of SDSS.Comment: 19 pages, low-resolution figures, accepted for publication in
Astronomy and Astrophysic
An XMM-Newton study of the sub-structure in M87's halo
The high signal to noise and good point spread function of XMM have allowed
the first detailed study of the interaction between the thermal and radio
emitting plasma in the central regions of M87. We show that the X-ray emitting
structure, previously seen by ROSAT, is thermal in nature and that the east and
southwest extensions in M87's X-ray halo have a significantly lower temperature
(kT= 1.5 keV) than the surrounding ambient medium (kT= 2.3 keV). There is
little or no evidence for non-thermal emission with an upper limit on the
contribution of a power law component of spectral index flatter than 3 being
less than 1% of the flux in the region of the radio lobes.Comment: 6 pages, 8 color figures, to be published in A&A, number 36
The Large Scale X-ray Emission from M87
We describe asymmetrical features in a long exposure X-ray map of M87 made
with the ROSAT High Resolution Imager (HRI). A bright triangular region is
marked by a linear `spur' along one edge. The structure of this spur suggests
an interpretation of a tangential view of a shock front 18 kpc long. None of
the brighter features are spatially coincident with radio or optical structures
so we concur with earlier investigators that most of the emission arises from
thermal processes.Comment: 6 pages latex, including 3 postscript figures. Uses psfig and
LAMUPHYS (Springer) macro. To be published in 'The M87 Ringberg Workshop',
September 1997, Springer Lecture Notes in Physics Series, Roeser and
Meisenheimer, ed
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