998 research outputs found
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
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
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
Optical and X-ray profiles in the REXCESS sample of galaxy clusters
Galaxy clusters' structure, dominated by dark matter, is traced by member
galaxies in the optical and hot intra-cluster medium (ICM) in X-rays. We
compare the radial distribution of these components and determine the
mass-to-light ratio vs. system mass relation.
We use 14 clusters from the REXCESS sample which is representative of
clusters detected in X-ray surveys. Photometric observations with the Wide
Field Imager on the 2.2m MPG/ESO telescope are used to determine the number
density profiles of the galaxy distribution out to . These are
compared to electron density profiles of the ICM obtained using XMM-Newton, and
dark matter profiles inferred from scaling relations and an NFW model.
While red sequence galaxies trace the total matter profile, the blue galaxy
distribution is much shallower. We see a deficit of faint galaxies in the
central regions of massive and regular clusters, and strong suppression of
bright and faint blue galaxies in the centres of cool-core clusters,
attributable to ram pressure stripping of gas from blue galaxies in high
density regions of ICM and disruption of faint galaxies due to galaxy
interactions. We find a mass-to-light ratio vs. mass relation within
of at
with slope , consistent with most
previous results
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
Particle acceleration in cooling flow clusters of galaxies: the case of Abell 2626
It has recently been proposed a theoretical model which accounts for the
origin of radio mini-halos observed in some cooling flow clusters as related to
electron re-acceleration by MHD turbulence (Gitti, Brunetti & Setti 2002). The
MHD turbulence is assumed to be frozen into the flow of the thermal ICM and
thus amplified in the cooling flow region. Here we present the application of
this model to a new mini-halo candidate, the cluster A2626, and compare the
results with those obtained for the mini-halo in the Perseus cluster. We
present VLA data at 330 MHz and 1.5 GHz of the diffuse radio emission observed
in A2626, and we show that its main properties can be explained by the model.
We find that the power necessary for the re-acceleration of the relic electron
population is only a factor ~ 0.7% of the maximum power that can be extracted
by the cooling flow (as estimated on the basis of the standard model). We also
discuss the observational properties of known mini-halos in connection with
those of host clusters, showing that the radio power of mini--halos increases
with the maximum power of cooling flows. This trend is expected in the
framework of the model. Possible effects of new Chandra and XMM-Newton
estimates of on this trend are considered: we conclude that even if
earlier derived cooling rates were overestimated, cooling flow powers are still
well above the radio powers emitted by mini-halos.Comment: 12 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
The effect of AGN feedback on the X-ray morphologies of clusters: Simulations vs. observations
Clusters of galaxies probe the large-scale distribution of matter and are a useful tool to test the cosmological models by constraining cosmic structure growth and the expansion of the Universe. It is the scaling relations between mass observables and the true mass of a cluster through which we obtain the cosmological constraints by comparing to theoretical cluster mass functions. These scaling relations are, however, heavily influenced by cluster morphology. The presence of the slight tension in recent cosmological constraints on and based on the CMB and clusters has boosted the interests in looking for possible sources for the discrepancy. Therefore we study here the effect of active galactic nucleus (AGN) feedback as one of the major mechanisms modifying the cluster morphology influencing scaling relations. It is known that AGN feedback injects energies up to 10 erg into the intracluster medium, controls the heating and cooling of a cluster, and re-distributes cold gas from the centre to outer radii. We have also learned that cluster simulations with AGN feedback can reproduce observed cluster properties, for example, the X-ray luminosity, temperature, and cooling rate at the centre better than without the AGN feedback. In this paper using cosmological hydrodynamical simulations we investigate how the AGN feedback changes the X-ray morphology of the simulated systems, and compare this to the observed Representative Cluster Structure Survey (REXCESS) clusters. We apply two substructure measures, centre shifts () and power ratios (e.g. /), to characterise the cluster morphology, and find that our simulated clusters are more substructured than the observed clusters based on the values of and /. We also show that the degree of this discrepancy is affected by the inclusion of AGN feedback. While the clusters simulated with the AGN feedback are in much better agreement with the REXCESS relation, they are also more substructured, which increases the tension with observations. When classified as non-relaxed or relaxed according to their and / values, we find that there are no relaxed clusters in the simulations with the AGN feedback. This suggests that not only global cluster properties, like and T, and radial profiles should be used to compare and to calibrate simulations with observations, but also substructure measures like centre shifts and power ratios. Finally, we discuss what changes in the simulations might ease the tension with observational constraints on these quantities.H.B. and G.C. acknowledge support from the DFG Transregio Program TR33 and the Munich Excellence Cluster “Structure and Evolution of the Universe”. G.C. acknowledges support by the DLR under grant No. 50 OR 1405. E.P. acknowledges support by the Kavli Foundation and the FP7 ERC Advanced Grant Emergence-320596
Observational evidence for a local underdensity in the Universe and its effect on the measurement of the Hubble constant
For precision cosmological studies it is important to know the local properties of the reference point from which we observe the Universe. Particularly for the determination of the Hubble constant with low-redshift distance indicators, the values observed depend on the average matter density within the distance range covered. In this study we used the spatial distribution of galaxy clusters to map the matter density distribution in the local Universe. The study is based on our CLASSIX galaxy cluster survey, which is highly complete and well characterised, where galaxy clusters are detected by their X-ray emission. In total, 1653 galaxy clusters outside the "zone of avoidance"fulfil the selection criteria and are involved in this study. We find a local underdensity in the cluster distribution of about 30-60% which extends about 85 Mpc to the north and ∼170 Mpc to the south. We study the density distribution as a function of redshift in detail in several regions in the sky. For three regions for which the galaxy density distribution has previously been studied, we find good agreement between the density distribution of clusters and galaxies. Correcting for the bias in the cluster distribution we infer an underdensity in the matter distribution of about -30 ± 15% (-20 ± 10%) in a region with a radius of about 100 (∼140) Mpc. Calculating the probability of finding such an underdensity through structure formation theory in a ΛCDM universe with concordance cosmological parameters, we find a probability characterised by σ-values of 1.3 - 3.7. This indicates low probabilities, but with values of around 10% at the lower uncertainty limit, the existence of an underdensity cannot be ruled out. Inside this underdensity, the observed Hubble parameter will be larger by about 5.5 +2.1-2.8%, which explains part of the discrepancy between the locally measured value of H0 compared to the value of the Hubble parameter inferred from the Planck observations of cosmic microwave background anisotropies. If distance indicators outside the local underdensity are included, as in many modern analyses, this effect is diluted. © H. Böhringer et al. 2019
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