223 research outputs found
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
The X-ray Luminosity - Velocity Dispersion relation in the REFLEX Cluster Survey
We present an estimate of the bolometric X-ray luminosity - velocity
dispersion L_x - sigma_v relation measured from a new, large and homogeneous
sample of 171 low redshift, X-ray selected galaxy clusters. The linear fitting
of log(L_x) - log(sigma_v) gives L_x = 10^{32.72 \pm 0.08} sigma^{4.1 \pm
0.3}_v erg s^{-1} h^{-2}_{50}. Furthermore, a study of 54 clusters, for which
the X-ray temperature of the intracluster medium T is available, allows us to
explore two other scaling relations, L_x -T and sigma_v -T. From this sample we
obtain L_x \propto T^{3.1 \pm 0.2} and sigma_v \propto T^{1.00 \pm 0.16}, which
are fully consistent with the above result for the L_x-sigma_v. The slopes of
L_x -T and sigma_v -T are incompatible with the values predicted by
self-similarity (L_x \propto T^{2} \propto \sigma_v^4), thus suggesting the
presence of non-gravitational energy sources heating up the intracluster
medium, in addition to the gravitational collapse, in the early stages of
cluster formation. On the other hand, the result on log(L_x) - log(sigma_v)
supports the self-similar model.Comment: 9 pages, accepted for publication in MNRA
Probing Turbulence in the Coma Galaxy Cluster
Spatially-resolved gas pressure maps of the Coma galaxy cluster are obtained
from a mosaic of XMM-Newton observations in the scale range between a
resolution of 20 kpc and an extent of 2.8 Mpc. A Fourier analysis of the data
reveals the presence of a scale-invariant pressure fluctuation spectrum in the
range between 40 and 90 kpc and is found to be well described by a projected
Kolmogorov/Oboukhov-type turbulence spectrum. Deprojection and integration of
the spectrum yields the lower limit of percent of the total
intracluster medium pressure in turbulent form. The results also provide
observational constraints on the viscosity of the gas.Comment: 12 pages, 13 figures (low resolution), version accepted by Astron.
Astrophy
Diffuse radio emission in a REFLEX cluster
Deep Very Large Array radio observations are presented for the REFLEX
clusters RXCJ0437.1+0043 and RXCJ1314.4-2515. They are at similar distance and
show similar X-ray luminosity, but they are quite different in X-ray structure.
Indeed RXCJ0437.1+0043 is regular and relaxed, whereas RXCJ1314.4-2515 is
characterized by substructure and possible merging processes. The radio images
reveal no diffuse emission in RXCJ0437.1+0043, and a complex diffuse structure
in RXCJ1314.4-2515. The diffuse source in the latter cluster consists of a
central radio halo which extends to the West toward the cluster periphery and
bends to the North to form a possible relic. Another extended source is
detected in the eastern cluster peripheral region. Although there could be
plausible optical identifications for this source, it might also be a relic
candidate owing to its very steep spectrum. The present results confirm the
tight link between diffuse cluster radio sources and cluster merger processes.Comment: Accepted for publication in A&A. Figures here have been degraded to
reduce their size. A version with full resolution figures is available at
http://www.ira.cnr.it/~lferetti/OUTGOING/papREFLEX.ps.g
Non-Markov Excursion Set Model of Dark Matter Halo Abundances
The excursion set model provides a convenient theoretical framework to derive
dark matter halo abundances. This paper generalizes the model by introducing a
more realistic merging and collapse process. A new parameter regulates the
influence of the environment and thus the coherence (non-Markovianity) of the
merging and the collapse of individual mass shells. The model mass function
also includes the effects of an ellipsoidal collapse. Analytic approximations
of the halo mass function are derived for scale-invariant power spectra with
the slopes . The mass function can be compared with the
results obtained from the `Hubble volume' simulations. A significant detection
of non-Markovian effects is found for an assumed accuracy of the simulated mass
function of 10%.Comment: 10 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
The 3D soft X-ray cluster-AGN cross-correlation function in the ROSAT NEP survey
X-ray surveys facilitate investigations of the environment of AGNs. Deep
Chandra observations revealed that the AGNs source surface density rises near
clusters of galaxies. The natural extension of these works is the measurement
of spatial clustering of AGNs around clusters and the investigation of relative
biasing between active galactic nuclei and galaxies near clusters.The major
aims of this work are to obtain a measurement of the correlation length of AGNs
around clusters and a measure of the averaged clustering properties of a
complete sample of AGNs in dense environments. We present the first measurement
of the soft X-ray cluster-AGN cross-correlation function in redshift space
using the data of the ROSAT-NEP survey. The survey covers 9x9 deg^2 around the
North Ecliptic Pole where 442 X-ray sources were detected and almost completely
spectroscopically identified. We detected a >3sigma significant clustering
signal on scales s<50 h70^-1 Mpc. We performed a classical maximum-likelihood
power-law fit to the data and obtained a correlation length s_0=8.7+1.2-0.3
h_70-1 Mpc and a slope gamma=1.7$^+0.2_-0.7 (1sigma errors). This is a strong
evidence that AGNs are good tracers of the large scale structure of the
Universe. Our data were compared to the results obtained by cross-correlating
X-ray clusters and galaxies. We observe, with a large uncertainty, that the
bias factor of AGN is similar to that of galaxies.Comment: 4 pages, 2 figure, proceedings of the Conference "At the edge of the
Universe", Sintra Portugal, October 2006. To be published on the Astronomical
Society of the Pacific Conference Series (ASPCS
Observational Constraints on General Relativistic Energy Conditions, Cosmic Matter Density and Dark Energy from X-Ray Clusters of Galaxies and Type-la Supernovae
New observational constraints on the cosmic matter density Ωm and an effectively redshift-independent equation
of state parameter wx of the dark energy are obtained while simultaneously testing the strong and null energy conditions of
general relativity on macroscopic scales. The combination of REFLEX X-ray cluster and type-Ia supernova data shows that
for a flat Universe the strong energy condition might presently be violated whereas the null energy condition seems to be
fulfilled. This provides another observational argument for the present accelerated cosmic expansion and the absence of exotic
physical phenomena related to a broken null energy condition. The marginalization of the likelihood distributions is performed
in a manner to include a large fraction of the recently discussed possible systematic errors involved in the application of
X-ray clusters as cosmological probes. This yields for a flat Universe, Ωm = 0.29+0.08 and wx = â0.95+0.30 (1Ï errors without â0.12 â0.35
cosmic variance). The scatter in the different analyses indicates a quite robust result around wx = â1, leaving little room for the introduction of new energy components described by quintessence-like models or phantom energy. The most natural interpretation of the data is a positive cosmological constant with wx = â1 or something like it
The REFLEX Galaxy Cluster Survey VII: Omega_m and sigma_8 from cluster abundance and large-scale clustering
For the first time the large-scale clustering and the mean abundance of
galaxy clusters are analysed simultaneously to get precise constraints on the
normalized cosmic matter density and the linear theory RMS
fluctuations in mass . A self-consistent likelihood analysis is
described which combines, in a natural and optimal manner, a battery of
sensitive cosmological tests where observational data are represented by the
(Karhunen-Lo\'{e}ve) eigenvectors of the sample correlation matrix. This method
breaks the degeneracy between and . The cosmological tests
are performed with the ROSAT ESO Flux-Limited X-ray (REFLEX) cluster sample.
The computations assume cosmologically flat geometries and a non-evolving
cluster population mainly over the redshift range . The REFLEX sample
gives the cosmological constraints and their random errors of
and . Possible systematic errors are evaluated by estimating the
effects of uncertainties in the value of the Hubble constant, the baryon
density, the spectral slope of the initial scalar fluctuations, the mass/X-ray
luminosity relation and its intrinsic scatter, the biasing scheme, and the
cluster mass density profile. All these contributions sum up to total
systematic errors of and
.Comment: 10 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
The REFLEX Cluster Survey: Probing the Mass Distribution in the Universe
We summarize some of the major results obtained so far from the REFLEX survey
of X-ray clusters of galaxies, concentrating on the latest measurements of the
cluster X-ray luminosity function and two-point correlation function. The
REFLEX luminosity function provides the most homogeneous census of the
distribution function of masses in the local Universe, representing a unique
zero-redshift reference quantity for evolutionary studies. On the other hand,
the observed clustering of REFLEX clusters is very well described both in
amplitude and shape by the correlation function computed for a low-Omega_M CDM
model. The bidimensional correlation map xi(r_p,pi) shows no stretching along
the line of sight, indicating negligible spurious effects in the sample, with
at the same time a clear compression of the contours as expected in the
presence of coherent large-scale motions.Comment: 8 pages, 9 PS figures. Uses newpasp.sty (included). To appear in
"Tracing Cosmic Evolution with Galaxy Clusters" (Sesto 2001), ASP Conference
Series, S. Borgani, M. Mezzetti & R. Valdarnini eds., in pres
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