80 research outputs found
Modelling the X-ray cluster dipole and cluster contribution to the soft X-ray background
We investigate the sampling and dipole convergence properties of flux-limited
samples of mock X-ray clusters in relation to their underlying ``parent''
cluster distribution. To this purpose, we resort to numerical simulations of
the cluster distribution and extract samples resembling the main observational
features of X-ray selected cluster samples. The flux-limited samples, being
quite sparse, underestimate the amplitude of the ``parent'' cluster dipole by
approximately 15 per cent on average for Local Group-like observers. However,
the general shape of their dipole amplitude profiles are in relatively good
agreement. We also calculate the expected contribution of clusters, selected
according to the relevant criteria, to the soft (0.1-2.4 keV) extragalactic
X-ray background, using the ESO Key Project X-ray luminosity function, assuming
a flat universe with vanishing cosmological constant. We obtain a value of
about 10 per cent of the observed XRB flux.Comment: Accepted by MNRAS. 12 pages, 6 figures. LaTeX in the MN styl
The X-ray Cluster Dipole
We estimate the dipole of the whole sky X-ray flux-limited sample of
Abell/ACO clusters (XBACs) and compare it to the optical Abell/ACO cluster
dipole. The X-ray cluster dipole is well aligned () with the
CMB dipole, while it follows closely the radial profile of its optical cluster
counterpart although its amplitude is per cent lower. In view of
the fact that the the XBACs sample is not affected by the volume incompleteness
and the projection effects that are known to exist at some level in the optical
parent Abell/ACO cluster catalogue, our present results confirm the previous
optical cluster dipole analysis that there are significant contributions to the
Local Group motion from large distances (Mpc). In order to
assess the expected contribution to the X-ray cluster dipole from a purely
X-ray selected sample we compare the dipoles of the XBACs and the Brightest
Cluster Sample (Ebeling et al. 1997a) in their overlap region. The resulting
dipoles are in mutual good aggreement with an indication that the XBACs sample
slightly underestimates the full X-ray dipole (by per cent) while the
Virgo cluster contributes about 10 - 15 per cent to the overall X-ray cluster
dipole. Using linear perturbation theory to relate the X-ray cluster dipole to
the Local group peculiar velocity we estimate the density parameter to be
.Comment: 16 pages, latex, + 4 ps figures, submitted to Ap
The Serendipitous XMM-Newton Cluster Athens Survey (SEXCLAS): Sample selection and the cluster log N - log S
In this paper we serendipitously identify X-ray cluster candidates using
XMM-Newton archival observations complemented by 5-band optical photometric
follow-up observations (r~23 mag) as part of the X-ray Identification (XID)
programme. Our sample covers an area of ~2.1 sq. deg (15 XMM-Newton fields) and
comprises a total of 21 (19 serendipitous + 2 target) extended X-ray sources to
the limit f(0.5-2keV) ~ 6x10^{-15} cgs with a high probability (> 99.9%) of
being extended on the XMM-Newton images. Of the 21 cluster candidates 7 are
spectroscopically confirmed in the literature. Exploiting the optical data
available for these fields we discover that 68% of the X-ray cluster candidates
are associated with optical galaxy overdensities. We also attempt to constrain
the redshifts of our cluster candidates using photometric methods. We thus
construct the photometric redshift distribution of galaxies in the vicinity of
each X-ray selected cluster candidate and search for statistically significant
redshift peaks against that of the background distribution of field galaxies.
Comparison of the photometric with spectroscopic redshift estimates for the
confirmed clusters suggest that our simple method is robust out to z~0.5. For
clusters at higher z, deeper optical data are required to estimate reliable
photometric redshifts. Finally, using the sample of the 19 serendipitous X-ray
selected cluster candidates we estimate their surface density down to
f(0.5-2keV) ~ 6x10^{-15} cgs and find it to be in fair agreement with previous
and recent studies.Comment: Submitted to the MNRAS, 8 page
Morphological Properties of Superclusters of Galaxies
We studied superclusters of galaxies in a volume-limited sample extracted
from the Sloan Digital Sky Survey Data Release 7 (SDSS/DR7) and from mock
catalogues based on a semi-analytical model of galaxy evolution in the
Millenium Simulation. A density field method was applied to a sample of
galaxies brighter than to identify superclusters,
taking into account selection and boundary effects. In order to evaluate the
influence of threshold density, we have chosen two thresholds: the first
maximizes the number of objects (D1), and the second constrains the maximum
supercluster size to 120~hMpc (D2). We have performed a
morphological analysis, using Minkowski Functionals, based on a parameter which
increases monotonically from filaments to pancakes. An anti-correlation was
found between supercluster richness (and total luminosity or size) and the
morphological parameter, indicating that filamentary structures tend to be
richer, larger and more luminous than pancakes in both observed and mock
catalogues. We have also used the mock samples to compare supercluster
morphologies identified in position and velocity spaces, concluding that our
morphological classification is not biased by the peculiar velocities. Monte
Carlo simulations designed to investigate the reliability of our results with
respect to random fluctuations show that these results are robust. Our analysis
indicates that filaments and pancakes present different luminosity and size
distributions.Comment: 12 pages, 17 figures Accepted to MNRA
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