242 research outputs found
Environments of Redshift Survey Compact Groups of Galaxies
Redshift Survey Compact Groups (RSCGs) are tight knots of N >= 3 galaxies
selected from the CfA2+SSRS2 redshift survey. The selection is based on
physical extent and association in redshift space alone. We measured 300 new
redshifts of fainter galaxies within 1 h^{-1} Mpc of 14 RSCGs to explore the
relationship between RSCGs and their environments. 13 of 14 RSCGs are embedded
in overdense regions of redshift space. The systems range from a loose group of
5 members to an Abell cluster. The remaining group, RSCG 64, appears isolated.
RSCGs are isolated and distinct from their surroundings to varying degrees, as
are the Hickson Compact Groups. Among the 13 embedded RSCGs, 3 are distinct
from their general environments (RSCG 9, RSCG 11 and RSCG 85).Comment: 35 pages, including 10 figures and 5 tables, accepted for publication
in the Astronomical Journa
A giant radio halo in the low luminosity X-ray cluster Abell 523
Radio halos are extended and diffuse non-thermal radio sources present at the
cluster center, not obviously associated with any individual galaxy. A strong
correlation has been found between the cluster X-ray luminosity and the halo
radio power. We observe and analyze the diffuse radio emission present in the
complex merging structure Abell 523, classified as a low luminosity X-ray
cluster, to discuss its properties in the context of the halo total radio power
versus X-ray luminosity correlation. We reduced VLA archive observations at 1.4
GHz to derive a deep radio image of the diffuse emission, and compared radio,
optical, and X-ray data. Low-resolution VLA images detect a giant radio halo
associated with a complex merging region. The properties of this new halo agree
with those of radio halos in general discussed in the literature, but its radio
power is about a factor of ten higher than expected on the basis of the cluster
X-ray luminosity. Our study of this giant radio source demonstrates that radio
halos can also be present in clusters with a low X-ray luminosity. Only a few
similar cases have so far been found . This result suggests that this source
represent a new class of objects, that cannot be explained by classical radio
halo models. We suggest that the particle reacceleration related to merging
processes is very efficient and/or the X-ray luminosity is not a good indicator
of the past merging activity of a cluster.Comment: 5 pages, 6 figures, Astronomy and Astrophysics Letter in pres
Mark correlations: relating physical properties to spatial distributions
Mark correlations provide a systematic approach to look at objects both
distributed in space and bearing intrinsic information, for instance on
physical properties. The interplay of the objects' properties (marks) with the
spatial clustering is of vivid interest for many applications; are, e.g.,
galaxies with high luminosities more strongly clustered than dim ones? Do
neighbored pores in a sandstone have similar sizes? How does the shape of
impact craters on a planet depend on the geological surface properties? In this
article, we give an introduction into the appropriate mathematical framework to
deal with such questions, i.e. the theory of marked point processes. After
having clarified the notion of segregation effects, we define universal test
quantities applicable to realizations of a marked point processes. We show
their power using concrete data sets in analyzing the luminosity-dependence of
the galaxy clustering, the alignment of dark matter halos in gravitational
-body simulations, the morphology- and diameter-dependence of the Martian
crater distribution and the size correlations of pores in sandstone. In order
to understand our data in more detail, we discuss the Boolean depletion model,
the random field model and the Cox random field model. The first model
describes depletion effects in the distribution of Martian craters and pores in
sandstone, whereas the last one accounts at least qualitatively for the
observed luminosity-dependence of the galaxy clustering.Comment: 35 pages, 12 figures. to be published in Lecture Notes of Physics,
second Wuppertal conference "Spatial statistics and statistical physics
Deep images of cluster radio halos
New radio data are presented for the clusters A401, A545, A754, A1914, A2219
and A2390, where the presence of diffuse radio emission was suggested from the
images of the NRAO VLA Sky Survey. Sensitive images of these clusters, obtained
with the Very Large Array (VLA)at 20 cm confirm the existence of the diffuse
sources and allow us to derive their fluxes and intrinsic parameters.The
correlation between the halo radio power and cluster X-ray luminosity is
derived for a large sample of halo clusters, and is briefly discussed.Comment: 9 pages, 13 figures, Astron. Astrophys. in pres
The intracluster magnetic field power spectrum in Abell 2382
The goal of this work is to put constraints on the strength and structure of
the magnetic field in the cluster of galaxies A2382. We investigate the
relationship between magnetic field and Faraday rotation effects in the
cluster, using numerical simulations as a reference for the observed
polarization properties. For this purpose we present Very Large Array
observations at 20 cm and 6 cm of two polarized radio sources embedded in
A2382, and we obtained detailed rotation measure images for both of them. We
simulated random three-dimensional magnetic field models with different power
spectra and thus produced synthetic rotation measure images. By comparing our
simulations with the observed polarization properties of the radio sources, we
can determine the strength and the power spectrum of intra-cluster magnetic
field fluctuations that best reproduce the observations. The data are
consistent with a power law magnetic field power spectrum with the Kolmogorov
index , while the outer scale of the magnetic field fluctuations is of
the order of 35 kpc. The average magnetic field strength at the cluster center
is about 3 G and decreases in the external region as the square root of
the electron gas density. The average magnetic field strength in the central 1
Mpc is about 1 G.Comment: Comments: 18 pages, 13 figures, accepted by A&A. For a version with
high quality figures, see http://erg.ca.astro.it/preprints/guidetti2007
Discovery of diffuse emission in the galaxy cluster A1689
The aim of this work is to investigate the possible presence of extended
diffuse synchrotron radio emission associated with the intracluster medium of
the complex galaxy cluster A1689. The radio continuum emission of A1689 has
been investigated by analyzing archival observations at 1.2 and 1.4 GHz
obtained with the Very Large Array in different configurations. We report the
detection of an extended, diffuse, low-surface brightness radio emission
located in the central region of A1689. The surface brightness profile of the
diffuse emission at 1.2 GHz indicates a central radio brightness of ~1.7 \mu
Jy/arcsec^2 and the 3\sigma radio isophothes reveal the largest linear size to
be 730 kpc. Given its central location, the low-level surface brightness, and
the comparatively large extension, we classify the diffuse cluster-wide
emission in A1689 as a small radio halo.Comment: 8 pages, 6 figures, A&A accepte
A2255: the First Detection of Filamentary Polarized Emission in a Radio Halo
A deep radio observation of the A2255 cluster of galaxies has been carried
out at 1.4 GHz with the Very Large Array synthesis telescope. Thanks to the
excellent (u,v) coverage and sensitivity achieved by our observation, the low
brightness diffuse extended sources in the cluster (radio halo and relic) have
been imaged with unprecedented resolution and dynamic range. We find that the
radio halo has filamentary structures that are strongly polarized. The
fractional linear polarization reaches levels of 2040% and the
magnetic fields appear ordered on scales of 400 kpc. This is the first
successful attempt to detect polarized emission from a radio halo and provides
strong evidence that in this cluster the magnetic field is ordered on large
scales.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and
Astrophysics Letter
The Apparent and Intrinsic Shape of the APM Galaxy Clusters
We estimate the distribution of intrinsic shapes of APM galaxy clusters from
the distribution of their apparent shapes. We measure the projected cluster
ellipticities using two alternative methods. The first method is based on
moments of the discrete galaxy distribution while the second is based on
moments of the smoothed galaxy distribution. We study the performance of both
methods using Monte Carlo cluster simulations covering the range of APM cluster
distances and including a random distribution of background galaxies. We find
that the first method suffers from severe systematic biases, whereas the second
is more reliable. After excluding clusters dominated by substructure and
quantifying the systematic biases in our estimated shape parameters, we recover
a corrected distribution of projected ellipticities. We use the non-parametric
kernel method to estimate the smooth apparent ellipticity distribution, and
numerically invert a set of integral equations to recover the corresponding
distribution of intrinsic ellipticities under the assumption that the clusters
are either oblate or prolate spheroids. The prolate spheroidal model fits the
APM cluster data best.Comment: 8 pages, including 7 figures, accepted for publication in MNRA
The Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey I: X-ray Properties of Clusters Detected as Extended X-ray Sources
In the construction of an X-ray selected sample of galaxy clusters for
cosmological studies, we have assembled a sample of 495 X-ray sources found to
show extended X-ray emission in the first processing of the ROSAT All-Sky
Survey. The sample covers the celestial region with declination and galactic latitude and comprises sources with
a count rate counts s and a source extent likelihood of 7. In
an optical follow-up identification program we find 378 (76%) of these sources
to be clusters of galaxies. ...Comment: 61 pages; ApJS in press; fixed bug in table file; also available at
(better image quality) http://www.xray.mpe.mpg.de/theorie/NORAS
- …