2,087 research outputs found
A substructure analysis of the A3558 cluster complex
The "algorithm driven by the density estimate for the identification of
clusters" (DEDICA, Pisani 1993, 1996) is applied to the A3558 cluster complex
in order to find substructures. This complex, located at the center of the
Shapley Concentration supercluster, is a chain formed by the ACO clusters
A3556, A3558 and A3562 and the two poor clusters SC 1327-312 and SC 1329-313.
We find a large number of clumps, indicating that strong dynamical processes
are active. In particular, it is necessary to use a fully three-dimensional
sample(i.e. using the galaxy velocity as third coordinate) in order to recover
also the clumps superimposed along the line of sight. Even if a great number of
detected substructures were already found in a previous analysis (Bardelli et
al. 1998), this method is more efficient and faster when compared with the use
of a wide battery of tests and permits the direct estimate of the detection
significance. Almost all subclusters previously detected by the wavelet
analyses found in the literature are recognized by DEDICA.
On the basis of the substructure analysis, we also briefly discuss the origin
of the A3558 complex by comparing two hypotheses: 1) the structure is a
cluster-cluster collision seen just after the first core-core encounter; 2)
this complex is the result of a series of incoherent group-group and
cluster-group mergings, focused in that region by the presence of the
surrounding supercluster. We studied the fraction of blue galaxies in the
detected substructures and found that the bluest groups reside between A3562
and A3558, i.e. in the expected position in the scenario of the cluster-cluster
collision.Comment: 10 pages with 12 encapsulated figures; MNRAS in pres
GMRT Radio Halo Survey in galaxy clusters at z = 0.2 -- 0.4. II.The eBCS clusters and analysis of the complete sample
We present the results of the GMRT cluster radio halo survey. The main
purposes of our observational project are to measure which fraction of massive
galaxy clusters in the redshift range z=0.2--0.4 hosts a radio halo, and to
constrain the expectations of the particle re--acceleration model for the
origin of the non--thermal radio emission. We selected a complete sample of 50
clusters in the X-ray band from the REFLEX (27) and the eBCS (23) catalogues.
In this paper we present Giant Metrewave Radio Telescope (GMRT) observations at
610 MHz for all clusters still lacking high sensitivity radio information, i.e.
16 eBCS and 7 REFLEX clusters, thus completing the radio information for the
whole sample. The typical sensitivity in our images is in the range 1Jy b. We found a radio halo in A697, a diffuse
peripheral source of unclear nature in A781, a core--halo source in Z7160, a
candidate radio halo in A1682 and ``suspect'' central emission in Z2661.
Including the literature information, a total of 10 clusters in the sample host
a radio halo. A very important result of our work is that 25 out of the 34
clusters observed with the GMRT do not host extended central emission at the
sensitivity level of our observations, and for 20 of them firm upper limits to
the radio power of a giant radio halo were derived. The GMRT Radio Halo Survey
shows that radio halos are not common, and our findings on the fraction of
giant radio halos in massive clusters are consistent with the statistical
expectations based on the re--acceleration model. Our results favour primary to
secondary electron models.Comment: A&A in press, 17 pages, 12 figures, 4 tables Version with high
quality figures available on web at
http://www.ira.inaf.it/~tventuri/pap/Venturi_web.pd
The cluster relic source in A521
We present high sensitivity radio observations of the merging cluster A521,
at a mean redsfhit z=0.247. The observations were carried out with the GMRT at
610 MHz and cover a region of 1 square degree, with a sensitivity limit
of = 35 Jy b. The most relevant result of these
observations is the presence of a radio relic at the cluster periphery, at the
edge of a region where group infalling into the main cluster is taking place.
Thanks to the wealth of information available in the literature in the optical
and X-ray bands, a multi--band study of the relic and its surroundings was
performed. Our analysis is suggestive of a connection between this source and
the complex ongoing merger in the A521 region. The relic might be ``revived'
fossil radio plasma through adiabatic compression of the magnetic field or
shock re--acceleration due to the merger events. We also briefly discussed the
possibility that this source is the result of induced ram pressure stripping of
radio lobes associated with the nearby cluster radio galaxy J0454--1016a.
Allowing for the large uncertainties due to the small statistics, the number of
radio emitting early--type galaxies found in A521 is consistent with the
expectations from the standard radio luminosity function for local (z0.09)
cluster ellipticals.Comment: 30 pages 8 figures, 5 tables, accepted by New Astronom
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