2,376 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
Conditions for electron-cyclotron maser emission in the solar corona
Context. The Sun is an active source of radio emission ranging from long
duration radio bursts associated with solar flares and coronal mass ejections
to more complex, short duration radio bursts such as solar S bursts, radio
spikes and fibre bursts. While plasma emission is thought to be the dominant
emission mechanism for most radio bursts, the electron-cyclotron maser (ECM)
mechanism may be responsible for more complex, short-duration bursts as well as
fine structures associated with long-duration bursts. Aims. We investigate the
conditions for ECM in the solar corona by considering the ratio of the electron
plasma frequency {\omega}p to the electron-cyclotron frequency {\Omega}e. The
ECM is theoretically possible when {\omega}p/{\Omega}e < 1. Methods.
Two-dimensional electron density, magnetic field, plasma frequency, and
electron cyclotron frequency maps of the off- limb corona were created using
observations from SDO/AIA and SOHO/LASCO, together with potential field
extrapolations of the magnetic field. These maps were then used to calculate
{\omega}p/{\Omega}e and Alfven velocity maps of the off-limb corona. Results.
We found that the condition for ECM emission ({\omega}p/{\Omega}e < 1) is
possible at heights < 1.07 R_sun in an active region near the limb; that is,
where magnetic field strengths are > 40 G and electron densities are greater
than 3x10^8 cm-3. In addition, we found comparatively high Alfv\'en velocities
(> 0.02 c or > 6000 km s-1) at heights < 1.07 R_sun within the active region.
Conclusions. This demonstrates that the condition for ECM emission is satisfied
within areas of the corona containing large magnetic fields, such as the core
of a large active region. Therefore, ECM could be a possible emission mechanism
for high-frequency radio and microwave bursts.Comment: 4 pages, 3 figure
Radio halos in merging clusters of galaxies
We present the preliminary results of 235 MHz, 327 MHz and 610 MHz
observations of the galaxy cluster A3562 in the core of the Shapley
Concentration. The purpose of these observations, carried out with the Giant
Metrewave Radio Telescope (GMRT, Pune, India) was to study the radio halo
located at the centre of A3562 and determine the shape of its radio spectrum at
low frequencies, in order to understand the origin of this source. In the
framework of the re--acceleration model, the preliminary analysis of the halo
spectrum suggests that we are observing a young source (few yrs) at the
beginning of the re--acceleration phase.Comment: 3 pages, 2 figures. Proceedings of IAU Colloquium 195 - Outskirts of
Galaxy Clusters: intense life in the suburb
Equality of Corecursive Streams Defined by Finitary Equational Systems
In recent work, non-regular streams have been defined corecursively, by representing them with finitary equational systems built on top of various operators, besides the standard constructor. With finitary equational systems based only on the stream constructor, one can use the free theory of regular (a.k.a. rational) trees to get a sound and complete procedure to decide whether two streams are equal. However, this is not the case if one allows other operators in equations, since the underlying equational theory becomes non-trivial, hence equality of regular trees is too strong to guarantee termination of corecursive functions defined even only with the constructor and tail operators. To overcome this problem, we provide a weaker definition of equality between streams denoted by finitary equational systems built on different stream operators, including tail operator and constructor, and prove its soundness
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