2,395 research outputs found
New Structure In The Shapley Supercluster
We present new radial velocities for 189 galaxies in a 91 sq. deg region of
the Shapley supercluster measured with the FLAIR-II spectrograph on the UK
Schmidt Telescope. The data reveal two sheets of galaxies linking the major
concentrations of the supercluster. The supercluster is not flattened in
Declination as was suggested previously and it may be at least 30 percent
larger than previously thought with a correspondingly larger contribution to
the motion of the Local Group.Comment: LaTex: 2 pages, 1 figure, includes conf_iap.sty style file. To appear
in proceedings of The 14th IAP Colloquium: Wide Field Surveys in Cosmology,
held in Paris, 1998 May 26--30, eds. S.Colombi, Y.Mellie
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
Radio emission at the centre of the galaxy cluster Abell 3560: evidence for core sloshing?
Previous radio observations of the galaxy cluster A3560 in the Shapley
Concentration showed complex radio emission associated with the brightest
cluster member.To understand its origin we observed it with the GMRT, the VLA
and ATCA at 240 and 610 MHz, 1.28,1.4, 2.3,4.8 and 8.4 GHz, and performed a
detailed morphological and spectral study of the radio emission associated with
the BCG. We also observed the cluster with XMM-Newton and Chandra to derive the
properties of the ICM. The radio emission of the N-E nucleus of the dumb-bell
BCG shows an active radio galaxy, plus aged diffuse emission, which is not
refurbished at present. Our Chandra data show that the radio active nucleus of
the BCG has extended X-ray emission, which we classify as a low-luminosity
corona. A residual image of the XMM-Newton brightness shows the presence of a
spiral-like feature, which we interpret as the signature of gas sloshing. The
presence of a subgroup is clear in the surface brightness residual map, and in
the XMM-Newton temperature analysis. The optical 2D analysis shows substructure
in A3560. A galaxy clump was found at the location of the X-ray subgroup, and
another group is present south of the cluster core, close to the spiral-like
feature. The aged part of the radio emission closely follows the spiral pattern
of the X-ray residual brightness distribution, while the two active radio lobes
are bent in a completely different direction. We conclude that the complex
radio emission associated with the cluster BCG is the result of a minor merger
event in A3560. The aged diffuse emission is strongly affected by the sloshing
motion in the ICM. On the other hand, the bent jets and lobes of the current
radio AGN activity may reflect a complex gas velocity field in the innermost
cluster regions and/or sloshing-induced oscillations in the motion of the cD
galaxy.Comment: 15 pages, 8 figures, 5 tables, A&A in pres
First Application of Pulse-Shape Analysis to Silicon Micro-Strip Detectors
The method of pulse-shape analysis (PSA) for particle identification (PID)
was applied to a double-sided silicon strip detector (DSSD) with a strip pitch
of 300 \{mu}m. We present the results of test measurements with particles from
the reactions of a 70 MeV 12C beam impinging on a mylar target. Good separation
between protons and alpha particles down to 3 MeV has been obtained when
excluding the interstrip events of the DSSD from the analysis.Comment: 7 pages, 6 figures, submitted to Nuclear Inst. and Methods in Physics
Research
Wiener Reconstruction of Large-Scale Structure from Peculiar Velocities
We present an alternative, Bayesian method for large-scale reconstruction
from observed peculiar velocity data. The method stresses a rigorous treatment
of the random errors and it allows extrapolation into poorly sampled regions in
real space or in k-space. A likelihood analysis is used to determine the
fluctuation power spectrum, followed by a Wiener Filter (WF) analysis to obtain
the minimum-variance mean fields of velocity and mass density. Constrained
Realizations (CR) are then used to sample the statistical scatter about the WF
mean field. The WF/CR method is applied as a demonstration to the Mark III data
with 1200 km/s, 900 km/s, and 500 km/s resolutions. The main reconstructed
structures are consistent with those extracted by the POTENT method. A
comparison with the structures in the distribution of IRAS 1.2Jy galaxies
yields a general agreement. The reconstructed velocity field is decomposed into
its divergent and tidal components relative to a cube of +/-8000 km/s centered
on the Local Group. The divergent component is very similar to the velocity
field predicted from the distribution of IRAS galaxies. The tidal component is
dominated by a bulk flow of 194 +/- 32 km/s towards the general direction of
the Shapley concentration, and it also indicates a significant quadrupole.Comment: 28 pages and 8 GIF figures, Latex (aasms4.sty), submitted to ApJ.
Postscript version of the figures can be obtained by anonymous ftp from:
ftp://alf.huji.ac.il/pub/saleem
Spectroscopic confirmation of clusters from the ESO imaging survey
We measure redshifts for 67 galaxies in the field of six cluster candidates
from the ESO Imaging Survey (EIS). The cluster candidates are selected in the
EIS patches C and D among those with estimated mean redshifts between 0.5 and
0.7. The observations were made with EFOSC2 at the 3.6m ESO telescope.
In the six candidate cluster fields, we identify 19 possible sets of 2 to 7
galaxies in redshift space. In order to establish which of the 19 sets are
likely to correspond to real dense systems we compare our counts with those
expected from a uniform distribution of galaxies with given luminosity
function. In order to take into account the effect of the Large Scale
Structure, we modulate the probability computed from the luminosity function
with random samplings of the Canada-France Redshift Survey.
We find that four out of six candidate EIS clusters are likely to correspond
to real systems in redshift space (> 95 % confidence level). Two of these
systems have mean redshift in agreement with the redshift estimate given by the
matched filter algorithm. The other two systems have significantly lower
redshifts.
We discuss the implications of our results in the context of our ongoing
research projects aimed at defining high-redshift optically-selected cluster
samples.Comment: To appear in A&A, main journal -- 12 pages, 9 figure
Excavation at Aguas Buenas, Robinson Crusoe Island, Chile, of a gunpowder magazine and the supposed campsite of Alexander Selkirk, together with an account of early navigational dividers
Excavations were undertaken of a ruined building at Aguas Buenas, identified as an 18th-century Spanish gunpowder magazine. Evidence was also found for the campsite of an early European occupant of the island. A case is made that this was Alexander Selkirk, a castaway here from 1704 to 1709. Selkirk was the model for Defoe’s Robinson Crusoe. A detailed discussion is given of a fragment of copper alloy identifi ed as being from a pair of navigational dividers
Shock acceleration as origin of the radio relic in A521?
We present new high sensitivity observations of the radio relic in A521
carried out with the Giant Metrewave Radio Telescope at 327 MHz and with the
Very Large Array at 4.9 and 8.5 GHz. We imaged the relic at these frequencies
and carried out a detailed spectral analysis, based on the integrated radio
spectrum between 235 MHz and 4.9 GHz, and on the spectral index image in the
frequency range 327-610 MHz. To this aim we used the new GMRT observations and
other proprietary as well as archival data. We also searched for a possible
shock front co-located with the relic on a short archival Chandra X-ray
observation of the cluster. The integrated spectrum of the relic is consistent
with a single power law; the spectral index image shows a clear trend of
steepening going from the outer portion of the relic toward the cluster centre.
We discuss the origin of the source in the light of the theoretical models for
the formation of cluster radio relics. Our results on the spectral properties
of the relic are consistent with acceleration of relativistic electrons by a
shock in the intracluster medium. This scenario is further supported by our
finding of an X-ray surface brightness edge coincident with the outer border of
the radio relic. This edge is likely a shock front.Comment: 13 pages, 12 figures, accepted for publication in A&
Evidence of Substructure in the Cluster of Galaxies A3558
We investigate the dynamical properties of the cluster of galaxies A3558
(Shapley 8). Studying a region of one square degree ( 3 Mpc) centered
on the cluster cD galaxy, we have obtained a statistically complete photometric
catalog with positions and magnitudes of 1421 galaxies (down to a limiting
magnitude of ). This catalog has been matched to the recent velocity
data obtained by Mazure et al. (1997) and from the literature, yielding a
radial velocity catalog containing 322 galaxies. Our analysis shows that the
position/velocity space distribution of galaxies shows significant
substructure. A central bimodal core detected previously in preliminary studies
is confirmed by using the Adaptive Kernel Technique and Wavelet Analysis. We
show that this central bimodal subtructure is nevertheless composed of a
projected feature, kinematically unrelated to the cluster, plus a group of
galaxies probably in its initial merging phase into a relaxed core. The cD
velocity offset with respect to the average cluster redshift, reported earlier
by several authors, is completely eliminated as a result of our dynamical
analysis. The untangling of the relaxed core component also allows a better,
more reliable determination of the central velocity dispersion, which in turn
eliminates the ``-problem'' for A3558. The cluster also shows a
``preferential'' distribution of subclumps coinciding with the direction of the
major axis position angle of the cD galaxy and of the central X-ray emission
ellipsoidal distribution, in agreement with an anisotropic merger scenario.Comment: 35 pages in latex, 17 figures in Postscript, accepted for publication
in the Astrophysical Journa
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