29 research outputs found
X-ray and Radio Interactions in the Cores of Cooling Flow Clusters
We present high resolution ROSAT x-ray and radio observations of three
cooling flow clusters containing steep spectrum radio sources at their cores.
All three systems exhibit strong signs of interaction between the radio plasma
and the hot intracluster medium. Two clusters, A133 and A2626, show enhanced
x-ray emission spatially coincident with the radio source whereas the third
cluster, A2052, exhibits a large region of x-ray excess surrounding much of the
radio source. Using 3-D numerical simulations, we show that a perturbed jet
propagating through a cooling flow atmosphere can give rise to amorphous radio
morphologies, particularly in the case where the jet was ``turned off'' and
allowed to age passively. In addition, the simulated x-ray surface brightness
produced both excesses and deficits as seen observationally.Comment: 25 pages, 10 figures, accepted for publication in A
GMRT observations of the Ophiuchus galaxy cluster
VLA observations at 1477 MHz revealed the presence of a radio mini-halo
surrounding the faint central point-like radio source in the Ophiuchus cluster
of galaxies. In this work we present a study of the radio emission from this
cluster of galaxies at lower radio frequencies. We observed the Ophiuchus
cluster at 153, 240, and 614 MHz with the GMRT. The mini-halo is clearly
detected at 153 and 240 MHz while it is not detected at 610 MHz. The most
prominent feature at low frequencies is a patch of diffuse steep spectrum
emission located at about 5' south-east from the cluster center. By combining
these images with that at 1477 MHz, we derived the spectral index of the
mini-halo. Globally, the mini-halo has a low-frequency spectral index of
alpha_240^153 ~1.4 +/- 0.3 and an high-frequency spectral index of
alpha_1477^240 ~ 1.60 +/- 0.05. Moreover, we measure a systematic increase of
the high-frequency spectral index with radius: the azimuthal radial average of
alpha_1477^240 increases from about 1.3, at the cluster center, up to about 2.0
in the mini-halo outskirts. The observed radio spectral index is in agreement
with that obtained by modeling the non-thermal hard X-ray emission in this
cluster of galaxies. We assume that the X-ray component arises from inverse
Compton scattering between the photons of the cosmic microwave background and a
population of non-thermal electrons which are isotropically distributed and
whose energy spectrum is a power law with index p. We derive that the electrons
energy spectrum should extend from a minimum Lorentz factor of gamma_min < 700
up to a maximum Lorentz factor of gamma_max =3.8 x 10^4 with an index p=3.8 +/-
0.4. The volume-averaged strength for a completely disordered intra-cluster
magnetic field is B_V ~0.3 +/- 0.1 micro-G.Comment: 14 pages, 8 figures, accepted for publication in Astronomy and
Astrophysics. For a version with high-quality figures see
http://erg.ca.astro.it/preprints/ophi_2010
Cluster Abell 520: a perspective based on member galaxies. A cluster forming at the crossing of three filaments?
The connection of cluster mergers with the presence of extended, diffuse
radio sources in galaxy clusters is still debated. An interesting case is the
rich, merging cluster Abell 520, containing a radio halo. A recent
gravitational analysis has shown in this cluster the presence of a massive dark
core suggested to be a possible problem for the current cold dark matter
paradigm.
We aim to obtain new insights into the internal dynamics of Abell 520
analyzing velocities and positions of member galaxies.
Our analysis is based on redshift data for 293 galaxies in the cluster field
obtained combining new redshift data for 86 galaxies acquired at the TNG with
data obtained by CNOC team and other few data from the literature. We also use
new photometric data obtained at the INT telescope. We combine galaxy
velocities and positions to select 167 cluster members around z~0.201. We
analyze the cluster structure using the weighted gap analysis, the KMM method,
the Dressler-Shectman statistics and the analysis of the velocity dispersion
profiles. We compare our results with those from X-ray, radio and gravitational
lensing analyses.
We find that Abell 520 is definitely a very complex system. Our results
suggest that we are looking at a cluster forming at the crossing of three
filaments of the large scale structure. In particular, we detect a filament
aligned with the LOS and projected onto the center of the forming cluster. It
might explain the apparent massive dark core shown by gravitational lensing
analysis.Comment: 18 pages, 16 figures, 4 tables. Accepted for publication on A&
Parsec-scale Properties of Brightest Cluster Galaxies
We present new VLBI observations at 5 GHz of a complete sample of Brightest
Cluster Galaxies (BCGs) in nearby Abell Clusters (distance class <3). Combined
with data from the literature, this provides parsec-scale information for 34
BCGs. Our analysis of their parsec scale radio emission and cluster X-ray
properties shows a possible dichotomy between BCGs in cool core clusters and
those in non cool core clusters. Among resolved sources, those in cool core
clusters tend to have two-sided parsec-scale jets, while those in less relaxed
clusters have predominantly one-sided parsec-scale jets. We suggest that this
difference could be the result of interplay between the jets and the
surrounding medium. The one-sided structure in non cool core clusters could be
due to Doppler boosting effects in relativistic, intrinsically symmetric jets;
two-sided morphology in cool core clusters is likely related to the presence of
heavy and mildly relativistic jets slowed down on the parsec-scale. Evidence of
recurrent activity are also found in BCGs in cool core clusters.Comment: 20 pages, 10 figures, accepted for publication in A&
Hard X-ray and radio observations of Abell 754
We present a long BeppoSAX observation of Abell 754 that reports a nonthermal
excess with respect to the thermal emission at energies greater than ~45 keV. A
VLA radio observation at 1.4 GHz definitely confirms the existence of diffuse
radio emission in the central region of the cluster, previously suggested by
images at 74 and 330 MHz (Kassim et al 2001), and reports additional features.
Besides, our observation determines a steeper radio halo spectrum in the
330-1400 MHz frequency range with respect to the spectrum detected at lower
frequencies, indicating the presence of a spectral cutoff. The presence of a
radio halo in A754, considered the prototype of a merging cluster, reinforces
the link between formation of Mpc-scale radio regions and very recent or
current merger processes. The radio results combined with the hard X-ray excess
detected by BeppoSAX give information on the origin of the electron population
responsible for nonthermal phenomena in galaxy clusters. We discuss also the
possibility that 26W20, a tailed radio galaxy with BL Lac characteristics
located in the field of view of the PDS, could be responsible for the observed
nonthermal hard X-ray emission.Comment: 7 pages, 3 figures; Astronomy & Astrophysics, in pres
Radio and X-ray diffuse emission in six clusters of galaxies
Deep Very Large Array radio observations confirm the presence of halo and
relic sources in six clusters of galaxies (A115, A520, A773, A1664, A2254,
A2744) where a wide diffuse emission was previously found in the NRAO VLA Sky
Survey. New images at 1.4 GHz of these six clusters of galaxies are presented
and X-ray data obtained from the ROSAT archive are analyzed. The properties of
clusters hosting radio halos and relics are analyzed and discussed. A
correlation between the halo radio power and the cluster gravitational mass is
presented.Comment: 18 pages, 18 ps figures, accepted by A&
Moving from a continuum to a community: reconceptualising the provision of support
The notion of the continuum is applied to special education in diverse contexts across many nations. This paper explores its conceptual underpinnings, drawing upon a systematic search of the literature to review recurring ideas associated with the notion and to explicate both its uses and short-comings. Through a thematic analysis of the literature the research team derived twenty-nine continua, situated within six broad groupings (space, students, staffing, support, strategies and systems). This provides a clear structure for reconsidering the issues which the notion of the continuum is supposed to describe and enables a reconceptualisation of how the delivery of services is represented. We present the initial underpinnings for a community of provision, in which settings and services work together to provide learning and support for all children and young people in their locality