1,744 research outputs found
Diffuse radio emission in the merging cluster MACS J0717.5+3745: the discovery of the most powerful radio halo
Hierarchical models of structure formation predict that galaxy clusters grow
via mergers of smaller clusters and galaxy groups, as well as through
continuous accretion of gas. MACS J0717.5+3745 is an X-ray luminous and complex
merging cluster, located at a redshift of 0.55. Here we present Giant Metrewave
Radio Telescope (GMRT) radio observations at 610 MHz of this cluster. The main
aim of the observations is to search for diffuse radio emission within the
galaxy cluster MACS J0717.5+3745 related to the ongoing merger. These GMRT
observations are complemented by Very Large Array (VLA) archival observations
at 1.4, 4.9 and 8.5 GHz. We have discovered a radio halo in the cluster MACS
J0717.5+3745 with a size of about 1.2 Mpc. The radio power P_1.4 GHz is 5 x
10^25 W/Hz, which makes it the most powerful radio halo known till date. A 700
kpc radio structure, which we classify as a radio relic, is located in between
the merging substructures of the system. The location of this relic roughly
coincides with regions of the intra-cluster medium (ICM) that have a
significant enhancement in temperature as shown by Chandra. The major axis of
the relic is also roughly perpendicular to the merger axis. This shows that the
relic might be the result of a merger-related shock wave, where particles are
accelerated via the diffuse shock acceleration (DSA) mechanism. Alternatively,
the relic might trace an accretion shock of a large-scale galaxy filament to
the south-west. The global spectral index of radio emission within the cluster
is found to be -1.24 +/-0.05 between 4.9 GHz and 610 MHz. We derive a value of
5.8 microGauss for the equipartition magnetic field strength at the location of
the radio halo. [abridged].Comment: 8 pages, 9 figures, accepted for publication in A&A on August 3, 200
Controlling the commoners
The making of rules by institutions of collective action, such as commons, has been and still is an instrument to promote desired behaviour and to prevent free-riding and other individual actions that might affect the collective interest negatively. In this article, we use the historical markeboeken of four Dutch commons (marken) to study the way in which commoners sought to guarantee the resilience and longevity of the common, by analysing the design of regulations against unauthorized use, the interaction of those rules with internal and external developments, and the effects that various forms of penalty may have had on the behaviour of commoners
Diffuse radio emission in the complex merging galaxy cluster Abell 2069
Galaxy clusters with signs for a recent merger show in many cases extended
diffuse radio features. This emission originates from relativistic electrons
which suffer synchrotron losses due to the intra-cluster magnetic field. The
mechanisms of the particle acceleration and the properties of the magnetic
field are still poorly understood. We search for diffuse radio emission in
galaxy clusters. Here, we study the complex galaxy cluster Abell 2069, for
which X-ray observations indicate a recent merger. We investigate the cluster's
radio continuum emission by deep Westerbork Synthesis Radio Telescope (WSRT)
observations at 346 MHz and a Giant Metrewave Radio Telescope (GMRT)
observation at 322 MHz. We find an extended diffuse radio feature roughly
coinciding with the main component of the cluster. We classify this emission as
a radio halo and estimate its lower limit flux density to 25 +/- 9 mJy.
Moreover, we find a second extended diffuse source located at the cluster's
companion and estimate its flux density to 15 +/- 2 mJy. We speculate that this
is a small halo or a mini-halo. If true, this cluster is the first example of a
double-halo in a single galaxy cluster.Comment: 6 pages, 3 figures, accepted for publication in A&
Particle Acceleration on Megaparsec Scales in a Merging Galaxy Cluster
Galaxy clusters form through a sequence of mergers of smaller galaxy clusters
and groups. Models of diffusive shock acceleration (DSA) suggest that in shocks
that occur during cluster mergers, particles are accelerated to relativistic
energies, similar to supernova remnants. Together with magnetic fields these
particles emit synchrotron radiation and may form so-called radio relics. Here
we report the detection of a radio relic for which we find highly aligned
magnetic fields, a strong spectral index gradient, and a narrow relic width,
giving a measure of the magnetic field in an unexplored site of the universe.
Our observations prove that DSA also operates on scales much larger than in
supernova remnants and that shocks in galaxy clusters are capable of producing
extremely energetic cosmic rays.Comment: Published in Science Express on 23 September 2010, 6 figures,
Supporting Online Material included. This is the author's version of the
work. It is posted here by permission of the AAAS for personal use, not for
redistribution. The definitive version was published in Science, volume 330,
15 October 201
New giant radio sources and underluminous radio halos in two galaxy clusters
The aim of this work is to analyse the radio properties of the massive and
dynamical disturbed clusters Abell 1451 and Zwcl 0634.1+4750, especially
focusing on the possible presence of diffuse emission. We present new GMRT 320
MHz and JVLA 1.5 GHz observations of these two clusters. We found that both
Abell 1451 and Zwcl 0634.1+4750 host a radio halo with a typical spectrum
(). Similarly to a few other cases reported in the recent
literature, these radio halos are significantly fainter in radio luminosity
with respect to the current radio power-mass correlations and they are smaller
than classical giant radio halos. These underluminous sources might contribute
to shed light on the complex mechanisms of formation and evolution of radio
halos. Furthermore, we detected a candidate radio relic at large distance from
the cluster center in Abell 1451 and a peculiar head tail radio galaxy in Zwcl
0634.1+4750, which might be interacting with a shock front.Comment: 15 pages, 13 figures, accepted for publication in A&
Why are central radio relics so rare?
In this paper we address the question why cluster radio relics that are
connected to shock acceleration, so-called radio gischt, have preferentially
been found in the outskirts of galaxy clusters. By identifying merger shock
waves in cosmological grid simulations, we explore several prescriptions for
relating the energy dissipated in shocks to the energy emitted in the radio
band. None of the investigated models produce detectable radio relics within
100-200 kpc from the cluster centre. All models cause > 50 per cent of the
detectable relic emission at projected distances > 800 kpc. Central radio
relics caused by shocks that propagate along the line-of-sight are rare events
for simple geometrical reasons, and they have a low surface brightness making
them elusive for current instruments. Our simulations show that the radial
distribution of observed relics can be explained by the radial trend of
dissipated kinetic energy in shocks, that increases with distance from the
cluster centre up until half of the virial radius.Comment: 6 pages, 4 figures. MNRAS accepte
XMM-Newton observations of the merging galaxy cluster CIZA J2242.8+5301
We studied the intracluster medium of the galaxy cluster CIZA J2242.8+5301
using deep XMM-Newton observations. The cluster hosts a remarkable 2-Mpc long,
~50-kpc wide radio relic that has been nicknamed the "Sausage". A smaller, more
irregular counter-relic is also present, along with a faint giant radio halo.
We analysed the distribution of the ICM physical properties, and searched for
shocks by trying to identify density and temperature discontinuities. East of
the southern relic, we find evidence of shock compression corresponding to a
Mach number of 1.3, and speculate that the shock extends beyond the length of
the radio structure. The ICM temperature increases at the northern relic. More
puzzling, we find a "wall" of hot gas east of the cluster centre. A partial
elliptical ring of hot plasma appears to be present around the merger. While
radio observations and numerical simulations predict a simple merger geometry,
the X-ray results point towards a more complex merger scenario.Comment: Extensively revised and expanded, with 18 pages and 17 figure
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