21 research outputs found
Using double radio relics to constrain galaxy cluster mergers: A model of double radio relics in CIZA J2242.8+5301
Galaxy clusters grow by mergers with other clusters and galaxy groups. These
mergers create shock waves within the intracluster medium (ICM) that can
accelerate particles to extreme energies. In the presence of magnetic fields,
relativistic electrons form large regions emitting synchrotron radiation,
so-called radio relics. Behind the shock front, synchrotron and inverse Compton
(IC) losses cause the radio spectral index to steepen away from the shock
front. An example of such a cluster is CIZA J2242.8+5301, where very clear
spectral steepening in the downstream region is observed. Here we present
hydrodynamical simulations of idealized binary cluster mergers with the aim of
constraining the merger scenario for this cluster. From our simulations, we
find that CIZA J2242.8+5301 is probably undergoing a merger in the plane of the
sky (less then 10 deg from edge-on) with a mass ratio of about 2:1, and an
impact parameter < 400 kpc. We find that the core passage of the clusters
happened about 1 Gyr ago. We conclude that double relics relics can set
constraints on the mass ratios, impact parameters, timescales, and viewing
geometry of binary cluster mergers, which is particularly useful when detailed
X-ray observations are not available. In addition, the presence of large radio
relics can be used to constrain the degree of clumping in the outskirts of the
ICM, which is important to constrain the baryon fraction, density and entropy
profiles, around the virial radius and beyond. We find that the amplitude of
density fluctuations, with sizes of < 200 kpc, in the relic in CIZA
J2242.8+5301 is not larger than 30%. [abridged]Comment: 14 pages, 8 figures, accepted for publication in MNRAS on July 20,
201
Radio relics in cosmological simulations
Radio relics have been discovered in many galaxy clusters. They are believed
to trace shock fronts induced by cluster mergers. Cosmological simulations
allow us to study merger shocks in detail since the intra-cluster medium is
heated by shock dissipation. Using high resolution cosmological simulations,
identifying shock fronts and applying a parametric model for the radio emission
allows us to simulate the formation of radio relics. We analyze a simulated
shock front in detail. We find a rather broad Mach number distribution. The
Mach number affects strongly the number density of relativistic electrons in
the downstream area, hence, the radio luminosity varies significantly across
the shock surface. The abundance of radio relics can be modeled with the help
of the radio power probability distribution which aims at predicting radio
relic number counts. Since the actual electron acceleration efficiency is not
known, predictions for the number counts need to be normalized by the observed
number of radio relics. For the characteristics of upcoming low frequency
surveys we find that about thousand relics are awaiting discovery.Comment: 10 pages, 4 figures, Invited talk at the conference "Diffuse
Relativistic Plasmas", Bangalore, 1-4 March 2011; in press in special issue
of Journal of Astrophysics and Astronom
A low-frequency radio halo survey of the South Pole Telescope SZ-selected clusters with the GMRT
Large scale structure and cosmolog
Diffuse radio emission in the galaxy cluster SPT-CL J2031-4037: a steep spectrum intermediate radio halo?
The advent of sensitive low frequency radio observations has revealed a
number of diffuse radio objects with peculiar properties that are challenging
our understanding about the physics of the intracluster medium. Here, we report
the discovery of a steep spectrum radio halo surrounding the central Brightest
Cluster Galaxy (BCG) in the galaxy cluster SPT-CL J2031-4037. This cluster is
morphologically disturbed yet has a weak cool core, an example of cool
core/non-cool core transition system, which harbours a radio halo of
Mpc in size. The halo emission detected at 1.7 GHz is less extended compared to
that in the 325 MHz observation, and the spectral index of the part of the halo
visible at 325 MHz to 1.7 GHz frequencies was found to be .
Also, was found to be W Hz
which falls in the region where radio mini-halos, halo upper limits and
ultra-steep spectrum (USS) halos are found in the plane. Additionally, simulations presented in the paper provide
support to the scenario of the steep spectrum. The diffuse radio emission found
in this cluster may be a steep spectrum "intermediate" or "hybrid" radio halo
which is transitioning into a mini-halo.Comment: 6 pages, 3 figures; Accepted for publication in MNRAS Lette
Magnetic Fields, Relativistic Particles, and Shock Waves in Cluster Outskirts
It is only now, with low-frequency radio telescopes, long exposures with
high-resolution X-ray satellites and gamma-ray telescopes, that we are
beginning to learn about the physics in the periphery of galaxy clusters. In
the coming years, Sunyaev-Zeldovich telescopes are going to deliver further
great insights into the plasma physics of these special regions in the
Universe. The last years have already shown tremendous progress with detections
of shocks, estimates of magnetic field strengths and constraints on the
particle acceleration efficiency. X-ray observations have revealed shock fronts
in cluster outskirts which have allowed inferences about the microphysical
structure of shocks fronts in such extreme environments. The best indications
for magnetic fields and relativistic particles in cluster outskirts come from
observations of so-called radio relics, which are megaparsec-sized regions of
radio emission from the edges of galaxy clusters. As these are difficult to
detect due to their low surface brightness, only few of these objects are
known. But they have provided unprecedented evidence for the acceleration of
relativistic particles at shock fronts and the existence of muG strength fields
as far out as the virial radius of clusters. In this review we summarise the
observational and theoretical state of our knowledge of magnetic fields,
relativistic particles and shocks in cluster outskirts.Comment: 34 pages, to be published in Space Science Review
Parametrizing Epoch of Reionization foregrounds: A deep survey of low-frequency point-source spectra with the Murchison Widefield Array
© 2016 The Authors. Experiments that pursue detection of signals from the Epoch of Reionization (EoR) are relying on spectral smoothness of source spectra at low frequencies. This article empirically explores the effect of foreground spectra on EoR experiments by measuring high-resolution full-polarization spectra for the 586 brightest unresolved sources in one of the Murchison Widefield Array (MWA) EoR fields using 45 h of observation. A novel peeling scheme is used to subtract 2500 sources from the visibilities with ionospheric and beam corrections, resulting in the deepest, confusion-limited MWA image so far. The resulting spectra are found to be affected by instrumental effects, which limit the constraints that can be set on source-intrinsic spectral structure. The sensitivity and power-spectrum of the spectra are analysed, and it is found that the spectra of residuals are dominated by point spread function sidelobes from nearby undeconvolved sources. We release a catalogue describing the spectral parameters for each measured source
A double radio relic in the merging galaxy cluster ZwCl 0008.8+5215
Contains fulltext :
91496.pdf (preprint version ) (Open Access
Probing the origin of diffuse radio emission in the cool core of the Phoenix galaxy cluster
Large scale structure and cosmolog