458 research outputs found
Rise and Fall of Radio Halos in Simulated Merging Galaxy Clusters
We present the first high resolution MHD simulation of cosmic-ray electron
reacceleration by turbulence in cluster mergers. We use an idealised model for
cluster mergers, combined with a numerical model for the injection, cooling and
reacceleration of cosmic-ray electrons, to investigate the evolution of cluster
scale radio emission in these objects. In line with theoretical expectations,
we for the first time, show in a simulation that reacceleration of CRe has the
potential to reproduce key observables of radio halos. In particular, we show
that clusters evolve being radio loud or radio quiet, depending on their
evolutionary stage during the merger. We thus recover the observed transient
nature of radio halos. In the simulation the diffuse emission traces the
complex interplay between spatial distribution of turbulence injected by the
halo infall and the spatial distribution of the seed electrons to reaccelerate.
During the formation and evolution of the halo the synchrotron emission spectra
show the observed variety: from power-laws with spectral index of 1 to 1.3 to
curved and ultra-steep spectra with index
Simulations of the merging cluster of galaxies Cygnus A
The archetype FR-II galaxy Cygnus A lies in a moderately rich cluster about
to undergo a major merger. We study the pre-merger Cygnus cluster environment
using smoothed particle hydrodynamics simulations constrained by 2Ms of Chandra
observations of the hot intracluster medium. The observations constrain the
total gravitating mass and concentration parameter, and the simulations provide
the quiescent and merger-enhanced temperature profiles of the pre- and post
merger of the cluster excluding the central active galactic nucleus. We present
the first detailed model of the sub cluster north west of Cygnus A, named
CygNW. We find a lower baryon fraction and higher concentration parameter for
CygA than expected from known scaling relations in the literature. The model
suggests the Cygnus cluster hosts a pre-merger with a progenitor mass ratio of
about 1.5:1 at the virial radius. We notice that the intra cluster medium is
heated as a result of the merger, but we find no evidence for a (pre-)merger
shock in the interstitial region between both cluster haloes. We attribute the
merger-induced heating to compression of the cluster outskirts. The smooth
model obtained from our simulations is subtracted from the observed cluster
state and shows residual temperature structure that is neither hydrostatic nor
merger-heated cluster gas. We speculate that this residual heating may result
from previous AGN activity over the last ~100 Myr
Measuring cosmic magnetic fields by rotation measure-galaxy cross-correlations in cosmological simulations
Using cosmological MHD simulations of the magnetic field in galaxy clusters
and filaments we evaluate the possibility to infer the magnetic field strength
in filaments by measuring cross-correlation functions between Faraday Rotation
Measures (RM) and the galaxy density field. We also test the reliability of
recent estimates considering the problem of data quality and Galactic
foreground (GF) removal in current datasets. Besides the two self-consistent
simulations of cosmological magnetic fields based on primordial seed fields and
galactic outflows analyzed here, we also explore a larger range of models
scaling up the resulting magnetic fields of one of the simulations. We find
that, if an unnormalized estimator for the cross-correlation functions and a GF
removal procedure is used, the detectability of the cosmological signal is only
possible for future instruments (e.g. SKA and ASKAP). However, mapping of the
observed RM signal to the underlying magnetization of the Universe (both in
space and time) is an extremely challenging task which is limited by the
ambiguities of our model parameters, as well as to the weak response of the RM
signal in low density environments. Therefore, we conclude that current data
cannot constrain the amplitude and distribution of magnetic fields within the
large scale structure and a detailed theoretical understanding of the build up
and distribution of magnetic fields within the Universe will be needed for the
interpretation of future observations.Comment: 11 pages, 11 figures, comparation between RM data and simulations in
fig. 8, submited to MNRAS
Magnetic Field Evolution in Giant Radio Relics using the example of CIZA J2242.8+5301
Giant radio relics are the arc-shaped diffuse radio emission regions observed
in the outskirts of some merging galaxy clusters. They are believed to trace
shock-waves in the intra-cluster medium. Recent observations demonstrated that
some prominent radio relics exhibit a steepening above 2 GHz in their radio
spectrum. This challenges standard theoretical models because shock
acceleration is expected to accelerate electrons to very high energies with a
power-law distribution in momentum. In this work we attempt to reconcile these
data with the shock-acceleration scenario. We propose that the spectral
steepening may be caused by the highest energy electrons emitting
preferentially in lower magnetic fields than the bulk of synchrotron bright
electrons in relics. Here, we focus on a model with an increasing mag- netic
field behind the shock front, which quickly saturates and then declines. We
derive the time-evolution of cosmic-ray electron spectra in time variable
magnetic fields and an expanding medium. We then apply the formalism on the
large radio relic in the cluster CIZA J2242.8+5301 (the Sausage relic). We show
that under favourable circumstances of magnetic field amplification downstream,
our model can explain the observed radio spectrum, the brightness profile and
the spectral index profile of the relic. A possible interpretation for the
required amplification of the magnetic field downstream is a dynamo acting
behind the shock with an injection scale of magnetic turbulence of about 10
kpc. Our models require injection efficiencies of CRe - which are in tension
with simple diffusive shock acceleration from the thermal pool. We show that
this problem can likely be alleviated considering pre-existing CRe.Comment: 18 pages, 3 tables, 14 figure
Is the Sunyaev-Zeldovich effect responsible for the observed steepening in the spectrum of the Coma radio halo ?
The spectrum of the radio halo in the Coma cluster is measured over almost
two decades in frequency. The current radio data show a steepening of the
spectrum at higher frequencies, which has implications for models of the radio
halo origin. There is an on-going debate on the possibility that the observed
steepening is not intrinsic to the emitted radiation, but is instead caused by
the SZ effect. Recently, the Planck satellite measured the SZ signal and its
spatial distribution in the Coma cluster allowing to test this hypothesis.
Using the Planck results, we calculated the modification of the radio halo
spectrum by the SZ effect in three different ways. With the first two methods
we measured the SZ-decrement within the aperture radii used for flux
measurements of the halo at the different frequencies. First we adopted the
global compilation of data from Thierbach et al. and a reference aperture
radius consistent with those used by the various authors. Second we used the
available brightness profiles of the halo at different frequencies to derive
the spectrum within two fixed apertures, and derived the SZ-decrement using
these apertures. As a third method we used the quasi-linear correlation between
the y and the radio-halo brightness at 330 MHz discovered by Planck to derive
the modification of the radio spectrum by the SZ-decrement in a way that is
almost independent of the adopted aperture radius. We found that the spectral
modification induced by the SZ-decrement is 4-5 times smaller than that
necessary to explain the observed steepening. Consequently a break or cut-off
in the spectrum of the emitting electrons is necessary to explain current data.
We also show that, if a steepening is absent from the emitted spectrum, future
deep observations at 5 GHz with single dishes are expected to measure a halo
flux in a 40 arcmin radius that would be 7-8 times higher than currently seen.Comment: 8 pages, 6 figures, accepted in Astronomy and Astrophysics (date of
acceptance 19/08/2013
Simulations of the Galaxy Cluster CIZA J2242.8+5301 I: Thermal Model and Shock Properties
The giant radio relic in CIZA J2242.8+5301 is likely evidence of a Mpc sized
shock in a massive merging galaxy cluster. However, the exact shock properties
are still not clearly determined. In particular, the Mach number derived from
the integrated radio spectrum exceeds the Mach number derived from the X-ray
temperature jump by a factor of two. We present here a numerical study, aiming
for a model that is consistent with the majority of observations of this galaxy
cluster. We first show that in the northern shock upstream X-ray temperature
and radio data are consistent with each other. We then derive progenitor masses
for the system using standard density profiles, X-ray properties and the
assumption of hydrostatic equilibrium. We find a class of models that is
roughly consistent with weak lensing data, radio data and some of the X-ray
data. Assuming a cool-core versus non-cool-core merger, we find a fiducial
model with a total mass of , a mass ratio of 1.76
and a Mach number that is consistent with estimates from the radio spectrum. We
are not able to match X-ray derived Mach numbers, because even low mass models
over-predict the X-ray derived shock speeds. We argue that deep X-ray
observations of CIZA J2242.8+5301 will be able to test our model and
potentially reconcile X-ray and radio derived Mach numbers in relics.Comment: 19 pages, 19 figure
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