189 research outputs found
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
Magnetic field evolution in giant radio relics using the example of CIZA J2242.8+5301
Article / Letter to editorSterrewach
Cluster magnetic fields through the study of polarized radio halos in the SKA era
Galaxy clusters are unique laboratories to investigate turbulent fluid
motions and large scale magnetic fields. Synchrotron radio halos at the center
of merging galaxy clusters provide the most spectacular and direct evidence of
the presence of relativistic particles and magnetic fields associated with the
intracluster medium. The study of polarized emission from radio halos is
extremely important to constrain the properties of intracluster magnetic fields
and the physics of the acceleration and transport of the relativistic
particles. However, detecting this polarized signal is a very hard task with
the current radio facilities.We use cosmological magneto-hydrodynamical
simulations to predict the expected polarized surface brightness of radio halos
at 1.4 GHz. We compare these expectations with the sensitivity and the
resolution reachable with the SKA1. This allows us to evaluate the potential
for studying intracluster magnetic fields in the surveys planned for SKA1.Comment: 11 pages, 4 figures; to appear as part of 'Cosmic Magnetism' in
Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14)10
The Coma cluster magnetic field from Faraday rotation measures
The aim of the present work is to constrain the Coma cluster magnetic field
strength, its radial profile and power spectrum by comparing Faraday Rotation
Measure (RM) images with numerical simulations of the magnetic field. We have
analyzed polarization data for seven radio sources in the Coma cluster field
observed with the Very Large Array at 3.6, 6 and 20 cm, and derived Faraday
Rotation Measures with kiloparsec scale resolution. Random three dimensional
magnetic field models have been simulated for various values of the central
intensity B_0 and radial power-law slope eta, where eta indicates how the field
scales with respect to the gas density profile. We derive the central magnetic
field strength, and radial profile values that best reproduce the RM
observations. We find that the magnetic field power spectrum is well
represented by a Kolmogorov power spectrum with minimum scale ~ 2 kpc and
maximum scale ~ 34 kpc. The central magnetic field strength and radial slope
are constrained to be in the range (B_0=3.9 microG; eta=0.4) and (B_0=5.4
microG; eta=0.7) within 1sigma. The best agreement between observations and
simulations is achieved for B_0=4.7 microG; eta=0.5. Values of B_0>7 microG and
1.0 are incompatible with RM data at
99 % confidence level.Comment: 23 pages, 21 figures. Higher resolution available at
http://www.ira.inaf.it/~bonafede/paper.pdf. A&A accepte
An improved SPH scheme for cosmological simulations
We present an implementation of smoothed particle hydrodynamics (SPH) with
improved accuracy for simulations of galaxies and the large-scale structure. In
particular, we combine, implement, modify and test a vast majority of SPH
improvement techniques in the latest instalment of the GADGET code. We use the
Wendland kernel functions, a particle wake-up time-step limiting mechanism and
a time-dependent scheme for artificial viscosity, which includes a high-order
gradient computation and shear flow limiter. Additionally, we include a novel
prescription for time-dependent artificial conduction, which corrects for
gravitationally induced pressure gradients and largely improves the SPH
performance in capturing the development of gas-dynamical instabilities. We
extensively test our new implementation in a wide range of hydrodynamical
standard tests including weak and strong shocks as well as shear flows,
turbulent spectra, gas mixing, hydrostatic equilibria and self-gravitating gas
clouds. We jointly employ all modifications; however, when necessary we study
the performance of individual code modules. We approximate hydrodynamical
states more accurately and with significantly less noise than standard SPH.
Furthermore, the new implementation promotes the mixing of entropy between
different fluid phases, also within cosmological simulations. Finally, we study
the performance of the hydrodynamical solver in the context of radiative galaxy
formation and non-radiative galaxy cluster formation. We find galactic disks to
be colder, thinner and more extended and our results on galaxy clusters show
entropy cores instead of steadily declining entropy profiles. In summary, we
demonstrate that our improved SPH implementation overcomes most of the
undesirable limitations of standard SPH, thus becoming the core of an efficient
code for large cosmological simulations.Comment: 21 figures, 2 tables, accepted to MNRA
Macromolecular-scale resolution in biological fluorescence microscopy
We demonstrate far-field fluorescence microscopy with a focal-plane resolution of 15â20 nm in biological samples. The 10- to 12-fold multilateral increase in resolution below the diffraction barrier has been enabled by the elimination of molecular triplet state excitation as a major source of photobleaching of a number of dyes in stimulated emission depletion microscopy. Allowing for relaxation of the triplet state between subsequent excitationâdepletion cycles yields an up to 30-fold increase in total fluorescence signal as compared with reported stimulated emission depletion illumination schemes. Moreover, it enables the reduction of the effective focal spot area by up to â140-fold below that given by diffraction. Triplet-state relaxation can be realized either by reducing the repetition rate of pulsed lasers or by increasing the scanning speed such that the build-up of the triplet state is effectively prevented. This resolution in immunofluorescence imaging is evidenced by revealing nanoscale protein patterns on endosomes, the punctuated structures of intermediate filaments in neurons, and nuclear protein speckles in mammalian cells with conventional optics. The reported performance of diffraction-unlimited fluorescence microscopy opens up a pathway for addressing fundamental problems in the life sciences
Radio Halos From Simulations And Hadronic Models II: The Scaling Relations of Radio Halos
We use results from a constrained, cosmological MHD simulation of the Local
Universe to predict radio halos and their evolution for a volume limited set of
galaxy clusters and compare to current observations. The simulated magnetic
field inside the clusters is a result of turbulent amplification within them,
with the magnetic seed originating from star-burst driven, galactic outflows.
We evaluate three models, where we choose different normalizations for the
Cosmic Ray proton population within clusters. Similar to our previous analysis
of the Coma cluster (Donnert et al. 2010), the radial profile and the
morphological properties of observed radio halos can not be reproduced, even
with a radially increasing energy fraction within the cosmic ray proton
population. Scaling relations between X-ray luminosity and radio power can be
reproduced by all models, however all models fail in the prediction of clusters
with no radio emission. Also the evolutionary tracks of our largest clusters in
all models fail to reproduce the observed bi-modality in radio luminosity. This
provides additional evidence that the framework of hadronic, secondary models
is disfavored to reproduce the large scale diffuse radio emission of galaxy
clusters. We also provide predictions for the unavoidable emission of
-rays from the hadronic models for the full cluster set. None of such
secondary models is yet excluded by the observed limits in -ray
emission, emphasizing that large scale diffuse radio emission is a powerful
tool to constrain the amount of cosmic ray protons in galaxy clusters
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