6,704 research outputs found
Testing the radio halo-cluster merger scenario. The case of RXCJ2003.5-2323
We present a combined radio, X-ray and optical study of the galaxy cluster
RXCJ2003.5-2323. The cluster hosts one of the largest, most powerful and
distant giant radio halos known to date, suggesting that it may be undergoing a
strong merger process. The aim of our multiwavelength study is to investigate
the radio-halo cluster merger scenario. We studied the radio properties of the
giant radio halo in RXCJ2003.5-2323 by means of new radio data obtained at 1.4
GHz with the Very Large Array, and at 240 MHz with the Giant Metrewave Radio
Telescope, in combination with previously published GMRT data at 610 MHz. The
dynamical state of the cluster was investigated by means of X-ray Chandra
observations and optical ESO--NTT observations. Our study confirms that
RXCJ2003.5-2323 is an unrelaxed cluster. The unusual filamentary and clumpy
morphology of the radio halo could be due to a combination of the filamentary
structure of the magnetic field and turbulence in the inital stage of a cluster
merger.Comment: 10 page, 10 figures, accepted for publication on A&
Revealing the magnetic field in a distant galaxy cluster: discovery of the complex radio emission from MACS J0717.5 +3745
Aims. To study at multiple frequencies the radio emission arising from the
massive galaxy cluster MACS J0717.5+3745 (z=0.55). Known to be an extremely
complex cluster merger, the system is uniquely suited for an investigation of
the phenomena at work in the intra-cluster medium (ICM) during cluster
collisions. Methods. We use multi-frequency and multi-resolution data obtained
with the Very Large Array radio telescope, and X-ray features revealed by
Chandra, to probe the non-thermal and thermal components of the ICM, their
relations and interactions. Results. The cluster shows highly complex radio
emission. A bright, giant radio halo is detected at frequencies as high as 4.8
GHz. MACS J0717.5+3745 is the most distant cluster currently known to host a
radio halo. This radio halo is also the most powerful ever observed, and the
second case for which polarized radio emission has been detected, indicating
that the magnetic field is ordered on large scales.Comment: 14 pages, 13 figures, Astronomy and Astrophysics, accepte
A radio minihalo in the extreme cool-core galaxy cluster RXCJ1504.1-0248
Aims. We report the discovery of a radio minihalo in RXCJ1504.1-0248, a
massive galaxy cluster that has an extremely luminous cool core. To date, only
9 radio minihalos are known, thus the discovery of a new one, in one of the
most luminous cool-core clusters, provides important information on this
peculiar class of sources and sheds light on their origin. Methods. The diffuse
radio source is detected using GMRT at 327 MHz and confirmed by pointed VLA
data at 1.46 GHz. The minihalo has a radius of 140 kpc. A Chandra gas
temperature map shows that the minihalo emission fills the cluster cool core
and has some morphological similarities to it, as has been previously observed
for other minihalos. Results. The Chandra data reveal two subtle cold fronts in
the cool core, likely created by sloshing of the core gas, as observed in most
cool-core clusters. Following previous work, we speculate that the origin of
the minihalo is related to sloshing. Sloshing may result in particle
acceleration by generating turbulence and/or amplifying the magnetic field in
the cool core, leading to the formation of a minihalo.Comment: 4 pages, 1 table, 3 color figures. Accepted for publication in A&A
Letter
Unravelling the origin of large-scale magnetic fields in galaxy clusters and beyond through Faraday Rotation Measures with the SKA
We investigate the possibility for the SKA to detect and study the magnetic
fields in galaxy clusters and in the less dense environments surrounding them
using Faraday Rotation Measures. To this end, we produce 3-dimensional magnetic
field models for galaxy clusters of different masses and in different stages of
their evolution, and derive mock rotation measure observations of background
radiogalaxies. According to our results, already in phase I, we will be able to
infer the magnetic field properties in galaxy clusters as a function of the
cluster mass, down to solar-masses. Moreover, using cosmological
simulations to model the gas density, we have computed the expected rotation
measure through shock-fronts that occur in the intra-cluster medium during
cluster mergers. The enhancement in the rotation measure due to the density
jump will permit to constraint the magnetic field strength and structure after
the shock passage. SKA observations of polarised sources located behind galaxy
clusters will answer several questions about the magnetic field strength and
structure in galaxy clusters, and its evolution with cosmic time.Comment: 9 pages, 4 Figures, to appear as part of 'Cosmic Magnetism' in
Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14
A giant radio halo in the massive and merging cluster Abell 1351
We report on the detection of diffuse radio emission in the X-ray luminous
and massive galaxy cluster A1351 (z=0.322) using archival Very Large Array data
at 1.4 GHz. Given its central location, morphology, and Mpc-scale extent, we
classify the diffuse source as a giant radio halo. X-ray and weak lensing
studies show A1351 to be a system undergoing a major merger. The halo is
associated with the most massive substructure. The presence of this source is
explained assuming that merger-driven turbulence may re-accelerate high-energy
particles in the intracluster medium and generate diffuse radio emission on the
cluster scale. The position of A1351 in the logP - logL plane
is consistent with that of all other radio-halo clusters known to date,
supporting a causal connection between the unrelaxed dynamical state of massive
() clusters and the presence of giant radio halos.Comment: 4 pages, 3 figures, proof corrections include
On the ICS interpretation of the Hard X-Ray Excesses in Galaxy Clusters: the case of Ophiuchus
(Abridged) High-E electrons produce Hard X-Ray (HXR) emission in galaxy
clusters by via Inverse Compton Scattering (ICS) of CMB photons. We derive the
ICS HXR emission of Ophiuchus under various scenarios: primary cosmic ray
model, secondary cosmic rays model and neutralino DM annihilation scenario. We
further discuss the predictions of the Warming Ray model for the cluster
atmosphere. Under the assumption to fit the observed HXR emission, we find that
the high-E electrons induce various consequences on the cluster atmosphere: i)
primary electrons can be marginally consistent with the data provided that
their spectrum is cutoff at E~30(90) MeV for spectral index of 3.5 (4.4); ii)
secondary electron models from pp collisions are inconsistent with gamma-ray
limits, cosmic ray protons produce too much heating of the IC gas and their
pressure at the cluster center largely exceeds the thermal one; iii) secondary
electron models from DM annihilation are inconsistent with gamma-ray and radio
limits and electrons produce too much heating of the IC gas at the cluster
center, unless the neutralino annihilation cross section is much lower than the
proposed value. We conclude that ICS by secondary electrons from both
neutralino DM annihilation and pp collisions cannot be the mechanism
responsible for the HXR excess emission; primary electrons are still a
marginally viable solution provided that their spectrum has a low-energy cutoff
at E~30-90 MeV. The WR model offers, so far, the best description of the
cluster in terms of temperature distribution, heating, pressure and spectral
energy distribution. Fermi observations of Ophiuchus will set further
constraints to this model.Comment: 10 pages, 9 figures, A&A in pres
The Evolution of the Stellar Hosts of Radio Galaxies
We present new near-infrared images of z>0.8 radio galaxies from the
flux-limited 7C-III sample of radio sources for which we have recently obtained
almost complete spectroscopic redshifts. The 7C objects have radio luminosities
about 20 times fainter than 3C radio galaxies at a given redshift. The absolute
magnitudes of the underlying host galaxies and their scale sizes are only
weakly dependent on radio luminosity. Radio galaxy hosts at z~2 are
significantly brighter than the hosts of radio-quiet quasars at similar
redshifts and the model AGN hosts of Kauffmann & Haehnelt (2000). There is no
evidence for strong evolution in scale size, which shows a large scatter at all
redshifts. The hosts brighten significantly with redshift, consistent with the
passive evolution of a stellar population that formed at z>~3. This scenario is
consistent with studies of host galaxy morphology and submillimeter continuum
emission, both of which show strong evolution at z>~2.5. The lack of a strong
``redshift cutoff'' in the radio luminosity function to z>4 suggests that the
formation epoch of the radio galaxy host population lasts >~1Gyr from z>~5 to
z~3. We suggest these facts are best explained by models in which the most
massive galaxies and their associated AGN form early due to high baryon
densities in the centres of their dark matter haloes.Comment: To appear in A
Rotation Measures of Radio Sources in Hot Galaxy Clusters
The goal of this work is to investigate the Faraday rotation measure (RM) of
radio galaxies in hot galaxy clusters in order to establish a possible
connection between the magnetic field strength and the gas temperature of the
intracluster medium. We performed Very Large Array observations at 3.6 cm and 6
cm of two radio galaxies located in A401 and Ophiuchus, a radio galaxy in
A2142, and a radio galaxy located in the background of A2065. All these galaxy
clusters are characterized by high temperatures. We obtained detailed RM images
at an angular resolution of 3'' for most of the observed radio galaxies. The RM
images are patchy and reveal fine substructures of a few kpc in size. Under the
assumption that the radio galaxies themselves have no effect on the measured
RMs, these structures indicate that the intracluster magnetic fields fluctuate
down to such small scales. These new data are compared with RM information
present in the literature for cooler galaxy clusters. For a fixed projected
distance from the cluster center, clusters with higher temperature show a
higher dispersion of the RM distributions (sigmaRM), mostly because of the
higher gas density in these clusters. Although the previously known relation
between the clusters X-ray surface brightness (Sx) at the radio galaxy location
and sigmaRM is confirmed, a possible connection between the sigmaRM-Sx relation
and the cluster temperature, if present, is very weak. Therefore, in view of
the current data, it is impossible to establish a strict link between the
magnetic field strength and the gas temperature of the intracluster medium.Comment: Accepted by Astronomy and Astrophysics, 26 pages, 19 figure
Observations of extended radio emission in clusters
We review observations of extended regions of radio emission in clusters;
these include diffuse emission in `relics', and the large central regions
commonly referred to as `halos'. The spectral observations, as well as Faraday
rotation measurements of background and cluster radio sources, provide the main
evidence for large-scale intracluster magnetic fields and significant densities
of relativistic electrons. Implications from these observations on acceleration
mechanisms of these electrons are reviewed, including turbulent and shock
acceleration, and also the origin of some of the electrons in collisions of
relativistic protons by ambient protons in the (thermal) gas. Improved
knowledge of non-thermal phenomena in clusters requires more extensive and
detailed radio measurements; we briefly review prospects for future
observations.Comment: 27 pages, 7 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 6; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
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
- …