2,577 research outputs found
Unveiling radio halos in galaxy clusters in the LOFAR era
Giant radio halos are mega-parsec scale synchrotron sources detected in a
fraction of massive and merging galaxy clusters. Radio halos provide one of the
most important pieces of evidence for non-thermal components in large scale
structure. Statistics of their properties can be used to discriminate among
various models for their origin. Therefore, theoretical predictions of the
occurrence of radio halos are important as several new radio telescopes are
about to begin to survey the sky at low frequencies with unprecedented
sensitivity. In this paper we carry out Monte Carlo simulations to model the
formation and evolution of radio halos in a cosmological framework. We extend
previous works on the statistical properties of radio halos in the context of
the turbulent re-acceleration model. First we compute the fraction of galaxy
clusters that show radio halos and derive the luminosity function of radio
halos. Then, we derive differential and integrated number count distributions
of radio halos at low radio frequencies with the main goal to explore the
potential of the upcoming LOFAR surveys. By restricting to the case of clusters
at redshifts <0.6, we find that the planned LOFAR all sky survey at 120 MHz is
expected to detect about 350 giant radio halos. About half of these halos have
spectral indices larger than 1.9 and substantially brighten at lower
frequencies. If detected they will allow for a confirmation that turbulence
accelerates the emitting particles. We expect that also commissioning surveys,
such as MSSS, have the potential to detect about 60 radio halos in clusters of
the ROSAT Brightest Cluster Sample and its extension (eBCS). These surveys will
allow us to constrain how the rate of formation of radio halos in these
clusters depends on cluster mass.Comment: 12 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Gamma ray emission and stochastic particle acceleration in galaxy clusters
FERMI (formely GLAST) will shortly provide crucial information on
relativistic particles in galaxy clusters. We discuss non-thermal emission in
the context of general calculations in which relativistic particles (protons
and secondary electrons due to proton-proton collisions) interact with MHD
turbulence generated in the cluster volume during cluster mergers. Diffuse
cluster-scale radio emission (Radio Halos) and hard X-rays are produced during
massive mergers while gamma ray emission, at some level, is expected to be
common in galaxy clusters.Comment: 4 pages, 2 Figure, proc. of the 4th Heidelberg International
Symposium on High Energy Gamma-ray Astronom
First evidence of diffuse ultra-steep-spectrum radio emission surrounding the cool core of a cluster
Diffuse synchrotron radio emission from cosmic-ray electrons is observed at the center of a number of galaxy clusters. These sources can be classified either as giant radio halos, which occur in merging clusters, or as mini halos, which are found only in cool-core clusters. In this paper, we present the first discovery of a cool-core cluster with an associated mini halo that also shows ultra-steep-spectrum emission extending well beyond the core that resembles radio halo emission. The large-scale component is discovered thanks to LOFAR observations at 144 MHz. We also analyse GMRT observations at 610 MHz to characterise the spectrum of the radio emission. An X-ray analysis reveals that the cluster is slightly disturbed, and we suggest that the steep-spectrum radio emission outside the core could be produced by a minor merger that powers electron re-acceleration without disrupting the cool core. This discovery suggests that, under particular circumstances, both a mini and giant halo could co-exist in a single cluster, opening new perspectives for particle acceleration mechanisms in galaxy clusters
Anxious to see you: Neuroendocrine mechanisms of social vigilance and anxiety during adolescence.
Social vigilance is a behavioral strategy commonly used in adverse or changing social environments. In animals, a combination of avoidance and vigilance allows an individual to evade potentially dangerous confrontations while monitoring the social environment to identify favorable changes. However, prolonged use of this behavioral strategy in humans is associated with increased risk of anxiety disorders, a major burden for human health. Elucidating the mechanisms of social vigilance in animals could provide important clues for new treatment strategies for social anxiety. Importantly, during adolescence the prevalence of social anxiety increases significantly. We hypothesize that many of the actions typically characterized as anxiety behaviors begin to emerge during this time as strategies for navigating more complex social structures. Here, we consider how the social environment and the pubertal transition shape neural circuits that modulate social vigilance, focusing on the bed nucleus of the stria terminalis and prefrontal cortex. The emergence of gonadal hormone secretion during adolescence has important effects on the function and structure of these circuits, and may play a role in the emergence of a notable sex difference in anxiety rates across adolescence. However, the significance of these changes in the context of anxiety is still uncertain, as not enough studies are sufficiently powered to evaluate sex as a biological variable. We conclude that greater integration between human and animal models will aid the development of more effective strategies for treating social anxiety
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&
The connection between radio halos and cluster mergers and the statistical properties of the radio halo population
We discuss the statistical properties of the radio halo population in galaxy
clusters. Radio bi-modality is observed in galaxy clusters: a fraction of
clusters host giant radio halos while the majority of clusters do not show
evidence of diffuse cluster-scale radio emission. The radio bi-modality has a
correspondence in terms of dynamical state of the hosting clusters showing that
merging clusters host radio halos and follow the well known radio--X-ray
correlation, while more relaxed clusters do not host radio halos and populate a
region well separated from that correlation. These evidences can be understood
in the framework of a scenario where merger-driven turbulence re-accelerate the
radio emitting electrons. We discuss the main statistical expectations of this
scenario underlining the important role of upcoming LOFAR surveys to test
present models.Comment: 11 pages, 2 figures, Invited talk at the conference "Diffuse
Relativistic Plasmas", Bangalore, 1-4 March 2011; accepted for publication in
the Journal of Astrophysics and Astronom
Entropy perturbations and large-scale magnetic fields
An appropriate gauge-invariant framework for the treatment of magnetized
curvature and entropy modes is developed. It is shown that large-scale magnetic
fields, present after neutrino decoupling, affect curvature and entropy
perturbations. The evolution of different magnetized modes is then studied
across the matter-radiation transition both analytically and numerically. From
the observation that, after equality (but before decoupling) the (scalar)
Sachs-Wolfe contribution must be (predominantly) adiabatic, constraints on the
magnetic power spectra are deduced. The present results motivate the
experimental analysis of more general initial conditions of CMB anisotropies
(i.e. mixtures of magnetized adiabatic and isocurvature modes during the
pre-decoupling phase). The role of the possible correlations between the
different components of the fluctuations is partially discussed.Comment: 43 pages, 9 figure
On the absence of radio halos in clusters with double relics
Pairs of radio relics are believed to form during cluster mergers, and are
best observed when the merger occurs in the plane of the sky. Mergers can also
produce radio halos, through complex processes likely linked to turbulent
re-acceleration of cosmic-ray electrons. However, only some clusters with
double relics also show a radio halo. Here, we present a novel method to derive
upper limits on the radio halo emission, and analyse archival X-ray Chandra
data, as well as galaxy velocity dispersions and lensing data, in order to
understand the key parameter that switches on radio halo emission. We place
upper limits on the halo power below the
correlation for some clusters, confirming that clusters with double relics have
different radio properties. Computing X-ray morphological indicators, we find
that clusters with double relics are associated with the most disturbed
clusters. We also investigate the role of different mass-ratios and
time-since-merger. Data do not indicate that the merger mass ratio has an
impact on the presence or absence of radio halos (the null hypothesis that the
clusters belong to the same group cannot be rejected). However, the data
suggests that the absence of radio halos could be associated with early and
late mergers, but the sample is too small to perform a statistical test. Our
study is limited by the small number of clusters with double relics. Future
surveys with LOFAR, ASKAP, MeerKat and SKA will provide larger samples to
better address this issue.Comment: 12 pages, 7 figures, MNRAS accepte
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