602 research outputs found
On the complexity and the information content of cosmic structures
The emergence of cosmic structure is commonly considered one of the most
complex phenomena in Nature. However, this complexity has never been defined
nor measured in a quantitative and objective way. In this work we propose a
method to measure the information content of cosmic structure and to quantify
the complexity that emerges from it, based on Information Theory. The emergence
of complex evolutionary patterns is studied with a statistical symbolic
analysis of the datastream produced by state-of-the-art cosmological
simulations of forming galaxy clusters. This powerful approach allows us to
measure how many bits of information are necessary to predict the evolution of
energy fields in a statistical way, and it offers a simple way to quantify
when, where and how the cosmic gas behaves in complex ways. The most complex
behaviors are found in the peripheral regions of galaxy clusters, where
supersonic flows drive shocks and large energy fluctuations over a few tens of
million years. Describing the evolution of magnetic energy requires at least a
twice as large amount of bits than for the other energy fields. When radiative
cooling and feedback from galaxy formation are considered, the cosmic gas is
overall found to double its degree of complexity. In the future, Cosmic
Information Theory can significantly increase our understanding of the
emergence of cosmic structure as it represents an innovative framework to
design and analyze complex simulations of the Universe in a simple, yet
powerful way.Comment: 15 pages, 14 figures. MNRAS accepted, in pres
Do radio relics challenge diffusive shock acceleration?
Radio relics in galaxy clusters are thought to be associated with powerful
shock waves that accelerate particles via diffusive shock acceleration (DSA).
Among the particles accelerated by DSA, relativistic protons should outnumber
electrons by a large factor. While the relativistic electrons emit synchrotron
emission detectable in the radio band, the protons interact with the thermal
gas to produce gamma-rays in hadronic interactions. Using simple models for the
propagation of shock waves through clusters, the distribution of thermal gas
and the efficiency of DSA, we find that the resulting hadronic -ray
emission lies very close or above the upper limits from the FERMI data on
nearby clusters. This suggests that the relative acceleration efficiency of
electrons and protons is at odds with predictions from DSA. The inclusion of
re-accelerated "fossil" particles does not seem to solve the problem. Our study
highlights a possible tension of the commonly assumed scenario for the
formation of radio relics and we discuss possible solutions to the problem.Comment: 7 pages, 3 figures. Updated version to match with the published
version in MNRA
Why are central radio relics so rare?
In this paper we address the question why cluster radio relics that are
connected to shock acceleration, so-called radio gischt, have preferentially
been found in the outskirts of galaxy clusters. By identifying merger shock
waves in cosmological grid simulations, we explore several prescriptions for
relating the energy dissipated in shocks to the energy emitted in the radio
band. None of the investigated models produce detectable radio relics within
100-200 kpc from the cluster centre. All models cause > 50 per cent of the
detectable relic emission at projected distances > 800 kpc. Central radio
relics caused by shocks that propagate along the line-of-sight are rare events
for simple geometrical reasons, and they have a low surface brightness making
them elusive for current instruments. Our simulations show that the radial
distribution of observed relics can be explained by the radial trend of
dissipated kinetic energy in shocks, that increases with distance from the
cluster centre up until half of the virial radius.Comment: 6 pages, 4 figures. MNRAS accepte
Turbulence in the ICM from mergers, cool-core sloshing and jets: results from a new multi-scale filtering approach
We have designed a simple multi-scale method that identifies turbulent
motions in hydrodynamical grid simulations. The method does not assmume ant
a-priori coherence scale to distinguish laminar and turbulent flows. Instead,
the local mean velocity field around each cell is reconstructed with a
multi-scale filtering technique, yielding the maximum scale of turbulent eddies
by means of iterations. The method is robust, fast and easily applicable to any
grid simulation. We present here the application of this technique to the study
of spatial and spectral properties of turbulence in the intra cluster medium,
measuring turbulent diffusion and anisotropy of the turbulent velocity field
for a variety of driving mechanisms: a) accretion of matter in galaxy clusters
(simulated with ENZO); b) sloshing motions around cool-cores (simulated with
FLASH); c) jet outflows from active galactic nuclei, AGN (simulated with
FLASH). The turbulent velocities driven by matter accretion in galaxy clusters
are mostly tangential in the inner regions (inside the cluster virial radius)
and isotropic in regions close to the virial radius. The same is found for
turbulence excited by cool core sloshing, while the jet outflowing from AGN
drives mostly radial turbulence motions near its sonic point and beyond.
Turbulence leads to a diffusivity in the range =10^29-10^30 cm^2/s in the intra
cluster medium. On average, the energetically dominant mechanism of turbulence
driving in the intra cluster medium is represented by accretion of matter and
major mergers during clusters evolution.Comment: 19 pages, 20 figures. Astronomy and Astrophysics, in pres
Thermal and non-thermal traces of AGN feedback: results from cosmological AMR simulations
We investigate the observable effects of feedback from Active Galactic Nuclei
(AGN) on non-thermal components of the intracluster medium (ICM). We have
modelled feedback from AGN in cosmological simulations with the adaptive mesh
refinement code ENZO, investigating three types of feedback that are sometimes
called quasar, jet and radio mode. Using a small set of galaxy clusters
simulated at high resolution, we model the injection and evolution of Cosmic
Rays, as well as their effects on the thermal plasma. By comparing, both, the
profiles of thermal gas to observed profiles from the ACCEPT sample, and the
secondary gamma-ray emission to the available upper limits from FERMI, we
discuss how the combined analysis of these two observables can constrain the
energetics and mechanisms of feedback models in clusters. Those modes of AGN
feedback that provide a good match to X-ray observations, yield a gamma-ray
luminosity resulting from secondary cosmic rays that is about below the
available upper limits from FERMI. Moreover, we investigate the injection of
turbulent motions into the ICM from AGN, and the detectability of these motions
via the analysis of line broadening of the Fe XXIII line. In the near future,
deeper observations/upper-limits of non-thermal emissions from galaxy clusters
will yield stringent constraints on the energetics and modes of AGN feedback,
even at early cosmic epochs.Comment: 24 pages, 20 figures. MNRAS accepted. A version of the paper with
higher quality figures can be found at this url:
http://www.ira.inaf.it/~vazza/papers/feedback_vazza.pd
Testing cosmic-ray acceleration with radio relics: a high-resolution study using MHD and tracers
Weak shocks in the intracluster medium may accelerate cosmic-ray protons and
cosmic-ray electrons differently depending on the angle between the upstream
magnetic field and the shock normal. In this work, we investigate how shock
obliquity affects the production of cosmic rays in high-resolution simulations
of galaxy clusters. For this purpose, we performed a magneto-hydrodynamical
simulation of a galaxy cluster using the mesh refinement code \enzo. We use
Lagrangian tracers to follow the properties of the thermal gas, the cosmic rays
and the magnetic fields over time. We tested a number of different acceleration
scenarios by varying the obliquity-dependent acceleration efficiencies of
protons and electrons, and by examining the resulting hadronic -ray and
radio emission. We find that the radio emission does not change significantly
if only quasi-perpendicular shocks are able to accelerate cosmic-ray electrons.
Our analysis suggests that radio emitting electrons found in relics have been
typically shocked many times before . On the other hand, the hadronic
-ray emission from clusters is found to decrease significantly if only
quasi-parallel shocks are allowed to accelerate cosmic-ray protons. This might
reduce the tension with the low upper limits on -ray emission from
clusters set by the \textit{Fermi}-satellite.Comment: 16 pages, 17 Figures, accepted for publication by MNRA
The challenge of detecting intracluster filaments with Faraday Rotation
The detection of filaments in the cosmic web will be crucial to distinguish
between the possible magnetogenesis scenarios and future large polarization
surveys will be able to shed light on their magnetization level. In this work,
we use numerical simulations of galaxy clusters to investigate their possible
detection. We compute the Faraday Rotation signal in intracluster filaments and
compare it to its surrounding environment. We find that the expected big
improvement in sensitivity with the SKA-MID will in principle allow the
detection of a large fraction of filaments surrounding galaxy clusters.
However, the contamination of the intrinsic Faraday Rotation of background
polarized sources will represent a big limiter to the number of objects that
can be significantly detected. We discuss possible strategies to minimize this
effect and increase the chances of detection of the cosmic web with the large
statistics expected from future surveys.Comment: 16 pages, accepted to Galaxie
On the amplification of magnetic fields in cosmic filaments and galaxy clusters
The amplification of primordial magnetic fields via a small-scale turbulent
dynamo during structure formation might be able to explain the observed
magnetic fields in galaxy clusters. The magnetisation of more tenuous
large-scale structures such as cosmic filaments is more uncertain, as it is
challenging for numerical simulations to achieve the required dynamical range.
In this work, we present magneto-hydrodynamical cosmological simulations on
large uniform grids to study the amplification of primordial seed fields in the
intracluster medium (ICM) and in the warm-hot-intergalactic medium (WHIM). In
the ICM, we confirm that turbulence caused by structure formation can produce a
significant dynamo amplification, even if the amplification is smaller than
what is reported in other papers. In the WHIM inside filaments, we do not
observe significant dynamo amplification, even though we achieve Reynolds
numbers of . The maximal amplification for large
filaments is of the order of for the magnetic energy, corresponding
to a typical field of a few starting from a primordial weak field
of G (comoving). In order to start a small-scale dynamo, we found
that a minimum of resolution elements across the virial radius of
galaxy clusters was necessary. In filaments we could not find a minimum
resolution to set off a dynamo. This stems from the inefficiency of supersonic
motions in the WHIM in triggering solenoidal modes and small-scale twisting of
magnetic field structures. Magnetic fields this small will make it hard to
detect filaments in radio observations.Comment: MNRAS accepted, in press. 18 pages, 18 Figures. New version to match
with the one published in MNRAS. Updated publication list and footnote added
to the title as obituary notic
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