1,310 research outputs found
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
Detecting shocked intergalactic gas with X-ray and radio observations
Detecting the thermal and non-thermal emission from the shocked cosmic gas
surrounding large-scale structures represents a challenge for observations, as
well as a unique window into the physics of the warm-hot intergalactic medium.
In this work, we present synthetic radio and X-ray surveys of large
cosmological simulations in order to assess the chances of jointly detecting
the cosmic web in both frequency ranges. We then propose best observing
strategies tailored for existing (LOFAR, MWA and XMM) or future instruments
(SKA-LOW and SKA-MID, ATHENA and eROSITA). We find that the most promising
targets are the extreme peripheries of galaxy clusters in an early merging
stage, where the merger causes the fast compression of warm-hot gas onto the
virial region. By taking advantage of a detection in the radio band, future
deep X-ray observations will probe this gas in emission, and help us to study
plasma conditions in the dynamic warm-hot intergalactic medium with
unprecedented detail.Comment: 22 pages, 25 Figures. A\&A accepted, in press. Moderate revision
compared to version 1, with a few new figure
Forecasts for the detection of the magnetised cosmic web from cosmological simulations
The cosmic web contains a large fraction of the total gas mass in the
universe but is difficult to detect at most wavelengths. Synchrotron emission
from shock-accelerated electrons may offer the chance of imaging the cosmic web
at radio wavelengths. In this work we use 3D cosmological ENZO-MHD simulations
(combined with a post-processing renormalisation of the magnetic field to
bracket for missing physical ingredients and resolution effects) to produce
models of the radio emission from the cosmic web. In post-processing we study
the capabilities of 13 large radio surveys to detect this emission. We find
that surveys by LOFAR, SKA1-LOW and MWA have a chance of detecting the cosmic
web, provided that the magnetisation level of the tenuous medium in filaments
is of the order of 1% of the thermal gas energy.Comment: 19 pages, 18 figures. A&A accepted, in press. The public repository
of radio maps for the full volumes studied in this work is available at
http://www.hs.uni-hamburg.de/DE/Ins/Per/Vazza/projects/Public_data.htm
The turbulent pressure support in galaxy clusters revisited
Due to their late formation in cosmic history, clusters of galaxies are not
fully in hydrostatic equilibrium and the gravitational pull of their mass at a
given radius is expected not to be entirely balanced by the thermal gas
pressure. Turbulence may supply additional pressure, and recent (X-ray and SZ)
hydrostatic mass reconstructions claim a pressure support of of
the total pressure at . In this work we show that, after carefully
disentangling bulk from small-scale turbulent motions in high-resolution
simulations of galaxy clusters, we can constrain which fraction of the gas
kinetic energy effectively provides pressure support in the cluster's
gravitational potential. While the ubiquitous presence of radial inflows in the
cluster can lead to significant bias in the estimate of the non-thermal
pressure support, we report that only a part of this energy effectively acts as
a source of pressure, providing a support of the order of of the
total pressure at .Comment: 5 pages, 5 pages, accepted, to appear in MNRAS Letter
Electron and proton acceleration efficiency by merger shocks in galaxy clusters
Radio relics in galaxy clusters are associated with powerful shocks that (re)accelerate relativistic electrons. It is widely believed that the acceleration proceeds via diffusive shock acceleration. In the framework of thermal leakage, the ratio of the energy in relativistic electrons to the energy in relativistic protons should be smaller than Ke/p∼10−2. The relativistic protons interact with the thermal gas to produce γ-rays in hadronic interactions. Combining observations of radio relics with upper limits from γ-ray observatories can constrain the ratio Ke/p. In this work, we selected 10 galaxy clusters that contain double radio relics, and derive new upper limits from the stacking of γ-ray observations by Fermi. We modelled the propagation of shocks using a semi-analytical model, where we assumed a simple geometry for shocks and that cosmic ray protons are trapped in the intracluster medium. Our analysis shows that diffusive shock acceleration has difficulties in matching simultaneously the observed radio emission and the constraints imposed by Fermi, unless the magnetic field in relics is unrealistically large ( ≫ 10 μG). In all investigated cases (also including realistic variations of our basic model and the effect of re-acceleration), the mean emission of the sample is of the order of the stacking limit by Fermi, or larger. These findings put tension on the commonly adopted model for the powering of radio relics, and imply that the relative acceleration efficiency of electrons and protons is at odds with predictions of diffusive shock acceleration, requiring Ke/p≥10 − 10−
Simulating the transport of relativistic electrons and magnetic fields injected by radio galaxies in the intracluster medium
Radio galaxies play an important role in the seeding of cosmic rays and
magnetic fields in galaxy clusters. Here, we simulate the evolution of
relativistic electrons injected into the intracluster medium by radio galaxies.
Using passive tracer particles added to magnetohydrodynamical adaptive-mesh
simulations, we calculate the evolution of the spectrum of relativistic
electrons taking into account energy losses and re-acceleration mechanisms
associated with the dynamics of the intracluster medium. Re-acceleration can
occur at shocks via diffusive shock acceleration, and in turbulent flows via
second-order Fermi re-acceleration. This study confirms that relativistic
electrons from radio galaxies can efficiently fill the intracluster medium over
scales of several , and that they create a stable reservoir of
fossil electrons that remains available for further re-acceleration by shock
waves and turbulent gas motions. Our results also show that late evolution of
radio lobes and remnant radio galaxies is significantly affected by the
dynamics of the surrounding intracluster medium. Here the diffusive
re-acceleration couples the evolution of relativistic particles to the gas
perturbations. In the near future, deep radio observations, especially at low
frequencies, can probe such mechanisms in galaxy clusters.Comment: 22 pages, 20 figures, A & A, in pres
Properties of cosmological filaments extracted from Eulerian simulations
Using a new parallel algorithm implemented within the VisIt framework, we analysed large cosmological grid simulations to study the properties of baryons in filaments. The procedure allows us to build large catalogues with up to∼3×104 filaments per simulated volume and to investigate the properties of cosmic filaments for very large volumes at high resolution (up to 3003 Mpc3 simulated with 20483 cells). We determined scaling relations for the mass, volume, length and temperature of filaments and compared them to those of galaxy clusters. The longest filaments have a total length of about 200 Mpc with a mass of several 1015 M⊙. We also investigated the effects of different gas physics. Radiative cooling significantly modifies the thermal properties of the warm-hot-intergalactic medium of filaments, mainly by lowering their mean temperature via line cooling. On the other hand, powerful feedback from active galactic nuclei in surrounding haloes can heat up the gas in filaments. The impact of shock-accelerated cosmic rays from diffusive shock acceleration on filaments is small and the ratio between cosmic ray and gas pressure within filaments is of the order of∼10-20 per cen
Turbulent pressure support and hydrostatic mass-bias in the intracluster medium
The degree of turbulent pressure support by residual gas motions in galaxy
clusters is not well known. Mass modelling of combined X-ray and Sunyaev
Zel'dovich observations provides an estimate of turbulent pressure support in
the outer regions of several galaxy clusters. Here, we test two different
filtering techniques to disentangle bulk from turbulent motions in
non-radiative high-resolution cosmological simulations of galaxy clusters using
the cosmological hydro code ENZO. We find that the radial behavior of the ratio
of non-thermal pressure to total gas pressure as a function of cluster-centric
distance can be described by a simple polynomial function. The typical
non-thermal pressure support in the centre of clusters is 5%, increasing
to 15% in the outskirts, in line with the pressure excess found in recent
X-ray observations. While the complex dynamics of the ICM makes it impossible
to reconstruct a simple correlation between turbulent motions and hydrostatic
bias, we find that a relation between them can be established using the median
properties of a sample of objects. Moreover, we estimate the contribution of
radial accelerations to the non-thermal pressure support and conclude that it
decreases moving outwards from 40% (in the core) to 15% (in the cluster's
outskirts). Adding this contribution to one provided by turbulence, we show
that it might account for the entire observed hydrostatic bias in the innermost
regions of the clusters, and for less than 80% of it at .Comment: 20 pages; 21 figures; Substantial Revision; MNRAS in pres
Abell 1033: birth of a radio phoenix
Extended steep-spectrum radio emission in a galaxy cluster is usually
associated with a recent merger. However, given the complex scenario of galaxy
cluster mergers, many of the discovered sources hardly fit into the strict
boundaries of a precise taxonomy. This is especially true for radio phoenixes
that do not have very well defined observational criteria. Radio phoenixes are
aged radio galaxy lobes whose emission is reactivated by compression or other
mechanisms. Here, we present the detection of a radio phoenix close to the
moment of its formation. The source is located in Abell 1033, a peculiar galaxy
cluster which underwent a recent merger. To support our claim, we present
unpublished Westerbork Synthesis Radio Telescope and Chandra observations
together with archival data from the Very Large Array and the Sloan Digital Sky
Survey. We discover the presence of two sub-clusters displaced along the N-S
direction. The two sub-clusters probably underwent a recent merger which is the
cause of a moderately perturbed X-ray brightness distribution. A steep-spectrum
extended radio source very close to an AGN is proposed to be a newly born radio
phoenix: the AGN lobes have been displaced/compressed by shocks formed during
the merger event. This scenario explains the source location, morphology,
spectral index, and brightness. Finally, we show evidence of a density
discontinuity close to the radio phoenix and discuss the consequences of its
presence.Comment: accepted MNRA
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