132 research outputs found
The Launching of Cold Clouds by Galaxy Outflows II: The Role of Thermal Conduction
We explore the impact of electron thermal conduction on the evolution of
radiatively-cooled cold clouds embedded in flows of hot and fast material, as
occur in outflowing galaxies. Performing a parameter study of three-dimensional
adaptive mesh refinement hydrodynamical simulations, we show that electron
thermal conduction causes cold clouds to evaporate, but it can also extend
their lifetimes by compressing them into dense filaments. We distinguish
between low column-density clouds, which are disrupted on very short times, and
high-column density clouds with much-longer disruption times that are set by a
balance between impinging thermal energy and evaporation. We provide fits to
the cloud lifetimes and velocities that can be used in galaxy-scale simulations
of outflows, in which the evolution of individual clouds cannot be modeled with
the required resolution. Moreover, we show that the clouds are only accelerated
to a small fraction of the ambient velocity because compression by evaporation
causes the clouds to present a small cross-section to the ambient flow. This
means that either magnetic fields must suppress thermal conduction, or that the
cold clouds observed in galaxy outflows are not formed of cold material carried
out from the galaxy.Comment: accepted by Ap
The Role of Turbulence in AGN Self-Regulation in Galaxy Clusters
Cool cores of galaxy clusters are thought to be heated by low-power active
galactic nuclei (AGN), whose accretion is regulated by feedback. However, the
interaction between the hot gas ejected by the AGN and the ambient intracluster
medium is extremely difficult to simulate, as it involves a wide range of
spatial scales and gas that is Rayleigh-Taylor (RT) unstable. Here we use a
subgrid model for RT-driven turbulence to overcome these problems and present
the first observationally-consistent hydrodynamical simulations of AGN
self-regulation in galaxy clusters. For a wide range of parameter choices the
cluster in our three-dimensional simulations regulates itself for at least
several Gyrs years. Heating balances cooling through a string of outbreaks with
a typical recurrence time of approximately 80 Myrs, a timescale that depends
only on the global cluster properties.Comment: 4 pages, 1 figure, To appear in proceedings of The Monster's Fiery
Breath: Feedback in Galaxies, Groups, and Clusters (AIP conference series
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
Evolution of vorticity and enstrophy in the intracluster medium
Turbulence generated by large-scale motions during structure formation
affects the evolution of the thermal and non-thermal components of the
intracluster medium.
As enstrophy is a measure of the magnitude of vorticity, we study the
generation and evolution of turbulence by analysing the Lagrangian history of
enstrophy. For this purpose we combine cosmological simulations carried out
with the ENZO-code with our Lagrangian post-processing tool CRaTer. This way we
are able to quantify the individual source terms of enstrophy in the course of
the accretion of groups onto galaxy clusters. Here we focus on the redshift
range from to . Finally, we measure the rate of dissipation of
turbulence and estimate the resulting amplification of intracluster magnetic
fields.
We find that compressive and baroclinic motions are the main sources of
enstrophy, while stretching motions and dissipation affect most of the ensuing
enstrophy evolution. The rate of turbulent dissipation is able to sustain the
amplification of intracluster magnetic fields to observed levels.Comment: 14 pages, 17 Figures, accepted for publication in MNRA
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
filaments per simulated volume and to investigate the
properties of cosmic filaments for very large volumes at high resolution (up to
simulated with 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 with a mass of several . 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 halos 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 of between cosmic ray and gas pressure within filaments is
of the order of percent.Comment: 27 pages, 24 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Main Journa
Dynamical evolution of magnetic fields in the intracluster medium
We investigate the evolution of magnetic fields in galaxy clusters starting
from constant primordial fields using highly resolved ()
cosmological MHD simulations. The magnetic fields in our sample exhibit
amplification via a small-scale dynamo and compression during structure
formation. In particular, we study how the spectral properties of magnetic
fields are affected by mergers, and we relate the measured magnetic energy
spectra to the dynamical evolution of the intracluster medium. The magnetic
energy grows by a factor of 40-50 in a time-span of Gyr and
equipartition between kinetic and magnetic energy occurs on a range of scales
( at all epochs) depending on the turbulence state of the
system. We also find that, in general, the outer scale of the magnetic field
and the MHD scale are not simply correlated in time. The effect of major
mergers is to shift the peak magnetic spectra to it smaller scales, whereas the
magnetic amplification only starts after 1 Gyr. In contrast,
continuous minor mergers promote the steady growth of the magnetic field. We
discuss the implications of these findings in the interpretation of future
radio observations of galaxy clusters.Comment: Accepted in MNRAS; 16 pages, 34 figure
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