1,591 research outputs found
3D simulations of shear instabilities in magnetized flows
We present results of three-dimensional (3D) simulations of the
magnetohydrodynamic Kelvin-Helmholtz instability in a stratified shear layer.
The magnetic field is taken to be uniform and parallel to the shear flow. We
describe the evolution of the fluid flow and the magnetic field for a range of
initial conditions. In particular, we investigate how the mixing rate of the
fluid depends on the Richardson number and the magnetic field strength. It was
found that the magnetic field can enhance as well as suppress mixing. Moreover,
we have performed two-dimensional (2D) simulations and discuss some interesting
differences between the 2D and 3D results.Comment: submitted to MNRAS, figures in colour and higher quality at
http://www.mpa-garching.mpg.de/~maria/greenreports/mpa00/reports_00.htm
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
Simulations of Buoyant Bubbles in Galaxy Clusters
It is generally argued that most clusters of galaxies host cooling flows in which radiative cooling in the centre causes a slow inflow. However, recent observations by Chandra and XMM conflict with the predicted cooling flow rates. Here we report highly resolved hydrodynamic simulations which show that buoyant bubbles can offset the cooling in the inner regions of clusters and can significantly delay the deposition of cold gas. The subsonic rise of bubbles uplifts colder material from the central regions of the cluster. This colder material appears as bright rims around the bubbles. The bubbles themselves appear as depressions in the X-ray surface brightness as observed in a growing number of clusters
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
Star formation in shocked cluster spirals and their tails
Recent observations of ram pressure stripped spiral galaxies in clusters
revealed details of the stripping process, i.e., the truncation of all
interstellar medium (ISM) phases and of star formation (SF) in the disk, and
multiphase star-forming tails. Some stripped galaxies, in particular in merging
clusters, develop spectacular star-forming tails, giving them a jellyfish-like
appearance. In merging clusters, merger shocks in the intra-cluster medium
(ICM) are thought to have overrun these galaxies, enhancing the ambient ICM
pressure and thus triggering SF, gas stripping and tail formation. We present
idealised hydrodynamical simulations of this scenario, including standard
descriptions for SF and stellar feedback. To aid the interpretation of recent
and upcoming observations, we focus on particular structures and dynamics in SF
patterns in the remaining gas disk and in the near tails, which are easiest to
observe. The observed jellyfish morphology is qualitatively reproduced for,
both, face-on and edge-on stripping. In edge-on stripping, the interplay
between the ICM wind and the disk rotation leads to asymmetries along the ICM
wind direction and perpendicular to it. The apparent tail is still part of a
highly deformed gaseous and young stellar disk. In both geometries, SF takes
place in knots throughout the tail, such that the stars in the tails show no
ordered age gradients. Significant SF enhancement in the disk occurs only at
radii where the gas will be stripped in due course.Comment: 6 pages, submitted to MNRAS Letter
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