365 research outputs found
Impact of tangled magnetic fields on AGN-blown bubbles
There is growing consensus that feedback from AGN is the main mechanism
responsible for stopping cooling flows in clusters of galaxies. AGN are known
to inflate buoyant bubbles that supply mechanical power to the intracluster gas
(ICM). High Reynolds number hydrodynamical simulations show that such bubbles
get entirely disrupted within 100 Myr, as they rise in cluster atmospheres,
which is contrary to observations. This artificial mixing has consequences for
models trying to quantify the amount of heating and star formation in cool core
clusters of galaxies. It has been suggested that magnetic fields can stabilize
bubbles against disruption. We perform MHD simulations of fossil bubbles in the
presence of tangled magnetic fields using the high order PENCIL code. We focus
on the physically-motivated case where thermal pressure dominates over magnetic
pressure and consider randomly oriented fields with and without maximum
helicity and a case where large scale external fields drape the bubble.We find
that helicity has some stabilizing effect. However, unless the coherence length
of magnetic fields exceeds the bubble size, the bubbles are quickly shredded.
As observations of Hydra A suggest that lengthscale of magnetic fields may be
smaller then typical bubble size, this may suggest that other mechanisms, such
as viscosity, may be responsible for stabilizing the bubbles. However, since
Faraday rotation observations of radio lobes do not constrain large scale ICM
fields well if they are aligned with the bubble surface, the draping case may
be a viable alternative solution to the problem. A generic feature found in our
simulations is the formation of magnetic wakes where fields are ordered and
amplified. We suggest that this effect could prevent evaporation by thermal
conduction of cold Halpha filaments observed in the Perseus cluster.Comment: accepted for publication in MNRAS, (downgraded resolution figures,
color printing recommended
Draping of Cluster Magnetic Fields over Bullets and Bubbles -- Morphology and Dynamic Effects
High-resolution X-ray observations have revealed cavities and `cold fronts'
with sharp edges in temperature, density, and metallicity within galaxy
clusters. Their presence poses a puzzle since these features are not expected
to be hydrodynamically stable, or to remain sharp in the presence of diffusion.
However, a moving core or bubble in even a very weakly magnetized plasma
necessarily sweeps up enough magnetic field to build up a dynamically important
sheath around the object; the layer's strength is set by a competition between
`plowing up' of field and field lines slipping around the core. We show that a
two-dimensional approach to the problem is quite generally not possible. In
three dimensions, we show with analytic arguments and in numerical experiments,
that this magnetic layer modifies the dynamics of a plunging core, greatly
modifies the effects of hydrodynamic instabilities on the core, modifies the
geometry of stripped material, and even slows the fall of the core through
magnetic tension. We derive an expression for the maximum magnetic field
strength, the thickness of the layer, and the opening angle of the magnetic
wake. The morphology of the magnetic draping layer implies the suppression of
thermal conduction across the layer, thus conserving strong temperature
gradients over the contact surface. The intermittent amplification of the
magnetic field as well as the injection of MHD turbulence in the wake of the
core is identified to be due to vorticity generation within the magnetic
draping layer. These results have important consequences for understanding the
physical properties and the complex gasdynamical processes of the intra-cluster
medium, and apply quite generally to motions through other magnetized
environments, e.g., the ISM.Comment: For version of this paper with interactive 3D graphics and
full-resolution figures, see http://www.cita.utoronto.ca/~ljdursi/draping/ .
19p, 26 figures, emulateapj format. Version accepted by ApJ - new references,
improved figure
Impact of Sodium Layer variations on the performance of the E-ELT MCAO module
Multi-Conjugate Adaptive Optics systems based on sodium Laser Guide Stars may
exploit Natural Guide Stars to solve intrinsic limitations of artificial
beacons (tip-tilt indetermination and anisoplanatism) and to mitigate the
impact of the sodium layer structure and variability. The sodium layer may also
have transverse structures leading to differential effects among Laser Guide
Stars. Starting from the analysis of the input perturbations related to the
Sodium Layer variability, modeled directly on measured sodium layer profiles,
we analyze, through a simplified end-to-end simulation code, the impact of the
low/medium orders induced on global performance of the European Extremely Large
Telescope Multi-Conjugate Adaptive Optics module MAORY.Comment: 7 pages, 5 figures, SPIE conference Proceedin
The role of cosmic ray pressure in accelerating galactic outflows
We study the formation of galactic outflows from supernova explosions (SNe)
with the moving-mesh code AREPO in a stratified column of gas with a surface
density similar to the Milky Way disk at the solar circle. We compare different
simulation models for SNe placement and energy feedback, including cosmic rays
(CR), and find that models that place SNe in dense gas and account for CR
diffusion are able to drive outflows with similar mass loading as obtained from
a random placement of SNe with no CRs. Despite this similarity, CR-driven
outflows differ in several other key properties including their overall
clumpiness and velocity. Moreover, the forces driving these outflows originate
in different sources of pressure, with the CR diffusion model relying on
non-thermal pressure gradients to create an outflow driven by internal pressure
and the random-placement model depending on kinetic pressure gradients to
propel a ballistic outflow. CRs therefore appear to be non-negligible physics
in the formation of outflows from the interstellar medium.Comment: 8 pages, 4 figures, accepted for publication in ApJL; movie of
simulated gas densities can be found here:
http://www.h-its.org/tap-images/galactic-outflows
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