450 research outputs found
Constrained-Transport Magnetohydrodynamics with Adaptive-Mesh-Refinement in CHARM
We present the implementation of a three-dimensional, second order accurate
Godunov-type algorithm for magneto-hydrodynamic (MHD), in the
adaptive-mesh-refinement (AMR) cosmological code {\tt CHARM}. The algorithm is
based on the full 12-solve spatially unsplit Corner-Transport-Upwind (CTU)
scheme. The fluid quantities are cell-centered and are updated using the
Piecewise-Parabolic-Method (PPM), while the magnetic field variables are
face-centered and are evolved through application of the Stokes theorem on cell
edges via a Constrained-Transport (CT) method. The multidimensional MHD source
terms required in the predictor step for high-order accuracy are applied in a
simplified form which reduces their complexity in three dimensions without loss
of accuracy or robustness. The algorithm is implemented on an AMR framework
which requires specific synchronization steps across refinement levels. These
include face-centered restriction and prolongation operations and a {\it
reflux-curl} operation, which maintains a solenoidal magnetic field across
refinement boundaries. The code is tested against a large suite of test
problems, including convergence tests in smooth flows, shock-tube tests,
classical two- and three-dimensional MHD tests, a three-dimensional shock-cloud
interaction problem and the formation of a cluster of galaxies in a fully
cosmological context. The magnetic field divergence is shown to remain
negligible throughout.Comment: 53 pages, 17 figs, under review by ApJ
Enabling Radiative Transfer on AMR grids in CRASH
We introduce CRASH-AMR, a new version of the cosmological Radiative Transfer
(RT) code CRASH, enabled to use refined grids. This new feature allows us to
attain higher resolution in our RT simulations and thus to describe more
accurately ionisation and temperature patterns in high density regions. We have
tested CRASH-AMR by simulating the evolution of an ionised region produced by a
single source embedded in gas at constant density, as well as by a more
realistic configuration of multiple sources in an inhomogeneous density field.
While we find an excellent agreement with the previous version of CRASH when
the AMR feature is disabled, showing that no numerical artifact has been
introduced in CRASH-AMR, when additional refinement levels are used the code
can simulate more accurately the physics of ionised gas in high density
regions. This result has been attained at no computational loss, as RT
simulations on AMR grids with maximum resolution equivalent to that of a
uniform cartesian grid can be run with a gain of up to 60% in computational
time.Comment: 19 pages, 17 figures. MNRAS, in pres
Evolution of shocks and turbulence in major cluster mergers
We performed a set of cosmological simulations of major mergers in galaxy
clusters to study the evolution of merger shocks and the subsequent injection
of turbulence in the post-shock region and in the intra-cluster medium (ICM).
The computations were done with the grid-based, adaptive mesh refinement hydro
code Enzo, using an especially designed refinement criteria for refining
turbulent flows in the vicinity of shocks. A substantial amount of turbulence
energy is injected in the ICM due to major merger. Our simulations show that
the shock launched after a major merger develops an ellipsoidal shape and gets
broken by the interaction with the filamentary cosmic web around the merging
cluster. The size of the post-shock region along the direction of shock
propagation is about 300 kpc h^-1, and the turbulent velocity dispersion in
this region is larger than 100 km s^-1. Scaling analysis of the turbulence
energy with the cluster mass within our cluster sample is consistent with
M^(5/3), i.e. the scaling law for the thermal energy in the self-similar
cluster model. This clearly indicates the close relation between virialization
and injection of turbulence in the cluster evolution. We found that the ratio
of the turbulent to total pressure in the cluster core within 2 Gyr after the
major merger is larger than 10%, and it takes about 4 Gyr to get relaxed, which
is substantially longer than typically assumed in the turbulent re-acceleration
models, invoked to explain the statistics of observed radio halos. Striking
similarities in the morphology and other physical parameters between our
simulations and the "symmetrical radio relics" found at the periphery of the
merging cluster A3376 are finally discussed. In particular, the interaction
between the merger shock and the filaments surrounding the cluster could
explain the presence of "notch-like" features at the edges of the double
relics.Comment: 16 pages, 19 figures, Published in Astrophysical Journal (online) and
printed version will be published on 1st January, 201
Probing Turbulence in the Coma Galaxy Cluster
Spatially-resolved gas pressure maps of the Coma galaxy cluster are obtained
from a mosaic of XMM-Newton observations in the scale range between a
resolution of 20 kpc and an extent of 2.8 Mpc. A Fourier analysis of the data
reveals the presence of a scale-invariant pressure fluctuation spectrum in the
range between 40 and 90 kpc and is found to be well described by a projected
Kolmogorov/Oboukhov-type turbulence spectrum. Deprojection and integration of
the spectrum yields the lower limit of percent of the total
intracluster medium pressure in turbulent form. The results also provide
observational constraints on the viscosity of the gas.Comment: 12 pages, 13 figures (low resolution), version accepted by Astron.
Astrophy
A Puzzling Merger in A3266: the Hydrodynamic Picture from XMM-Newton
Using the mosaic of nine XMM-Newton observations, we study the hydrodynamic
state of the merging cluster of galaxies Abell 3266. The high quality of the
spectroscopic data and large field of view of XMM-Netwon allow us to determine
the thermodynamic conditions of the intracluster medium on scales of order of
50 kpc. A high quality entropy map reveals the presence of an extended region
of low entropy gas, running from the primary cluster core toward the northeast
along the nominal merger axis. The mass of the low entropy gas amounts to
approximately 2e13 solar masses, which is comparable to the baryonic mass of
the core of a rich cluster. We test the possibility that the origin of the
observed low entropy gas is either related to the disruption a preexisting
cooling core in Abell 3266 or to the stripping of gas from an infalling
subcluster companion. We find that both the radial pressure and entropy
profiles as well as the iron abundance of Abell 3266 do not resemble those in
other known cooling core clusters (Abell 478). Thus we conclude that the low
entropy region is subcluster gas in the process of being stripped off from its
dark matter halo. In this scenario the subcluster would be falling onto the
core of A3266 from the foreground. This would also help interpret the observed
high velocity dispersion of the galaxies in the cluster center, provided that
the mass of the subcluster is at most a tenth of the mass of the main cluster.Comment: 6 pages, ApJ sub
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