157 research outputs found
Fast simulations of gas sloshing and cold front formation
We present a simplified and fast method for simulating minor mergers between
galaxy clusters. Instead of following the evolution of the dark matter halos
directly by the N-body method, we employ a rigid potential approximation for
both clusters. The simulations are run in the rest frame of the more massive
cluster and account for the resulting inertial accelerations in an optimised
way. We test the reliability of this method for studies of minor merger induced
gas sloshing by performing a one-to-one comparison between our simulations and
hydro+N-body ones. We find that the rigid potential approximation reproduces
the sloshing-related features well except for two artefacts: the temperature
just outside the cold fronts is slightly over-predicted, and the outward motion
of the cold fronts is delayed by typically 200 Myr. We discuss reasons for both
artefacts.Comment: 14 pages, 15 figures. Accepted by MNRA
Cluster Core Heating from Merging Subclusters
Though feedback from central active galactic nuclei provides an attractive
solution to the problem of overcooling in galaxy cluster cores, another
possible source of heating may come from ``sloshing'' of the cluster core gas
initiated by mergers. We present a set of simulations of galaxy cluster mergers
with subclusters in order to determine the amount of heating provided by the
mechanism of sloshing, exploring a parameter space over mass ratio, impact
parameter, and viscosity of the intracluster medium (ICM). Our results show
that for sloshing caused by mergers with gasless subclusters cooling may be
partially offset by heating from sloshing, but this mechanism is less effective
if the ICM is viscous.Comment: To appear in proceedings of "The Monster's Fiery Breath", Eds.
Sebastian Heinz & Eric Wilcots (AIP conference series). 4 pages, 3 figure
Simulating Astro-H Observations of Sloshing Gas Motions in the Cores of Galaxy Clusters
Astro-H will be the first X-ray observatory to employ a high-resolution
microcalorimeter, capable of measuring the shift and width of individual
spectral lines to the precision necessary for estimating the velocity of the
diffuse plasma in galaxy clusters. This new capability is expected to bring
significant progress in understanding the dynamics, and therefore the physics,
of the intracluster medium. However, because this plasma is optically thin,
projection effects will be an important complicating factor in interpreting
future Astro-H measurements. To study these effects in detail, we performed an
analysis of the velocity field from simulations of a galaxy cluster
experiencing gas sloshing, and generated synthetic X-ray spectra, convolved
with model Astro-H Soft X-ray Spectrometer (SXS) responses. We find that the
sloshing motions produce velocity signatures that will be observable by Astro-H
in nearby clusters: the shifting of the line centroid produced by the
fast-moving cold gas underneath the front surface, and line broadening produced
by the smooth variation of this motion along the line of sight. The line shapes
arising from inviscid or strongly viscous simulations are very similar,
indicating that placing constraints on the gas viscosity from these
measurements will be difficult. Our spectroscopic analysis demonstrates that,
for adequate exposures, Astro-H will be able to recover the first two moments
of the velocity distribution of these motions accurately, and in some cases
multiple velocity components may be discerned. The simulations also confirm the
importance of accurate treatment of PSF scattering in the interpretation of
Astro-H/SXS spectra of cluster plasmas.Comment: 27 pages, 20 figures, submitted to the Astrophysical Journa
Sloshing of the Magnetized Cool Gas in the Cores of Galaxy Clusters
X-ray observations of many clusters of galaxies reveal the presence of edges in surface brightness and temperature, known as "cold fronts". In relaxed clusters with cool cores, these edges have been interpreted as evidence for the "sloshing" of the core gas in the cluster's gravitational potential. The smoothness of these edges has been interpreted as evidence for the stabilizing effect of magnetic fields "draped" around the front surfaces. To check this hypothesis, we perform high-resolution magnetohydrodynamics simulations of magnetized gas sloshing in galaxy clusters initiated by encounters with subclusters. We go beyond previous works on the simulation of cold fronts in a magnetized intracluster medium by simulating their formation in realistic, idealized mergers with high resolution ((Delta)x approx. 2 kpc). Our simulations sample a parameter space of plausible initial magnetic field strengths and field configurations. In the simulations, we observe strong velocity shears associated with the cold fronts amplifying the magnetic field along the cold front surfaces, increasing the magnetic field strength in these layers by up to an order of magnitude, and boosting the magnetic pressure up to near-equipartition with thermal pressure in some cases. In these layers, the magnetic field becomes strong enough to stabilize the cold fronts against Kelvin-Helmholtz instabilities, resulting in sharp, smooth fronts as those seen in observations of real clusters. These magnetic fields also result in strong suppression of mixing of high and low-entropy gas in the cluster, seen in our simulations of mergers in the absence of a magnetic field. As a result, the heating of the core due to sloshing is very modest and is unable to stave off a cooling catastrophe
Suppression of local heat flux in a turbulent magnetized intracluster medium
X-ray observations of hot gas in galaxy clusters often show steeper
temperature gradients across cold fronts -- contact discontinuities, driven by
the differential gas motions. These sharp (a few kpc wide) surface
brightness/temperature discontinuities would be quickly smeared out by the
electron thermal conduction in unmagnetized plasma, suggesting significant
suppression of the heat flow across the discontinuities. In fact, the character
of the gas flow near cold fronts is favorable for suppression of conduction by
aligning magnetic field lines along the discontinuities. We argue that a
similar mechanism is operating in the bulk of the gas. Generic 3D random
isotropic and incompressible motions increase the temperature gradients (in
some places) and at the same time suppress the local conduction by aligning the
magnetic field lines perpendicular to the local temperature gradient. We show
that the suppression of the effective conductivity in the bulk of the gas can
be linked to the increase of the frozen magnetic field energy density. On
average the rate of decay of the temperature fluctuations decreases as .Comment: 13 pages, 10 figures, published in MNRA
Turbulence and Radio Mini-halos in the Sloshing Cores of Galaxy Clusters
A number of relaxed, cool-core galaxy clusters exhibit diffuse,
steep-spectrum radio sources in their central regions, known as radio
mini-halos. It has been proposed that the relativistic electrons responsible
for the emission have been reaccelerated by turbulence generated by the
sloshing of the cool core gas. We present a high-resolution MHD simulation of
gas sloshing in a galaxy cluster coupled with subgrid simulations of
relativistic electron acceleration to test this hypothesis. Our simulation
shows that the sloshing motions generate turbulence on the order of 50-200 km s on spatial scales of 50-100 kpc and below in the
cool core region within the envelope of the sloshing cold fronts, whereas
outside the cold fronts, there is negligible turbulence. This turbulence is
potentially strong enough to reaccelerate relativistic electron seeds (with
initial ) to via damping of
magnetosonic waves and non-resonant compression. The seed electrons could
remain in the cluster from, e.g., past AGN activity. In combination with the
magnetic field amplification in the core, these electrons then produce diffuse
radio synchrotron emission that is coincident with the region bounded by the
sloshing cold fronts, as indeed observed in X-rays and the radio. The result
holds for different initial spatial distributions of preexisting relativistic
electrons. The power and the steep spectral index () of the
resulting radio emission are consistent with observations of minihalos, though
the theoretical uncertainties of the acceleration mechanisms are high. We also
produce simulated maps of inverse-Compton hard X-ray emission from the same
population of relativistic electrons.Comment: 28 pages, 29 figures, in emulateapj format. Revised version accepted
by the referee, conclusions unchange
- …