343 research outputs found
Cold fronts: probes of plasma astrophysics in galaxy clusters
The most massive baryonic component of galaxy clusters is the “intracluster medium” (ICM), a diffuse, hot, weakly magnetized plasma that is most easily observed in the X-ray band. Despite being observed for decades, the macroscopic transport properties of the ICM are still not well-constrained. A path to determine macroscopic ICM properties opened up with the discovery of “cold fronts”. These were observed as sharp discontinuities in surface brightness and temperature in the ICM, with the property that the denser side of the discontinuity is the colder one. The high spatial resolution of the Chandra X-ray Observatory revealed two puzzles about cold fronts. First, they should be subject to Kelvin-Helmholtz instabilities, yet in many cases they appear relatively smooth and undisturbed. Second, the width of the interface between the two gas phases is typically narrower than the mean free path of the particles in the plasma, indicating negligible thermal conduction. It was thus realized that these special characteristics of cold fronts may be used to probe the properties of the cluster plasma. In this review, we will discuss the recent simulations of cold fronts in galaxy clusters, focusing on those which have attempted to use these features to constrain ICM physics. In particular, we will examine the effects of magnetic fields, viscosity, and thermal conductivity on the stability properties and long-term evolution of cold fronts. We conclude with a discussion on what important questions remain unanswered, and the future role of simulations and the next generation of X-ray observatories
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
Sloshing of Galaxy Cluster Core Plasma in the Presence of Self-Interacting Dark Matter
The "sloshing" of the cold gas in the cores of relaxed clusters of galaxies
is a widespread phenomenon, evidenced by the presence of spiral-shaped "cold
fronts" in X-ray observations of these systems. In simulations, these flows of
cold gas readily form by interactions of the cluster core with small
subclusters, due to a separation of the cold gas from the dark matter (DM), due
to their markedly different collisionalities. In this work, we use numerical
simulations to investigate the effects of increasing the DM collisionality on
sloshing cold fronts in a cool-core cluster. For clusters in isolation, the
formation of a flat DM core via self-interactions results in modest adiabatic
expansion and cooling of the core gas. In merger simulations, cold fronts form
in the same manner as in previous simulations, but the flattened potential in
the core region enables the gas to expand to larger radii in the initial
stages. Upon infall, the subcluster's DM mass decreases via collisions,
reducing its influence on the core. Thus, the sloshing gas moves slower,
inhibiting the growth of fluid instabilities relative to simulations where the
DM cross section is zero. This also inhibits turbulent mixing and the increase
in entropy that would otherwise result. For values of the cross section
, subclusters do not survive as self-gravitating structures for
more than two core passages. Additionally, separations between the peaks in the
X-ray emissivity and thermal Sunyaev-Zeldovich effect signals during sloshing
may place constraints on DM self-interactions.Comment: 20 pages, 14 figures, submitted to Ap
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
Mapping the Gas Turbulence in the Coma Cluster: Predictions for Astro-H
Astro-H will be able for the first time to map gas velocities and detect
turbulence in galaxy clusters. One of the best targets for turbulence studies
is the Coma cluster, due to its proximity, absence of a cool core, and lack of
a central active galactic nucleus. To determine what constraints Astro-H will
be able to place on the Coma velocity field, we construct simulated maps of the
projected gas velocity and compute the second-order structure function, an
analog of the velocity power spectrum. We vary the injection scale, dissipation
scale, slope, and normalization of the turbulent power spectrum, and apply
measurement errors and finite sampling to the velocity field. We find that even
with sparse coverage of the cluster, Astro-H will be able to measure the Mach
number and the injection scale of the turbulent power spectrum--the quantities
determining the energy flux down the turbulent cascade and the diffusion rate
for everything that is advected by the gas (metals, cosmic rays, etc.). Astro-H
will not be sensitive to the dissipation scale or the slope of the power
spectrum in its inertial range, unless they are outside physically motivated
intervals. We give the expected confidence intervals for the injection scale
and the normalization of the power spectrum for a number of possible pointing
configurations, combining the structure function and velocity dispersion data.
Importantly, we also determine that measurement errors on the line shift will
bias the velocity structure function upward, and show how to correct this bias.Comment: 18 pages, 13 figures. Matches published ApJ version, except that it
fixes an error in the left panel of Figure 5 that is being addressed in an
ApJ erratu
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
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