36 research outputs found
Turbulence decay in the density-stratified intracluster medium
Turbulence evolution in a density-stratified medium differs from that of
homogeneous isotropic turbulence described by the Kolmogorov picture. We
evaluate the degree of this effect in the intracluster medium (ICM) with
hydrodynamical simulations. We find that the buoyancy effect induced by ICM
density stratification introduces qualitative changes to the turbulence energy
evolution, morphology, and the density fluctuation - turbulence Mach number
relation, and likely explains the radial dependence of the ICM turbulence
amplitude as found previously in cosmological simulations. A new channel of
energy flow between the kinetic and the potential energy is opened up by
buoyancy. When the gravitational potential is kept constant with time, this
energy flow leaves oscillations to the energy evolution, and leads to a
balanced state of the two energies where both asymptote to power-law time
evolution with slopes shallower than that for the turbulence kinetic energy of
homogeneous isotropic turbulence. We discuss that the energy evolution can
differ more significantly from that of homogeneous isotropic turbulence when
there is a time variation of the gravitational potential. Morphologically, ICM
turbulence can show a layered vertical structure and large horizontal vortical
eddies in the central regions with the greatest density stratification. In
addition, we find that the coefficient in the linear density fluctuation -
turbulence Mach number relation caused by density stratification is in general
a variable with position and time.Comment: 10 pages, 9 figures, published in MNRA
Offsets between the X-ray and the Sunyaev-Zel'dovich-effect peaks in merging galaxy clusters and their cosmological implications
Observations reveal that the peaks of the X-ray map and the
Sunyaev-Zel'dovich (SZ) effect map of some galaxy clusters are offset from each
other. In this paper, we perform a set of hydrodynamical simulations of mergers
of two galaxy clusters to investigate the spatial offset between the maxima of
the X-ray and the SZ surface brightness of the merging clusters. We find that
significantly large SZ-X-ray offsets (>100kpc) can be produced during the major
mergers of galaxy clusters. The significantly large offsets are mainly caused
by a `jump effect' occurred between the primary and secondary pericentric
passages of the two merging clusters, during which the X-ray peak may jump to
the densest gas region located near the center of the small cluster, but the SZ
peak remains near the center of the large one. Our simulations show that
merging systems with higher masses and larger initial relative velocities may
result in larger offset sizes and longer offset time durations; and only nearly
head-on mergers are likely to produce significantly large offsets. We further
investigate the statistical distribution of the SZ-X-ray offset sizes and find
that (1) the number distribution of the offset sizes is bimodal with one peak
located at low offsets ~0 and the other at large offsets ~350-450kpc/h, but the
objects with intermediate offsets are scarce; and (2) the probabilities of the
clusters in the mass range higher than 2x10^{14}Msun/h that have offsets larger
than 20, 50, 200, 300, and 500kpc/h are 34.0%, 11.1%, 8.0%, 6.5%, and 2.0%
respectively at z=0.7. The probability is sensitive to the underlying pairwise
velocity distribution and the merger rate of clusters. Future observations on
the offsets for a large number of clusters may put strong constraints on the
cosmic velocity fields on the cluster scale and the cluster merger rate.
(Abridged)Comment: 25 pages, 15 figure
Runaway Merger Shocks in Galaxy Cluster Outskirts and Radio Relics
Moderately strong shocks arise naturally when two subclusters merge. For
instance, when a smaller subcluster falls into the gravitational potential of a
more massive cluster, a bow shock is formed and moves together with the
subcluster. After pericenter passage, however, the subcluster is decelerated by
the gravity of the main cluster, while the shock continues moving away from the
cluster center. These shocks are considered as promising candidates for
powering radio relics found in many clusters. The aim of this paper is to
explore the fate of such shocks when they travel to the cluster outskirts, far
from the place where the shocks were initiated. In a uniform medium, such a
"runaway" shock should weaken with distance. However, as shocks move to large
radii in galaxy clusters, the shock is moving down a steep density gradient
that helps the shock to maintain its strength over a large distance.
Observations and numerical simulations show that, beyond , gas density
profiles are as steep as, or steeper than, , suggesting that there
exists a "Habitable zone" for moderately strong shocks in cluster outskirts
where the shock strength can be maintained or even amplified. A characteristic
feature of runaway shocks is that the strong compression, relative to the
initial state, is confined to a narrow region just behind the shock. Therefore,
if such a shock runs over a region with a pre-existing population of
relativistic particles, then the boost in radio emissivity, due to pure
adiabatic compression, will also be confined to a narrow radial shell.Comment: 9 pages, 8 figures; published in MNRA
Standoff Distance of Bow Shocks in Galaxy Clusters as Proxy for Mach Number
X-ray observations of merging clusters provide many examples of bow shocks
leading merging subclusters. While the Mach number of a shock can be estimated
from the observed density jump using Rankine-Hugoniot condition, it reflects
only the velocity of the shock itself and is generally not equal to the
velocity of the infalling subcluster dark matter halo or to the velocity of the
contact discontinuity separating gaseous atmospheres of the two subclusters.
Here we systematically analyze additional information that can be obtained by
measuring the standoff distance, i.e. the distance between the leading edge of
the shock and the contact discontinuity that drives this shock. The standoff
distance is influenced by a number of additional effects, e.g. (1) the
gravitational pull of the main cluster (causing acceleration/deceleration of
the infalling subcluster), (2) the density and pressure gradients of the
atmosphere in the main cluster, (3) the non-spherical shape of the subcluster,
and (4) projection effects. The first two effects tend to bias the standoff
distance in the same direction, pushing the bow shock closer to (farther away
from) the subcluster during the pre- (post-)merger stages. Particularly, in the
post-merger stage, the shock could be much farther away from the subcluster
than predicted by a model of a body moving at a constant speed in a uniform
medium. This implies that a combination of the standoff distance with
measurements of the Mach number from density/temperature jumps can provide
important information on the merger, e.g. differentiating between the pre- and
post-merger stages.Comment: 11 pages, 12 figures. Including major revision and matched to
accepted version in MNRA
Pairs of Giant Shock Waves (N-Waves) in Merging Galaxy Clusters
When a subcluster merges with a larger galaxy cluster, a bow shock is driven
ahead of the subcluster. At a later merger stage, this bow shock separates from
the subcluster, becoming a "runaway" shock that propagates down the steep
density gradient through the cluster outskirts and approximately maintains its
strength and the Mach number. Such shocks are plausible candidates for
producing radio relics in the periphery of clusters. We argue that, during the
same merger stage, a secondary shock is formed much closer to the main cluster
center. A close analog of this structure is known in the usual hydrodynamics as
N-waves, where the trailing part of the "N" is the result of the non-linear
evolution of a shock. In merging clusters, spherical geometry and
stratification could further promote its development. Both the primary and the
secondary shocks are the natural outcome of a single merger event and often
both components of the pair should be present. However, in the radio band, the
leading shock could be more prominent, while the trailing shock might
conversely be more easily seen in X-rays. The latter argument implies that for
some of the (trailing) shocks found in X-ray data, it might be difficult to
identify their "partner" leading shocks or the merging subclusters, which are
farther away from the cluster center. We argue that the Coma cluster and A2744
could be two examples in a post-merger state with such well-separated shock
pairs.Comment: 9 pages, 8 figures, submitted to MNRAS. Comments are welcom
Generation of Internal Waves by Buoyant Bubbles in Galaxy Clusters and Heating of Intracluster Medium
Buoyant bubbles of relativistic plasma in cluster cores plausibly play a key
role in conveying the energy from a supermassive black hole to the intracluster
medium (ICM) - the process known as radio-mode AGN feedback. Energy
conservation guarantees that a bubble loses most of its energy to the ICM after
crossing several pressure scale heights. However, actual processes responsible
for transferring the energy to the ICM are still being debated. One attractive
possibility is the excitation of internal waves, which are trapped in the
cluster's core and eventually dissipate. Here we show that a sufficient
condition for efficient excitation of these waves in stratified cluster
atmospheres is flattening of the bubbles in the radial direction. In our
numerical simulations, we model the bubbles phenomenologically as rigid bodies
buoyantly rising in the stratified cluster atmosphere. We find that the
terminal velocities of the flattened bubbles are small enough so that the
Froude number . The effects of stratification make the
dominant contribution to the total drag force balancing the buoyancy force. In
particular, clear signs of internal waves are seen in the simulations. These
waves propagate horizontally and downwards from the rising bubble, spreading
their energy over large volumes of the ICM. If our findings are scaled to the
conditions of the Perseus cluster, the expected terminal velocity is
near the cluster cores, which is in broad
agreement with direct measurements by the Hitomi satellite.Comment: 15 pages, 13 figures, submitted to MNRA
Evolution of Splashback Boundaries and Gaseous Outskirts: Insights from Mergers of Self-similar Galaxy Clusters
A self-similar spherical collapse model predicts a dark matter (DM)
splashback and accretion shock in the outskirts of galaxy clusters while misses
a key ingredient of structure formation - processes associated with mergers. To
fill this gap, we perform simulations of merging self-similar clusters and
investigate their DM and gas evolution in an idealized cosmological context.
Our simulations show that the cluster rapidly contracts during the major merger
and the splashback radius decreases, approaching the virial radius
. While correlates with a smooth mass accretion rate
(MAR) parameter in the self-similar model, our simulations
show a similar trend with the total MAR (includes both
mergers and ). The scatter of the relation indicates a generally low in
clusters in cosmological simulations. In contrast to the DM, the hot gaseous
atmospheres significantly expand by the merger-accelerated (MA-) shocks formed
when the runaway merger shocks overtake the outer accretion shock. After a
major merger, the MA-shock radius is larger than by a factor of up
to for and is for
. This implies that (1) mergers could easily generate
the MA-shock-splashback offset measured in cosmological simulations, and (2)
the smooth MAR is small in regions away from filaments where MA-shocks reside.
We further discuss various shocks and contact discontinuities formed at
different epochs of the merger, the ram pressure stripping in cluster
outskirts, and the dependence of member galaxies' splashback feature on their
orbital parameters.Comment: 25 pages, 24 figures, submitted to MNRAS. Comments are welcom
Encounters of Merger and Accretion Shocks in Galaxy Clusters and their Effects on Intracluster Medium
Several types/classes of shocks naturally arise during formation and
evolution of galaxy clusters. One such class is represented by accretion
shocks, associated with deceleration of infalling baryons. Such shocks,
characterized by a very high Mach number, are present even in 1D models of
cluster evolution. Another class is composed of "runaway merger shocks", which
appear when a merger shock, driven by a sufficiently massive infalling
subcluster, propagates away from the main-cluster center. We argue that, when
the merger shock overtakes the accretion shock, a new long-living shock is
formed that propagates to large distances from the main cluster (well beyond
its virial radius) affecting the cold gas around the cluster. We refer to these
structures as Merger-accelerated Accretion shocks (MA-shocks) in this paper. We
show examples of such MA-shocks in 1D and 3D simulations and discuss their
characteristic properties. In particular, (1) MA-shocks shape the boundary
separating the hot intracluster medium (ICM) from the unshocked gas, giving
this boundary a "flower-like" morphology. In 3D, MA-shocks occupy space between
the dense accreting filaments. (2) Evolution of MA-shocks highly depends on the
Mach number of the runaway merger shock and the mass accretion rate parameter
of the cluster. (3) MA-shocks may lead to the misalignment of the ICM boundary
and the splashback radius.Comment: 10 pages, 9 figures; published in MNRA