1,910 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
The role of the Rayleigh-Taylor instability in ram pressure stripped disk galaxies
Ram pressure stripping, i.e. the removal of a galaxy's gas disk due to its
motion through the intracluster medium of a galaxy cluster, appears to be a
common phenomenon. Not every galaxy, however, is completely stripped of its gas
disk. If the ram pressure is insufficiently strong, only the outer parts of the
gas disk are removed, and the inner gas disk is retained by the galaxy. One
example of such a case is the Virgo spiral NGC 4402. Observations of NGC 4402
(Crowl et al. 2005) reveal structures at the leading edge of the gas disk,
which resemble the characteristic finger-like structures produced by the
Rayleigh-Taylor (RT) instability. We argue, however, that the RT instability is
unlikely to be responsible for these structures. We demonstrate that the
conditions under which a galaxy's disk gas experiences ram pressure stripping
are identical to those that lead to RT instability. If the galaxy's gravity
prevents ram pressure stripping of the inner disk, it also prevents the RT
instability. In contrast, the stripped gas could still be subject to RT
instability, and we discuss consequences for the stripped gas.Comment: accepted by A&
Kelvin-Helmholtz instabilities at the sloshing cold fronts in the Virgo cluster as a measure for the effective ICM viscosity
Sloshing cold fronts (CFs) arise from minor merger triggered gas sloshing.
Their detailed structure depends on the properties of the intra-cluster medium
(ICM): hydrodynamical simulations predict the CFs to be distorted by
Kelvin-Helmholtz instabilities (KHIs), but aligned magnetic fields, viscosity,
or thermal conduction can suppress the KHIs. Thus, observing the detailed
structure of sloshing CFs can be used to constrain these ICM properties. Both
smooth and distorted sloshing CFs have been observed, indicating that the KHI
is suppressed in some clusters, but not in all. Consequently, we need to
address at least some sloshing clusters individually before drawing general
conclusions about the ICM properties. We present the first detailed attempt to
constrain the ICM properties in a specific cluster from the structure of its
sloshing CF. Proximity and brightness make the Virgo cluster an ideal target.
We combine observations and Virgo-specific hydrodynamical sloshing simulations.
Here we focus on a Spitzer-like temperature dependent viscosity as a mechanism
to suppress the KHI, but discuss the alternative mechanisms in detail. We
identify the CF at 90 kpc north and north-east of the Virgo center as the best
location in the cluster to observe a possible KHI suppression. For viscosities
10% of the Spitzer value KHIs at this CF are suppressed. We describe
in detail the observable signatures at low and high viscosities, i.e. in the
presence or absence of KHIs. We find indications for a low ICM viscosity in
archival XMM-Newton data and demonstrate the detectability of the predicted
features in deep Chandra observations.Comment: Accepted for ApJ; 15 pages, 11 figures. A movie can be found here:
http://www.hs.uni-hamburg.de/DE/Ins/Per/Roediger/research.html#Virgo-viscou
Shock heating by FR I radio sources in galaxy clusters
Feedback by active galactic nuclei (AGN) is frequently invoked to explain the
cut-off of the galaxy luminosity function at the bright end and the absence of
cooling flows in galaxy clusters. Meanwhile, there are recent observations of
shock fronts around radio-loud AGN. Using realistic 3D simulations of jets in a
galaxy cluster, we address the question what fraction of the energy of active
galactic nuclei is dissipated in shocks. We find that weak shocks that
encompass the AGN have Mach numbers of 1.1-1.2 and dissipate at least 2% of the
mechanical luminosity of the AGN. In a realistic cluster medium, even a
continuous jet can lead to multiple shock structures, which may lead to an
overestimate of the AGN duty cycles inferred from the spatial distribution of
waves.Comment: accepted by MNRAS Letter
Ram pressure stripping of disk galaxies
(Abridged) We perform high resolution 2D hydrodynamical simulations of
face-on ram pressure stripping (RPS) of disk galaxies to compile a
comprehensive parameter study varying galaxy properties (mass, vertical
structure of the gas disk) and covering a large range of ICM conditions,
reaching from high density environments like in cluster centres to low density
environments typical for cluster outskirts or groups. We find that the ICM-ISM
interaction proceeds in three phases: firstly the instantaneous stripping
phase, secondly the dynamic intermediate phase, thirdly the quasi-stable
continuous viscous stripping phase. The stripping efficiency depends slightly
on the Mach number of the flow, however, the main parameter is the ram
pressure. The stripping efficiency does not depend on the vertical structure
and thickness of the gas disk. We discuss uncertainties in the classic estimate
of the stripping radius of \citet{gunn72}, and adapt the estimate used by
\cite{mori00} for spherical galaxies, (comparison of central pressure with ram
pressure). We find that the latter estimate predicts the radius and mass of the
gas disk remaining at the end of the second phase very well, and better than
the \citet{gunn72} criterion. From our simulations we conclude that gas disks
of galaxies in high density environments are heavily truncated or even
completely stripped, but also the gas disks of galaxies in low density
environments are disturbed by the flow and back-falling material, so that they
should also be pre-processed.Comment: 25 pages, 32 figures, accepted for publication in A&A, high
resolution version at
http://www.astrophysik.uni-kiel.de/~eroediger/publications.htm
Star formation in shocked cluster spirals and their tails
Recent observations of ram pressure stripped spiral galaxies in clusters
revealed details of the stripping process, i.e., the truncation of all
interstellar medium (ISM) phases and of star formation (SF) in the disk, and
multiphase star-forming tails. Some stripped galaxies, in particular in merging
clusters, develop spectacular star-forming tails, giving them a jellyfish-like
appearance. In merging clusters, merger shocks in the intra-cluster medium
(ICM) are thought to have overrun these galaxies, enhancing the ambient ICM
pressure and thus triggering SF, gas stripping and tail formation. We present
idealised hydrodynamical simulations of this scenario, including standard
descriptions for SF and stellar feedback. To aid the interpretation of recent
and upcoming observations, we focus on particular structures and dynamics in SF
patterns in the remaining gas disk and in the near tails, which are easiest to
observe. The observed jellyfish morphology is qualitatively reproduced for,
both, face-on and edge-on stripping. In edge-on stripping, the interplay
between the ICM wind and the disk rotation leads to asymmetries along the ICM
wind direction and perpendicular to it. The apparent tail is still part of a
highly deformed gaseous and young stellar disk. In both geometries, SF takes
place in knots throughout the tail, such that the stars in the tails show no
ordered age gradients. Significant SF enhancement in the disk occurs only at
radii where the gas will be stripped in due course.Comment: 6 pages, submitted to MNRAS Letter
Viscous Kelvin-Helmholtz instabilities in highly ionised plasmas
Transport coefficients in highly ionised plasmas like the intra-cluster
medium (ICM) are still ill-constrained. They influence various processes, among
them the mixing at shear flow interfaces due to the Kelvin-Helmholtz
instability (KHI). The observed structure of potential mixing layers can be
used to infer the transport coefficients, but the data interpretation requires
a detailed knowledge of the long-term evolution of the KHI under different
conditions. Here we present the first systematic numerical study of the effect
of constant and temperature-dependent isotropic viscosity over the full range
of possible values. We show that moderate viscosities slow down the growth of
the KHI and reduce the height of the KHI rolls and their rolling-up.
Viscosities above a critical value suppress the KHI. The effect can be
quantified in terms of the Reynolds number Re = U{\lambda}/{\nu}, where U is
the shear velocity, {\lambda} the perturbation length, and {\nu} the kinematic
viscosity. We derive the critical Re for constant and temperature dependent,
Spitzer-like viscosities, an empirical relation for the viscous KHI growth time
as a function of Re and density contrast, and describe special behaviours for
Spitzer-like viscosities and high density contrasts. Finally, we briefly
discuss several astrophysical situations where the viscous KHI could play a
role, i.e., sloshing cold fronts, gas stripping from galaxies, buoyant
cavities, ICM turbulence, and high velocity clouds.Comment: Accepted by MNRAS. 22 pages, 21 figure
Stripped elliptical galaxies as probes of ICM physics: I. Tails, wakes, and flow patterns in and around stripped ellipticals
Elliptical cluster galaxies are progressively stripped of their atmospheres
due to their motion through the intra-cluster medium (ICM). Deep X-ray
observations reveal the fine-structure of the galaxy's remnant atmosphere and
its gas tail and wake. This fine-structure depends on dynamic conditions
(galaxy potential, initial gas contents, orbit through the host cluster),
orbital stage (early infall, pre-/post-pericenter passage), and ICM plasma
properties (thermal conductivity, viscosity, magnetic field structure). We aim
to disentangle dynamic and plasma effects in order to use stripped ellipticals
as probes of ICM plasma properties. This first paper of a series investigates
the hydrodynamics of progressive gas stripping by means of inviscid
hydrodynamical simulations. We distinguish a long-lasting initial relaxation
phase and a quasi-steady stripping phase. During quasi-steady stripping, the
ICM flow around the remnant atmosphere resembles the flow around solid bodies,
including a `deadwater' region in the near wake. Gas is stripped from the
remnant atmosphere predominantly at its sides via Kelvin-Helmholtz
instabilities. The downstream atmosphere is largely shielded from the ICM wind
and thus shaped into a tail. Observationally, both, this `remnant tail' and the
stripped gas in the wake can appear as a `tail', but only in the wake can
galactic gas mix with the ambient ICM. While the qualitative results are
generic, the simulations presented here are tailored to the Virgo elliptical
galaxy M89 (NGC 4552) for the most direct comparison to observations. Papers II
and III of this series describe the effect of viscosity and compare to Chandra
and XMM-Newton observations, respectively.Comment: ApJ, in press. 19 pages, 13 figures. Clarifications added, text
restructured. Conclusions unchange
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