Shocks and cold fronts in galaxy clusters

By M. Markevitch, A. Vikhlinin. 2007

563 citations recorded by the source corpus.

Abstract The currently operating X-ray imaging observatories provide us with an exquisitely detailed view of the Megaparsec-scale plasma atmospheres in nearby galaxy clusters. At z 0.05 , the Chandra's 1 ″ angular resolution corresponds to linear resolution of less than a kiloparsec, which is smaller than some interesting linear scales in the intracluster plasma. This enables us to study the previously unseen hydrodynamic phenomena in clusters: classic bow shocks driven by the infalling subclusters, and the unanticipated “cold fronts,” or sharp contact discontinuities between regions of gas with different entropies. The ubiquitous cold fronts are found in mergers as well as around the central density peaks in “relaxed” clusters. They are caused by motion of cool, dense gas clouds in the ambient higher-entropy gas. These clouds are either remnants of the infalling subclusters, or the displaced gas from the cluster's own cool cores. Both shock fronts and cold fronts provide novel tools to study the intracluster plasma on microscopic and cluster-wide scales, where the dark matter gravity, thermal pressure, magnetic fields, and ultrarelativistic particles are at play. In particular, these discontinuities provide the only way to measure the gas bulk velocities in the plane of the sky. The observed temperature jumps at cold fronts require that thermal conduction across the fronts is strongly suppressed. Furthermore, the width of the density jump in the best-studied cold front is smaller than the Coulomb mean free path for the plasma particles. These findings show that transport processes in the intracluster plasma can easily be suppressed. Cold fronts also appear less prone to hydrodynamic instabilities than expected, hinting at the formation of a parallel magnetic field layer via magnetic draping. This may make it difficult to mix different gas phases during a merger. A sharp electron temperature jump across the best-studied shock front has shown that the electron–proton equilibration timescale is much shorter than the collisional timescale; a faster mechanism has to be present. To our knowledge, this test is the first of its kind for any astrophysical plasma. We attempt a systematic review of these and other results obtained so far (experimental and numerical), and mention some avenues for further studies.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/376633335a005cb8fb5b7ab502a22e46dd891495

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Citations in the corpus

On the Structure of Relativistic Collisionless Shocks in Electron-Ion Plasmas (2007, background): “Recently, the interest in understanding the electron-ion temperature ratio in collisionless shocks has increased due to the new observations of Te/Ti in supernova remnant shocks (Rakowski 2006) and in cluster shocks (Markevitch & Vikhlinin 2007).”
The colours of satellite galaxies in groups and clusters (2008, background): “…disks) could also be relevant (Cowie & Songaila 1977), but observational studies of bubbles and cold fronts in X-ray groups and clusters have placed strong constraints on the efficiency of conduction, concluding that it is strongly suppressed (Markevitch & Vikhlinin 2007; McNamara & Nulsen 2007).”
Draping of Cluster Magnetic Fields over Bullets and Bubbles—Morphology and Dynamic Effects (2007, background): “…of very sharp ‘cold fronts’ in galaxy clusters raise unanswered questions in the hydrodynamics of galaxy clusters (see for instance the review of Markevitch and Vikhlinin 2007), for such abrupt transitions are not expected to be stable against either hydrodynamical motions or diffusion for…”
The MCXC: a meta-catalogue of x-ray detected clusters of galaxies (2010, background): “…in-depth study of the hierarchical assembly process Send offprint requests to: R. Piffaretti, e-mail: rocco.piffaretti@cea.fr through merging (e.g., Markevitch & Vikhlinin 2007) and the physical mechanisms associated with feedback and its impact on structure formation (e.g., McNamara & Nulsen…”
A characteristic scale for cold gas (2016, background): “Swept up magnetic fields draped across a cloudlet enable a contact discontinuity to be maintained, as observed in galaxy cluster “cold fronts” (Markevitch & Vikhlinin, 2007).”
The speed of the ‘bullet’ in the merging galaxy cluster 1E0657−56 (2007, background): “The temperature maps show the strong heating of the gas by the shock front, while the bullet itself is seen to be quite cold, revealing that the wedge-like edge of the bullet is really a cold front (see Markevitch & Vikhlinin 2007, for a review).”
Diffuse Radio Emission from Galaxy Clusters (2019, background): “This energy is dissipated through lowMach number shocks and turbulence, heating the ICM (e.g., Markevitch & Vikhlinin 2007).”
Shock waves in Eulerian cosmological simulations: main properties and acceleration of cosmic rays (2008, unknown): “Journal compilation C© 2009 RAS Downloaded from https://academic.oup.com/mnras/article-abstract/395/3/1333/997891 by guest on 29 July 2018 Sarazin & Vikhlinin 1999; Markevitch et al. 2002; Belsole et al. 2004; Markevitch & Vikhlinin 2007).”
Cosmological Parameters from Observations of Galaxy Clusters (2011, background): “In the cores of even the most dynamically relaxed clusters, small-scale departures from hydrostatic equilibrium are often apparent (Fabian et al. 2003; Markevitch & Vikhlinin 2007; Allen et al. 2008).”
ASTROPHYSICAL GYROKINETICS: KINETIC AND FLUID TURBULENT CASCADES IN MAGNETIZED WEAKLY COLLISIONAL PLASMAS (2007, background): “Both isothermal (Fabian et al. 2006; Sanders & Fabian 2006) and non-isothermal (Markevitch & Vikhlinin 2007) coherent structures that appear to be shocks are observed.”
Total mass biases in X-ray galaxy clusters (2008, background): “The validity of the numerical approach is supported by X-ray observations, which show the existence of complex thermal structures and of merging activity (see Markevitch & Vikhlinin 2007, for a review).”
Cosmology and astrophysics from relaxed galaxy clusters - II. Cosmological constraints (2014, background): “…range where the signal to noise ratio is generally adequate, but which deliberately excludes the innermost regions, where complex structure (e.g. associated with sloshing or AGN-induced cavities) is ubiquitous, even in the most relaxed clusters (McNamara & Nulsen 2007; Markevitch & Vikhlinin 2007).”
Creation of cosmic structure in the complex galaxy cluster merger Abell 2744 (2011, background): “One possible explanation was suggested by Owers et al. (2011), who describe it as a rampressure slingshot (e.g. Markevitch & Vikhlinin 2007).”
RESIDUAL GAS MOTIONS IN THE INTRACLUSTER MEDIUM AND BIAS IN HYDROSTATIC MEASUREMENTS OF MASS PROFILES OF CLUSTERS (2009, background): “Observationally, little is known about gas motions in clusters except that in many systems there are indirect indications of bulk gas motions associated with mergers and motions of the central cluster galaxies (see Markevitch & Vikhlinin 2007, for a review).”
X-ray–optical classification of cluster mergers and the evolution of the cluster merger fraction (2011, background): “…makes head-on massive mergers particularly suitable also for quantitative studies of the collision of the gaseous cluster components which leads to shock fronts, collisional heating, and the gradual destruction of cool cores (Markevitch & Vikhlinin 2007; Million et al. 2010; Ehlert et al. 2011).”
STIRRING UP THE POT: CAN COOLING FLOWS IN GALAXY CLUSTERS BE QUENCHED BY GAS SLOSHING? (2009, background): “…(“cold fronts”), concentric with respect to the brightness peak of the cluster (e.g., Mazzotta et al. 2001; Markevitch et al. 2001, 2003b; Churazov et al. 2003; Ascasibar & Markevitch 2006, hereafter AM06; for a review of observations and simulations see Markevitch & Vikhlinin 2007).”