Merging Processes in Galaxy Clusters

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Merger shocks in galaxy clusters A and A and their relation to radio halos - Markevitch, Maxim et al. Shocks and cold fronts in galaxy clusters - Markevitch, Maxim et al. Markevitch M. A Textbook example of a bow shock in the merging galaxy cluster 1E - Markevitch, M. Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E - Markevitch, Maxim et al. Gasdynamics and starbursts in major mergers - Mihos, J Christopher et al. The Integrated Sunyaev-Zeldovich effect as the superior method for measuring the mass of clusters of galaxies - Motl, Patrick M.

The Structure of cold dark matter halos - Navarro, Julio F. Formation and evolution of galactic halos in clusters of galaxies - Okamoto, T. On the non thermal emission and acceleration of electrons in coma and other clusters of galaxies - Petrosian, Vahe Astrophys. Pinkney J. Pisani A.. Planck results. Randall S.

Off-axis cluster mergers: Effects of a strongly peaked dark matter profile - Ricker, Paul M. Rossetti M.

Hierarchical Merging

Ghizzardi S.. Russell H. Chandra observation of two shock fronts in the merging galaxy cluster Abell - Russell, H. A merger mystery: no extended radio emission in the merging cluster Abell - Russell, H. Sarazin, C.. Merging processes in galaxy clusters - Feretti, I. Giovannini eds. The Speed of the bullet in the merging galaxy cluster 1E - Springel, Volker et al. Stuble M. This is what we see in today's elliptical galaxies, very little molecular gas and very few young stars. It is thought that this is because elliptical galaxies are the end products of major mergers which use up the majority of gas during the merger, and thus further star formation after the merger is quenched.

Galaxy mergers can be simulated in computers, to learn more about galaxy formation. Galaxy pairs initially of any morphological type can be followed, taking into account all gravitational forces , and also the hydrodynamics and dissipation of the interstellar gas, the star formation out of the gas, and the energy and mass released back in the interstellar medium by supernovae.

The team also analyzed different orbits for the galaxies, possible collision impacts, and how galaxies were oriented to each other. In all, the group came up with 57 different merger scenarios and studied the mergers from 10 different viewing angles. It may form one of the largest galaxies in the Universe.

Galaxy mergers can be classified into distinct groups due to the properties of the merging galaxies , such as their number, their comparative size and their gas richness. One study found that large galaxies merged with each other on average once over the past 9 billion years. Small galaxies were coalescing with large galaxies more frequently.

The merging of these galaxies would classify as major as they have similar sizes. The result would therefore be an elliptical galaxy.

In the standard cosmological model, any single galaxy is expected to have formed from a few or many successive mergers of dark matter haloes , in which gas cools and forms stars at the centres of the haloes, becoming the optically visible objects historically identified as galaxies during the twentieth century. Modelling the mathematical graph of the mergers of these dark matter haloes and in turn the corresponding star formation was initially treated either by analysing purely gravitational N -body simulations. In a observational cosmology conference in Milan , [14] Roukema, Quinn and Peterson showed the first merger history trees of dark matter haloes extracted from cosmological N -body simulations.

These merger history trees were combined with formulae for star formation rates and evolutionary population synthesis, yielding synthetic luminosity functions of galaxies statistics of how many galaxies are intrinsically bright or faint at different cosmological epochs.

Roukema's group chose to define this relation by requiring the halo at the later time step to contain strictly more than 50 percent of the particles in the halo at the earlier time step; this guaranteed that between two time steps, any halo could have at most a single descendant. Independently, Lacey and Cole showed at the same conference [18] how they used the Press—Schechter formalism combined with dynamical friction to statistically generate Monte Carlo realisations of dark matter halo merger history trees and the corresponding formation of the stellar cores galaxies of the haloes.

Some of the galaxies that are in the process of merging or are believed to have formed by merging are:. From Wikipedia, the free encyclopedia. Play media. Merging galaxies. NGC — late stage merging of two galaxies. Galaxy twistings — possible merger. Markarian — possible merger. Ancient galaxy mega-merger artist concept. Retrieved 16 April Retrieved 15 September Two types of cold fronts have been identified. One is interpreted as a result of sloshing of the ICG in the main cluster with a cool core due to its displacement from equilibrium in its DM potential well due to minor mergers.

In this case the gas on the two sides of the cold font belong to the same cluster, and their motion is tangential Keshet et al. The other type of cold front separates the two regions of ICG of the main and infalling subcluster following the bow shock in the main cluster due to major mergers. X-ray observations suggest that the width of cold fronts may be significantly smaller than the Coulomb free mean path. Similar results were obtained analyzing contact discontinuities in other clusters e.

In galaxy group merger RXJ The width of contact discontinuities suggest a large gradient of the temperature across them within less than the Coulomb mean free path. Eckert et al. The first explanation for the narrow width of cold fronts and shocks was that the flow of the ambient ICM around the dense subcluster core will stretch the initially tangled magnetic field lines to form a draping layer with a magnetic field parallel to the front.

Transport processes are significantly suppressed perpendicular to magnetic field lines in a plasma because charged particles get trapped and circle around the field liens with a very small gyroradius, much smaller than the Coulomb mean free path. This assumption was used to explain the narrow width of contact discontinuities and shocks in merging clusters and groups: in A Vikhlinin et al.

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This explanation was supported by MHD numerical simulations including anisotropic heat conduction and assuming a uniform magnetic field perpendicular to the path of the infalling cluster carried out by Asai et al. They suggested that the magnetic field lines wrapping around the infalling subcluster suppress the heat conduction across them. Asai et al. Comparing the observations with MHD numerical simulations, the presence of Kelvin-Helmholtz instability would imply that the effective viscosity of the ICG is suppressed by more than an order of magnitude with respect to the isotropic Spitzer viscosity Werner et al.

Detailed numerical simulations question the magnetic draping scenario for suppression of transport processes at cold fronts and shocks in the ICG. MHD simulations suggest that magnetic draping suppresses the conduction only by a factor of a few ZuHone et al.

Galaxy clusters caught in the first moment of collision | University of Strathclyde

Simulations also suggest, that magnetic draping is broken near the cold front due to tangled magnetic fields and the increased magnetic pressure Ruszkowski et al. A more straightforward explanation for the significantly suppressed transport processes is that a thin shock surface is developed where the kinetic energy of the colliding plasma is dissipated due to collective plasma instabilities. Fluctuations in the electron and ion distributions lead to electric currents which generate magnetic fields even in an unmagnetized plasma.

As a consequence of these processes we expect the width of a shock in the ICG to be in the order of the ion inertial length,. We expect the width of cold fronts and shocks to be a few times this inertial length, which is many orders of magnitude less than the Coulomb mean free path Equation In addition, if external magnetic fields exist, the width of the shocks will be limited perpendicular to the magnetic field, by the gyroradius, for an ion is. As a consequence of the hierarchical structure formation, merging of massive objects generate large scale turbulence and bulk motion.

Similar results were found in numerical simulations of binary cluster mergers e. Turbulence plays an important role in determining the structure of clusters by transporting heat, and providing non-thermal pressure to the ICG.

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Numerical simulations suggest that turbulent motions of the ICG caused by mergers and shocks provide a significant non-thermal pressure support which varies as a function of radius e. Observations confirm that non-thermal pressure support is important in clusters at the core and the outer regions e. The contributions to non-thermal pressure support from turbulence and residual bulk motion generated by hierarchical structure formation, if not accounted for, introduce bias in cluster mass determinations, and therefore in cosmological parameters derived using clusters.

Turbulence may also accelerate relativistic electrons, which are responsible for the emission in diffuse radio sources in clusters and amplify magnetic fields Dolag et al. Turbulence generated by cluster mergers and AGN feedback is thought to be able to heat the cluster cores of cool core clusters and prevent them from overcooling and forming too many stars. Hydrodynamical flows become turbulent, when their Reynolds number is high.

The Reynolds number is defined as the ratio of inertial forces to viscous forces.

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With the new generation of high angular resolution X-ray telescopes on board Chandra and XMM-Newton it has become possible to study turbulence in the ICG for the first time. Schuecker et al. It is very difficult to characterize turbulence in the ICG based on observations of X-ray emission lines. The different ionization stages depending on the temperature of the ICG of Iron seems to be the best choice to study turbulence.

Turbulence causes line broadening and a unique line profile, as well as shifts in the line centroid Inogamov and Sunyaev, Turbulence suppresses the optical depth of resonant lines due to a shift in frequency caused by the Doppler effect, thus observations of resonant lines can also be used to study turbulence Churazov et al. We illustrate the effect of turbulence on emission lines in the left panel in Figure 2 from Zhuravleva et al.

In this panel we show simulated spectra of the emerging line from the Perseus cluster with and without resonant scattering turbulence.

Astronomers spot ‘ridge’ of plasma linking galaxy clusters

Figure 2. Comparison between different line shapes from clusters due to turbulence and resonant line scatterings. Right panel: Shapes of emerging lines due to resonant scatterings with different optical depths from Monte Carlo simulations.