Multi-scale analysis of turbulence evolution in the density stratified intracluster medium

The diffuse hot medium inside clusters of galaxies typically exhibits turbulent motions whose amplitude increases with radius, as revealed by cosmological hydrodynamical simulations. However, its physical origin remains unclear. It could either be due to an excess injection of turbulence at large radii, or faster turbulence dissipation at small radii. We investigate this by studying the time evolution of turbulence in the intracluster medium (ICM) after major mergers, using the Omega500 non-radiative hydrodynamical cosmological simulations. By applying a novel wavelet analysis to study the radial dependence of the ICM turbulence spectrum, we discover that faster turbulence dissipation in the inner high density regions leads to the increasing turbulence amplitude with radius. We also find that the ICM turbulence at all radii decays in two phases after a major merger: an early fast decay phase followed by a slow secular decay phase. The buoyancy effects resulting from the ICM density stratification becomes increasingly important during turbulence decay, as revealed by a decreasing turbulence Froude number $Fr \sim \mathcal{O}(1)$. Our results indicate that the stronger density stratification and smaller eddy turn-over time are the likely causes of the faster turbulence dissipation rate in the inner regions of the cluster.Comment: 8 pages, 7 figures, accepted to MNRA

Cluster Merger Shock Constraints on Particle Acceleration and Nonthermal Pressure in the Intracluster Medium

X-ray observations of galaxy cluster merger shocks can be used to constrain nonthermal processes in the intracluster medium (ICM). The presence of nonthermal pressure components in the ICM, as well as the shock acceleration of particles and their escape, all affect shock jump conditions in distinct ways. Therefore, these processes can be constrained using X-ray surface brightness and temperature maps of merger shock fronts. Here we use these observations to place constraints on particle acceleration efficiency in intermediate Mach number (M ~ 2-3) shocks and explore the potential to constrain the contribution of nonthermal components (e.g., cosmic rays, magnetic field, and turbulence) to ICM pressure in cluster outskirts. We model the hydrodynamic jump conditions in merger shocks discovered in the galaxy clusters A520 (M ~ 2) and 1E 0657-56 (M ~ 3) using a multifluid model comprised of a thermal plasma, a nonthermal plasma, and a magnetic field. Based on the published X-ray spectroscopic data alone, we find that the fractional contribution of cosmic rays accelerated in these shocks is lower than about 10% of the shock downstream pressure. Current observations do not constrain the fractional contribution of nonthermal components to the pressure of the undisturbed shock upstream. Future X-ray observations, however, have the potential to either detect particle acceleration in these shocks through its effect on the shock dynamics, or to place a lower limit on the nonthermal pressure contributions in the undisturbed ICM. We briefly discuss implications for models of particle acceleration in collisionless shocks and the estimates of galaxy cluster masses derived from X-ray and Sunyaev-Zel'dovich effect observations.Comment: 10 pages, 4 figures, comments welcom

The Effect of Baryons on Halo Shapes

Observational evidence indicates a mismatch between the shapes of collisionless dark matter (DM) halos and those of observed systems. Using hydrodynamical cosmological simulations we investigate the effect of baryonic dissipation on halo shapes. We show that dissipational simulations produce significantly rounder halos than those formed in equivalent dissipationless simulations. Gas cooling causes an average increase in halo principal axis ratios of ~ 0.2-0.4 in the inner regions and a systematic shift that persists out to the virial radius, alleviating any tension between theory and observations. Although the magnitude of the effect may be overestimated due to overcooling, cluster formation simulations designed to reproduce the observed fraction of cold baryons still produce substantially rounder halos. Subhalos also exhibit a trend of increased axis ratios in dissipational simulations. Moreover, we demonstrate that subhalos are generally rounder than corresponding field halos even in dissipationless simulations. Lastly, we analyze a series of binary, equal-mass merger simulations of disk galaxies. Collisionless mergers reveal a strong correlation between DM halo shape and stellar remnant morphology. In dissipational mergers, the combination of strong gas inflows and star formation leads to an increase of the DM axis ratios in the remnant. All of these results highlight the vital role of baryonic processes in comparing theory with observations and warn against over-interpreting discrepancies with collisionless simulations on small scales.Comment: 8 pages, 3 figures. To appear in the proceedings of the XXIst IAP Colloquium "Mass Profiles and Shapes of Cosmological Structures", Paris 4-9 July 2005, France, (Eds.) G. Mamon, F. Combes, C. Deffayet, B. Fort, EAS Publications Serie