Massive and refined

Abstract

We study the properties of chaotic motions in the intra cluster medium using a set of 20 galaxy clusters simulated with large dynamical range, using the adaptive mesh refinement code ENZO. The adopted setup allows us to study the spectral and spatial properties of turbulent motions in galaxy clusters with unprecedented detail, achieving an maximum available Reynolds number of the order of R e ∼ 500−1000 for the largest eddies. We investigated the correlations between the energy of these motions in the intra cluster medium and the dynamical state of the host systems. We find that the statistical properties of turbulent motions and their evolution with time imply that major merger events are responsible for the injection of the bulk of turbulent kinetic energy into the cluster. Turbulence is found to account for ∼20−30 per cent of the thermal energy in merging clusters, and ∼5 per cent in relaxed clusters. We compare the energies of turbulence and motions in our simulated clusters with upper-limits for real nearby clusters derived from XMM-Newton data. When turbulent motions are compared on the same spatial scales, the data from simulations are well within the range presently allowed by observations. Finally, we comment on the possibility that turbulence may accelerate relativistic particles leading to the formation of giant radio halos in turbulent (merging) clusters. On the basis of our simulations, we confirm the conclusions of previous semi-analytical studies that the fraction of turbulent clusters appears to be consistent with that of clusters hosting radio halos.

Turbulence and vorticity in Galaxy clusters generated by structure formation

“…The character of the ICM gas flows and the presence of turbulence in our simulated cluster is consistent with other highresolution simulation studies in the literature (e.g., Dolag et al 2005;Vazza et al 2011a;Miniati 2014;Schmidt et al 2014), with quantitative differences related to the specific analysis techniques. In the limited context of high-resolution grid-based simulations of non-radiative clusters, the pressure support from turbulence in the cluster core has, for example, been estimated in the range ∼ 5 − 15% of the gas pressure using the total, global gas velocity dispersion (e.g., Lau et al 2009;Nelson et al 2014), or, say ∼ 5 − 20% from large-scale, incoherent, but fixed scale velocity distributions, Vazza et al (2011a).…”

“…Our results here, as well as previous cluster simulations are roughly consistent with the behaviour of solenoidal turbulence fol-lowing the classic, Kolmogorov picture in which |δ v| ∝ L 1/3 n ((e.g., Ryu et al 2008;Xu et al 2011;Vazza et al 2011a;Miniati 2014). Consequently, while the r.m.s turbulent velocity or the total turbulent pressure depend on Ln, the solenoidal kinetic energy cascade flux, defined as:…”

“…For that we computed the 3-D power spectra within increasing volumes around the centre of it903 at z = 0.32, assuming, for that exercise, periodic boundaries in application of FFTs. The spectra for the total velocity field show the typical power-law behaviour of clusters simulated in this way (e.g., Vazza et al 2011a), up to nearly two orders of magnitude in scale when the cluster virial volume(5.76 Mpc) 3 is considered. The spectrum flattens at low k-values, but the powerlaw is close to E(k) ∝ k −2 (i.e.…”

“…Gaspari et al 2014;Schmidt et al 2014) or structure functions (Vazza et al 2011a;Miniati 2014Miniati , 2015.…”

“…These motions may be driven by processes originating on galactic scales (e.g., star burst winds, AGN outflows and bubbles, (e.g., O'Neill et al 2009;Morsony et al 2010;Mendygral et al 2012;Gaspari et al 2012)), possibly ICM-based magneto-thermal instabilities (e.g., Kunz et al 2011;ZuHone et al 2013), but especially by cluster-scale processes associated with cluster formation out of cosmological, large-⋆ Email:franco.vazza@hs.uni-hamburg.de scale structure (e.g., Dolag et al 2005;Vazza et al 2006;Ryu et al 2008;Lau et al 2009;Vazza et al 2011a;ZuHone 2011;Miniati 2014;Schmidt et al 2014). …”