Turbulence and vorticity in Galaxy clusters generated by structure formation

Abstract

Turbulence is a key ingredient for the evolution of the intracluster medium, whose properties can be predicted with high resolution numerical simulations. We present initial results on the generation of solenoidal and compressive turbulence in the intracluster medium during the formation of a small-size cluster using highly resolved, non-radiative cosmological simulations, with a refined monitoring in time. In this first of a series of papers, we closely look at one simulated cluster whose formation was distinguished by a merger around z ∼ 0.3. We separate laminar gas motions, turbulence and shocks with dedicated filtering strategies and distinguish the solenoidal and compressive components of the gas flows using Hodge-Helmholtz decomposition. Solenoidal turbulence dominates the dissipation of turbulent motions (∼ 95%) in the central cluster volume at all epochs. The dissipation via compressive modes is found to be more important (∼ 30% of the total) only at large radii ( 0.5 r vir ) and close to merger events. We show that enstrophy (vorticity squared) is good proxy of solenoidal turbulence. All terms ruling the evolution of enstrophy (i.e. baroclinic, compressive, stretching and advective terms) are found to be significant, but in amounts that vary with time and location. Two important trends for the growth of enstrophy in our simulation are identified: first, enstrophy is continuously accreted into the cluster from the outside, and most of that accreted enstrophy is generated near the outer accretion shocks by baroclinic and compressive processes. Second, in the cluster interior vortex stretching is dominant, although the other terms also contribute substantially.

On the complexity and the information content of cosmic structures

“…These result can be interpreted by noticing that the scale at which the efficiency of prediction of the simulated ICM is maximum is tentatively close to the typical scale of turbulent eddies in the simulated ICM (e.g. Vazza et al 2011aVazza et al , 2012Miniati & Beresnyak 2015;Vazza et al 2016b), to the typical correlation scale of observed and simulated magnetic fields (e.g. Xu et al 2009;Bonafede et al 2013), as well as to the scale of measured projected density fluctuations in X-ray (e.g.…”

“…The ICM is known to host volume filling subsonic turbulence at all epochs, as result of gravity-driven random motions (e.g. Vazza et al 2011a;Miniati & Beresnyak 2015;Vazza et al 2016b). The kinetic energy is thus subject to complex fluctuations due to turbulence even on the ≈ 7.3 × 10 7 yr timescale.…”

Simulations of extragalactic magnetic fields and of their observables

“…Vazza et al, 2017; in the simulations presented here we use a fixed resolution approach, using large (≥ 1024…”

“…Ryu et al, 2008) and the vorticity squared (also known as ???enstrophy?? ?, (∇ × v) 2 ≡ ω ) gives a convenient way of estimating the turbulent dissipation rate on the fly in our simulations as F turb η t ρ 3 ω /L , where η t = 0.014 and L is the stencil to compute the vorticity (Jones et al, 2011;Porter et al, 2015;Vazza et al, 2017;Wittor, Jones, Vazza and Brüggen, 2017). The fraction of this turbulent kinetic power which is ultimately converted into magnetic energy, dyn , sets the amplified magnetic energy as E B,dyn = dyn (M)F turb ∆t.…”

Relativistic protons in the Coma galaxy cluster: first gamma-ray constraints ever on turbulent reacceleration

“…Numerical simulations suggest that incompressible turbulence is important in the ICM (Miniati 2014;Vazza et al 2017). Turbulent reacceleration by large-scale incompressible turbulence in the ICM has been investigated in Brunetti & Lazarian (2016).…”