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 per cent) in the central cluster volume at all epochs. The dissipation via compressive modes is found to be more important (∼30 per cent of the total) only at large radii (≥0.5rvir) 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. Secondly, in the cluster interior vortex, stretching is dominant, although the other terms also contribute substantially.
Highlights
The rarefied media in galaxy clusters are highly dynamic and likely to be turbulent, with strong motions on many scales that can significantly influence a wide range of ICM physical processes (e.g. Schekochihin & Cowley 2006; Subramanian, Shukurov & Haugen 2006; Brunetti & Lazarian 2007; Jones et al 2011)
We look at the processes that generate enstrophy described in equations (9) and (10), which allows a deeper understanding of the sources and amplification of turbulence in the cluster over time
We surmise that the reduced compressive turbulence role we found is mostly due to our procedure of extracting turbulent motions from the filtered, small-scale uncorrelated velocity field (Section 3.2), rather than from the total velocity field, and from our removal of contributions from stronger shocks to the turbulent motions on the grounds that most of those strong shocks are not directly involved in the uncorrelated motions
Summary
The rarefied media in galaxy clusters (intracluster mediums, ICMs) are highly dynamic and likely to be turbulent, with strong motions on many scales that can significantly influence a wide range of ICM physical processes (e.g. Schekochihin & Cowley 2006; Subramanian, Shukurov & Haugen 2006; Brunetti & Lazarian 2007; Jones et al 2011). Schekochihin & Cowley 2006; Subramanian, Shukurov & Haugen 2006; Brunetti & Lazarian 2007; Jones et al 2011). These motions may be driven by processes originating on galactic scales [e.g. starburst winds, active galactic nuclei outflows and bubbles, (e.g. O’Neill, De Young & Jones 2009; Morsony et al 2010; Gaspari, Ruszkowski & Sharma 2012; Mendygral, Jones & Dolag 2012)], possibly ICM-based magneto-thermal instabilities The compressive turbulence component will, itself, produce weak shocks that can, in turn, generate solenoidal motions (e.g. Porter et al 2015). Both compressive and solenoidal turbulent components may accelerate cosmic rays through second-order Fermi processes (e.g. Fujita, Takizawa & Sarazin 2003; Brunetti & Blasi 2005; Brunetti & Lazarian 2007, 2016)
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