The relation between gas density and velocity power spectra in galaxy clusters: High-resolution hydrodynamic simulations and the role of conduction

Abstract

Exploring the power spectrum of fluctuations and velocities in the intracluster medium (ICM) can help us to probe the gas physics of galaxy clusters. Using high-resolution 3D plasma simulations, we study the statistics of the velocity field and its intimate relation with the ICM thermodynamic perturbations. The normalization of the ICM spectrum (related to density, entropy, or pressure fluctuations) is linearly tied to the level of large-scale motions, which excite both gravity and sound waves due to stratification. For a low 3D Mach number M ∼ 0.25, gravity waves mainly drive entropy perturbations, which are traced by preferentially tangential turbulence. For M > 0.5, sound waves start to significantly contribute and pass the leading role to compressive pressure fluctuations, which are associated with isotropic (or slightly radial) turbulence. Density and temperature fluctuations are then characterized by the dominant process: isobaric (low M), adiabatic (high M), or isothermal (strong conduction). Most clusters reside in the intermediate regime, showing a mixture of gravity and sound waves, hence drifting toward isotropic velocities. Remarkably, regardless of the regime, the variance of density perturbations is comparable to the 1D Mach number, M 1D ∼ δρ/ρ. This linear relation allows us to easily convert between gas motions and ICM perturbations (δρ/ρ < 1), which can be exploited by the available Chandra, XMM data and by the forthcoming Astro-H mission. At intermediate and small scales (10-100 kpc), the turbulent velocities develop a tight Kolmogorov cascade. The thermodynamic perturbations (which can be generally described by log-normal distributions) act as effective tracers of the velocity field, in broad agreement with the Kolmogorov-Obukhov-Corrsin advection theory. The cluster radial gradients and compressive features induce a flattening in the cascade of the perturbations. Thermal conduction, on the other hand, acts to damp the thermodynamic fluctuations, washing out the filamentary structures and steepening the spectrum, while leaving the velocity cascade unaltered. The ratio of the velocity and density spectrum thus inverts the downtrend shown by the non-diffusive models, as it widens up to ∼5. This new key diagnostic can robustly probe the presence of conductivity in the ICM. We produce X-ray images of the velocity field, showing how future missions (e.g. Astro-H, Athena) can detect velocity dispersions of a few 100 km s −1 (M > 0.1 in massive clusters), allowing us to calibrate the linear relation and to constrain relative perturbations down to just a few percent.

The stripping of a galaxy group diving into the massive cluster A2142

“…The discovery of the infalling group in A2142 thus provides evidence that thermal conduction is largely suppressed (by a factor f −1 400) in the cluster's environment. Similar conclusions have been reached in the past few years for special environments, such as cold fronts (Ettori & Fabian 2000) and filamentary structures in cluster cores (Forman et al 2007;Sanders et al 2013) and from the power spectrum of density fluctuations (Gaspari & Churazov 2013;Gaspari et al 2014). This is however the first time that this result is confirmed in a direct way outside the cluster core.…”

Metal distribution in sloshing galaxy clusters: the case of A496

“…4.2, the lifetime of the cold fronts and the spiral structure in A496 is about ∼0.6 Gyr, three orders of magnitude longer than the time required to erase the cold-front features and one order of magnitude longer than the time necessary to reach thermal equilibrium and erase the spiral structure: the spiral would be quickly destroyed if conduction were not suppressed along all the spiral edge. While suppression of conduction at cold fronts has been reported by several authors (e.g., Ettori & Fabian 2000;Vikhlinin et al 2001a;Gaspari & Churazov 2013;Gaspari et al 2014), this is, to the best of our knowledge, the first time that suppression is reported between gas within and outside of the spiral. Magnetic fields are probably responsible for this suppression.…”

Turbulence and vorticity in Galaxy clusters generated by structure formation

“…ATHENA) should be able eventually to detect directly the driving-scale turbulent motions in the ICMs of multiple clusters (Nagai et al 2013;Ettori et al 2013;Zhuravleva et al 2013;). In the meantime, turbulent motions in the ICM induce moderate pressure fluctuations that may be detected in X-rays (e.g., Schuecker et al 2004;Sanders & Fabian 2012;Churazov et al 2012;Gaspari et al 2014;Zhuravleva et al 2014), or through the S-Z effect (e.g., Khatri & Gaspari 2016). …”

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