Probing viscosity of the intracluster medium using ram-pressure stripping

TL;DR

This work probes how the viscosity of the intracluster medium (ICM) influences ram-pressure stripping (RPS) tails by performing 3D Braginskii-MHD simulations with four viscosity models: inviscid, unsuppressed isotropic, unsuppressed anisotropic, and anisotropic viscosity suppressed by plasma instabilities. The simulations reveal that isotropic viscosity most effectively suppresses Kelvin-Helmholtz instabilities, yielding long, coherent tails with strong viscous heating, while the inviscid case shows vigorous mixing and shorter tails. Anisotropic viscosity lies between these extremes, but when plasma-instability saturation is included (S), the effective viscosity is drastically reduced (by roughly a factor of $20$–$30$) and tail turbulence approaches the inviscid case, reducing heating relative to the fully anisotropic model. These findings demonstrate that microphysical plasma processes in the ICM critically affect RPS morphology and associated observables, such as X-ray emission, and provide a framework for constraining ICM viscosity from RPS tails.

Abstract

Galaxies falling into galaxy clusters can leave imprints on both the corona of galaxies and the intracluster medium (ICM) of galaxy clusters. Throughout this infall process, the galaxy's atmosphere is subjected to ram pressure from a headwind, leading to the stripping morphology observed in its tail. The morphological evolution is affected by the properties of the surrounding ICM such as magnetic fields and viscosity. In this Letter, we perform 3D Braginskii-magnetohydrodynamic simulations using the FLASH code with varied ICM viscosity models. Specifically, we explore four models: an inviscid case, unsuppressed isotropic viscosity, unsuppressed anisotropic viscosity, and anisotropic viscosity suppressed by plasma instabilities. Our findings indicate that the isotropic viscosity case effectively suppresses hydrodynamic instabilities and shows strong viscous heating and the least mixing with the ICM, enabling the formation of long, coherent tails. The inviscid model has the shortest tail due to vigorous mixing, and the models with anisotropic viscosity are in between. The model with suppressed anisotropic viscosity due to plasma instabilities exhibits enhanced turbulence in the galactic tail and a concurrent limitation in viscous heating compared to the model neglecting plasma instabilities. These findings highlight the significant impact of ICM plasma physics on the processes of ram pressure stripping of galaxies.

Figures (6)

• Figure 1: Density slice plots of four ICM viscosity models at $t=350$ Myr. The slice plot is cut through the plane $x=0$. The bifurcated tails resulting from the magnetic draping layer are clearly seen.
• Figure 2: Evolution of the turbulent velocity of the four cases. Among the scenarios considered, the turbulent velocity is greatest for case (N), followed by case (S), (A), and lowest for case (I) for $t < 270$ Myr. This trend is consistent with the expectation that a higher level of viscosity would suppress KH instabilities and thus result in lower turbulent velocities.
• Figure 3: Slice plots of gas temperature of the four cases at $t = 350$ Myr. Case (I) exhibits the most pronounced viscous heating at the tail-ICM interface. Case (A) shows higher temperatures along its tails due to viscous heating compared to case (S).
• Figure 4: Slice plots of the Reynolds number ($Re$) for three cases at $t = 350$ Myr. Case (N) is omitted, as it has no viscosity and therefore no well-defined Reynolds number. For case (I) at about $z = -25$ kpc, $Re$ is the smallest among the four scenarios, indicating significant viscosity-driven suppression of KH instabilities. In contrast, the other three cases display insignificant differences in their $Re$ values, suggesting a limited impact of viscosity.
• Figure 5: Simulated X-ray images in the 0.7-10 keV band at $t=350$ Myr for each model. The upper left, upper right, bottom left, and bottom right panels show the (N), (I), (A), and (S) cases, respectively. Among these, case (I) exhibits the highest brightness, followed by case (S), with cases (A) and (N) appearing comparatively fainter.