(abridged) We perform hydrodynamical simulations of minor-merger induced gas sloshing and the subsequent formation of cold fronts in the Virgo cluster. We show for the first time that sloshing reproduces all characteristics of the observed cold front
s quantitatively, and we suggest a third cold front at 20 kpc NW of the Virgo core. We identify several new features typical for sloshing cold fronts, most importantly a large-scale brightness asymmetry. We can trace these new features not only in Virgo, but also in other sloshing cold front clusters. By comparing synthetic and real observations, we estimate that the original minor merger event took place about 1.5 Gyr ago when a subcluster of 2-4 times 10^13 Modot passed the Virgo core at 100 to 400 kpc distance, where a smaller mass corresponds to a smaller pericentre distance, and vice versa. From the merger geometry, we derive the current location of the disturbing subcluster to be about 1-2 Mpc E of the Virgo core. A possible candidate is M60. Additionally, we quantify the metal redistribution by sloshing and discuss its importance. We verify that the subcluster required to produce the observed cold fronts could be completely ram pressure stripped before reaching the Virgo centre, and discuss the conditions required for this to be achieved. Finally, we demonstrate that the bow shock of a fast galaxy passing the Virgo cluster at ~ 400 kpc distance also causes sloshing and leads to very similar cold front structures. The responsible galaxy would be located about 2 Mpc north of the Virgo centre. A possible candidate is M85.
In this paper we investigate whether Smoothed Particle Hydrodynamics (SPH), equipped with artificial conductivity, is able to capture the physics of density/energy discontinuities in the case of the so-called shearing layers test, a test for examinin
g Kelvin-Helmholtz (KH) instabilities. We can trace back each failure of SPH to show KH rolls to two causes: i) shock waves travelling in the simulation box and ii) particle clumping, or more generally, particle noise. The probable cause of shock waves is the Local Mixing Instability (LMI), previously identified in the literature. Particle noise on the other hand is a problem because it introduces a large error in the SPH momentum equation. We also investigate the role of artificial conductivity (AC). Including AC is necessary for the long-term behavior of the simulation (e.g. to get $lambda=1/2, 1$ KH rolls). In sensitive hydrodynamical simulations great care is however needed in selecting the AC signal velocity, with the default formulation leading to too much energy diffusion. We present new signal velocities that lead to less diffusion. The effects of the shock waves and of particle disorder become less important as the time-scale of the physical problem (for the shearing layers problem: lower density contrast and higher Mach numbers) decreases. At the resolution of current galaxy formation simulations mixing is probably not important. However, mixing could become crucial for next-generation simulations.
