Kelvin-Helmholtz instabilities with Godunov SPH

Numerical simulations for the non-linear development of Kelvin-Helmholtz instability in two different density layers have been performed with the particle-based method (Godunov SPH) developed by Inutsuka (2002). The Godunov SPH can describe the Kelvin-Helmholtz instability even with a high density contrast, while the standard SPH shows the absence of the instability across a density gradient (Agertz et al. 2007). The interaction of a dense blob with a hot ambient medium has been performed also. The Godunov SPH describes the formation and evolution of the fingers due to the combinations of Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholtz instabilities. The blob test result coincides well with the results of the grid-based codes. An inaccurate handling of a density gradient in the standard SPH has been pointed out as the direct reason of the absence of the instabilities. An unphysical force happens at the density gradient even in a pressure equilibrium, and repulses particles from the initial density discontinuity. Therefore, the initial perturbation damps, and a gap forms at the discontinuity. The unphysical force has been studied in terms of the consistency of a numerical scheme. Contrary to the standard SPH, the momentum equation of the Godunov SPH doesnt use the particle approximation, and has been derived from the kernel convolution or a new Lagrangian function. The new Lagrangian function used in the Godunov SPH is more analogous to the real Lagrangian function for continuum. The momentum equation of the Godunov SPH has much better linear consistency, so the unphysical force is greatly reduced compared to the standard SPH in a high density contrast.Comment: 11 pages, 7 figures, Accepted for publication in MNRA

The dynamics and high-energy emission of conductive gas clouds in supernova-driven galactic superwinds

In this paper we present high-resolution hydrodynamical models of warm ionized clouds embedded in a superwind, and compare the OVI and soft X-ray properties to the existing observational data. These models include thermal conduction, which we show plays an important role in shaping both the dynamics and radiative properties of the resulting wind/cloud interaction. Heat conduction stabilizes the cloud by inhibiting the growth of K-H and R-T instabilities, and also generates a shock wave at the cloud's surface that compresses the cloud. This dynamical behaviour influences the observable properties. We find that while OVI emission and absorption always arises in cloud material at the periphery of the cloud, most of the soft X-ray arises in the region between the wind bow shock and the cloud surface, and probes either wind or cloud material depending on the strength of conduction and the relative abundances of the wind with respect to the cloud. In general only a small fraction (<1%) of the wind mechanical energy intersecting a cloud is radiated away at UV and X-ray wavelengths, with more wind energy going into accelerating the cloud. Models with heat conduction at Spitzer-levels are found to produce observational properties closer to those observed in superwinds than models with no thermal conduction, in particular in terms of the OVI-to-X-ray luminosity ratio, but cloud life times are uncomfortably short (<1Myr) compared to the dynamical ages of real winds. We experimented with reducing the thermal conductivity and found that even when we reduced conduction by a factor of 25 that the simulations retained the beneficial hydrodynamical stability and low O{\sc vi}-to-X-ray luminosity ratio found in the Spitzer-level conductive models, while also having reduced evaporation rates.Comment: 27 pages, 12 figures (4 in color), MNRAS accepte

The Weak Shock in the Core of the Perseus Cluster

The dissipation of energy from sound waves and weak shocks is one of the most promising mechanisms for coupling AGN activity to the surrounding intracluster medium (ICM), and so offsetting cooling in cluster cores. We present a detailed analysis of the weak shock found in deep Chandra observations of the Perseus cluster core. A comparison of the spectra either side of the shock front shows that they are very similar. By performing a deprojection analysis of a sector containing the shock, we produce temperature and density profiles across the shock front. These show no evidence for a temperature jump coincident with the density jump. To understand this result, we model the shock formation using 1D hydrodynamic simulations including models with thermal conduction and gamma < 5/3 gas. These models do not agree well with the data, suggesting that further physics is needed to explain the shock structure. We suggest that an interaction between the shock and the H-alpha filaments could have a significant effect on cooling the post-shock gas. We also calculate the thermal energy liberated by the weak shock. The total energy in the shocked region is about 3.5 times the work needed to inflate the bubbles adiabatically, and the power of the shock is around 6x10^44 erg/s per bubble, just over 10^45 erg/s in total.Comment: 12 pages, 13 figures, accepted by MNRA