Instability of supersonic cold streams feeding Galaxies – III. Kelvin–Helmholtz instability in three dimensions

Abstract

We study the effects of Kelvin-Helmholtz instability (KHI) on the cold\nstreams that feed high-redshift galaxies through their hot haloes, generalizing\nour earlier analyses of a 2D slab to a 3D cylinder, but still limiting our\nanalysis to the adiabatic case with no gravity. We combine analytic modeling\nand numerical simulations in the linear and non-linear regimes. For subsonic or\ntransonic streams with respect to the halo sound speed, the instability in 3D\nis qualitatively similar to 2D, but progresses at a faster pace. For supersonic\nstreams, the instability grows much faster in 3D and can be qualitatively\ndifferent due to azimuthal modes, which introduce a strong dependence on the\ninitial width of the stream-background interface. Using analytic toy models and\napproximations supported by high-resolution simulations, we apply our idealized\nhydrodynamical analysis to the astrophysical scenario. The upper limit for the\nradius of a stream that disintegrates prior to reaching the central galaxy is\n~70% larger than the 2D estimate; it is in the range (0.5-5)% of the halo\nvirial radius, decreasing with increasing stream density and velocity. Stream\ndisruption generates a turbulent mixing zone around the stream with velocities\nat the level of ~20% of the initial stream velocity. KHI can cause significant\nstream deceleration and energy dissipation in 3d, contrary to 2D estimates. For\ntypical streams, up to (10-50)% of the gravitational energy gained by inflow\ndown the dark-matter halo potential can be dissipated, capable of powering\nLyman-alpha blobs if most of it is dissipated into radiation.\n