Instability of Supersonic Cold Streams Feeding Galaxies III: Kelvin-Helmholtz Instability in Three Dimensions.

We study the effects of Kelvin-Helmholtz instability (KHI) on the cold streams that feed high-redshift galaxies through their hot haloes, generalizing our earlier analyses of a 2D slab to a 3D cylinder. Our analysis combines analytic modeling and numerical simulations in the linear and non-linear regimes. For streams that are subsonic or transonic with respect to the halo sound speed, the instability in 3D is qualitatively similar to that in 2D, except that it progresses at a faster pace. For supersonic streams, the instability grows much faster in 3D and it can be qualitatively different due to 3D azimuthal modes, which introduce a strong dependence on the initial width of the transition region between the stream and the background. Overall, the upper limit for the radius of a stream that disintegrates prior to reaching the central galaxy is larger by 70% than the 2D estimate; it is in the range 0.5-5% of the halo virial radius, decreasing with increasing stream density and velocity. Stream disruption generates a turbulent mixing zone around the stream with velocities at the level of 40% of the initial stream velocity. Contrary to the 2D estimates, we find that in 3D the KHI can lead to significant stream deceleration and energy dissipation. For typical streams, 30-70% of the gravitational energy gained by the inflow down the dark-matter halo potential well can be dissipated, capable of powering Lyman-alpha blobs.