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

TL;DR

This study investigates Kelvin-Helmholtz instability effects on cold gas streams feeding high-redshift galaxies, revealing faster growth, larger disintegration radii, and significant energy dissipation in 3D compared to 2D models, with implications for galaxy formation.

Contribution

It extends previous 2D analyses of KHI to 3D, providing new insights into stream stability, disruption scales, and energy dissipation relevant to galaxy feeding processes.

Findings

3D KHI grows faster than 2D, especially for supersonic streams.

Maximum stable stream radius is ~70% larger in 3D, ranging from 0.5-5% of halo virial radius.

Up to 50% of gravitational energy can be dissipated, powering Lyman-alpha blobs.

Abstract

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, but still limiting our analysis to the adiabatic case with no gravity. We combine analytic modeling and numerical simulations in the linear and non-linear regimes. For subsonic or transonic streams with respect to the halo sound speed, the instability in 3D is qualitatively similar to 2D, but progresses at a faster pace. For supersonic streams, the instability grows much faster in 3D and can be qualitatively different due to azimuthal modes, which introduce a strong dependence on the initial width of the stream-background interface. Using analytic toy models and approximations supported by high-resolution simulations, we apply our idealized hydrodynamical analysis to the astrophysical scenario. The upper limit for the radius of a stream that disintegrates prior to reaching the central galaxy is ~70% larger 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 ~20% of the initial stream velocity. KHI can cause significant stream deceleration and energy dissipation in 3d, contrary to 2D estimates. For typical streams, up to (10-50)% of the gravitational energy gained by inflow down the dark-matter halo potential can be dissipated, capable of powering Lyman-alpha blobs if most of it is dissipated into radiation.