Massive galaxies at high redshift are predicted to be fed from the cosmic web by narrow, dense streams of cold gas that penetrate through the hot medium encompassed by a stable shock near the virial radius of the dark-matter halo. Our long-term goal is to explore the heating and dissipation rate of the streams and their fragmentation and possible breakup, in order to understand how galaxies are fed, and how this affects their star-formation rate and morphology. We present here the first step, where we analyze the linear Kelvin-Helmholtz instability (KHI) of a cold, dense slab or cylinder in 3D flowing supersonically through a hot, dilute medium. The current analysis is limited to the adiabatic case with no gravity. By analytically solving the linear dispersion relation, we find a transition from a dominance of the familiar rapidly growing surface modes in the subsonic regime to more slowly growing body modes in the supersonic regime. The system is parametrized by three parameters: the density contrast between stream and medium, the Mach number of stream velocity with respect to the medium, and the stream width with respect to the halo virial radius. A realistic choice for these parameters places the streams near the mode transition, with the KHI exponential-growth time in the range 0.01-10 virial crossing times for a perturbation wavelength comparable to the stream width. We confirm our analytic predictions with idealized hydrodynamical simulations. Our linear estimates thus indicate that KHI may be effective in the evolution of streams before they reach the galaxy. More definite conclusions await the extension of the analysis to the nonlinear regime and the inclusion of cooling, thermal conduction, the halo potential well, self-gravity and magnetic fields.
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.
As part of our long-term campaign to understand how cold streams feed massive galaxies at high redshift, we study the Kelvin-Helmholtz instability (KHI) of a supersonic, cold, dense gas stream as it penetrates through a hot, dilute circumgalactic medium (CGM). A linear analysis (Paper I) showed that, for realistic conditions, KHI may produce nonlinear perturbations to the stream during infall. Therefore, we proceed here to study the nonlinear stage of KHI, still limited to a two-dimensional slab with no radiative cooling or gravity. Using analytic models and numerical simulations, we examine stream breakup, deceleration and heating via surface modes and body modes. The relevant parameters are the density contrast between stream and CGM (δ), the Mach number of the stream velocity with respect to the CGM (M b ) and the stream radius relative to the halo virial radius (R s /R v ). We find that sufficiently thin streams disintegrate prior to reaching the central galaxy. The condition for breakup ranges from R s < 0.03R v for (M b ∼ 0.75, δ ∼ 10) to R s < 0.003R v for (M b ∼ 2.25, δ ∼ 100). However, due to the large stream inertia, KHI has only a small effect on the stream inflow rate and a small contribution to heating and subsequent Lyman-α cooling emission.
Virial shocks at edges of cosmic-web structures are a clear prediction of standard structure formation theories. We derive a criterion for the stability of the post-shock gas and of the virial shock itself in spherical, filamentary and planar infall geometries. When gas cooling is important, we find that shocks become unstable, and gas flows uninterrupted towards the center of the respective halo, filament or sheet. For filaments, we impose this criterion on self-similar infall solutions. We find that instability is expected for filament masses between 10 11 − 10 13 M ⊙ Mpc −1 . Using a simplified toy model, we then show that these filaments will likely feed halos with 10 10 M ⊙ M halo 10 13 M ⊙ at redshift z = 3, as well as 10 12 M ⊙ M halo 10 15 M ⊙ at z = 0.The instability will affect the survivability of the filaments as they penetrate gaseous halos in a non-trivial way. Additionally, smaller halos accreting onto non-stable filaments will not be subject to ram-pressure inside the filaments. The instreaming gas will continue towards the center, and stop either once its angular momentum balances the gravitational attraction, or when its density becomes so high that it becomes self-shielded to radiation.
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