Kelvin-Helmholtz Instability in Self-Gravitating Streams

Abstract: Self-gravitating gaseous filaments exist on many astrophysical scales, from sub-pc filaments in the interstellar medium to Mpc scale streams feeding galaxies from the cosmic web. These filaments are often subject to Kelvin-Helmotz Instability (KHI) due to shearing against a confining background medium. We study the nonlinear evolution of KHI in pressure-confined self-gravitating gas streams initially in hydrostatic equilibrium, using analytic models and hydrodynamic simulations, not including radiative cooling. We derive a critical line-mass as a function of the stream Mach number and density contrast with respect to the background, $\mu_{cr}(M_b,\delta_c)\le 1$, where $\mu=1$ is normalized to the maximal line mass for which initial hydrostatic equilibrium is possible. For $\mu<\mu_{cr}$, KHI dominates the stream evolution. A turbulent shear layer expands into the background and leads to stream deceleration at a similar rate to the non-gravitating case. However, with gravity, penetration of the shear layer into the stream is halted at roughly half the initial stream radius by stabilizing buoyancy forces, significantly delaying total stream disruption. Streams with $\mu_{cr}<\mu\le 1$ fragment and form round, long-lived clumps by gravitational instability (GI), with typical separations roughly 8 times the stream radius, similar to the case without KHI. When KHI is still somewhat effective, these clumps are below the spherical Jeans mass and are partially confined by external pressure, but they approach the Jeans mass as $\mu\rightarrow 1$ and GI dominates. We discuss potential applications of our results to filaments in the ISM and dense streams feeding galaxies at high redshift.