Kelvin-Helmholtz Instability in Self-Gravitating Streams
  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.
