Instability of supersonic cold streams feeding galaxies-II. Non-linear evolution of surface and body modes of Kelvin-Helmholtz instability

As part of our long-term campaign to understand how cold streams feed massive galaxies at high redshift,we study theKelvin-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 non-linear perturbations to the stream during infall. Therefore, we proceed here to study the non-linear stage of KHI, still limited to a 2D slab with no radiative cooling or gravity. Using analytic models and numerical simulations, we examine stream break-up, deceleration, and heating via surface modes and body modes. The relevant parameters are the density contrast between stream and CGM (d), 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 break-up 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-a cooling emission.