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

Abstract

As part of our long-term campaign to understand how cold streams feed massive\ngalaxies at high redshift, we study the Kelvin-Helmholtz instability (KHI) of a\nsupersonic, cold, dense gas stream as it penetrates through a hot, dilute\ncircumgalactic medium (CGM). A linear analysis (Paper I) showed that, for\nrealistic conditions, KHI may produce nonlinear perturbations to the stream\nduring infall. Therefore, we proceed here to study the nonlinear stage of KHI,\nstill limited to a two-dimensional slab with no radiative cooling or gravity.\nUsing analytic models and numerical simulations, we examine stream breakup,\ndeceleration and heating via surface modes and body modes. The relevant\nparameters are the density contrast between stream and CGM ($\\delta$), the Mach\nnumber of the stream velocity with respect to the CGM ($M_{\\rm b}$) and the\nstream radius relative to the halo virial radius ($R_{\\rm s}/R_{\\rm v}$). We\nfind that sufficiently thin streams disintegrate prior to reaching the central\ngalaxy. The condition for breakup ranges from $R_{\\rm s} < 0.03 R_{\\rm v}$ for\n$(M_{\\rm b} \\sim 0.75, \\delta \\sim 10)$ to $R_{\\rm s} < 0.003 R_{\\rm v}$ for\n$(M_{\\rm b} \\sim 2.25, \\delta \\sim 100)$. However, due to the large stream\ninertia, KHI has only a small effect on the stream inflow rate and a small\ncontribution to heating and subsequent Lyman-$\\alpha$ cooling emission.\n