Instability of supersonic cold streams feeding galaxies – I. Linear Kelvin–Helmholtz instability with body modes

Abstract

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.

The impact of magnetic fields on cold streams feeding galaxies

“…The key difference between our models of cold streams and the ones studied by Mandelker et al (2016), Padnos et al (2018) and Mandelker et al (2019) is that we include a magnetic field and treat the system with ideal MHD (see e.g. Freidberg 2014).…”

“…If the motion is sufficiently supersonic, disturbances on the interface are removed by the fast flow before they can grow. The precise velocity at which this stabilization occurs depends on the density contrast, δ, and is given by equation 22 in Mandelker et al (2016) who generalized the result of Landau (1944).…”

“…In a supersonic stream, however, a different version of the KHI can arise in which waves that are trapped inside the body of the stream grow in amplitude as they are reflected back and forth between the inside surfaces. In 2D hydrodynamics, the body modes are the dominant type of instability in the supersonic regime (Mandelker et al 2016).…”

“…The analyses in Mandelker et al (2016), Padnos et al (2018) and Mandelker et al (2019), while extremely comprehensive, considered the KHI using ideal hydrodynamics, and thus neglected potentially important influences on stream behavior from physical effects such as radiative cooling, thermal conduction, gravity and magnetic fields. In this paper, we present an idealized model of magnetized cold streams in order to understand the impact of magnetic fields on cold streams feeding galaxies.…”

The density distribution of accreting cosmic filaments as shaped by Kelvin–Helmholtz instability

“…where v fil is the velocity of the filament and c halo is the sound speed of the halo. In setting our initial conditions we closely follow the approach of Mandelker et al (2016).…”