RSAA Colloquia / Seminars / Feast-of-Facts: Thursday, 24 November 2016, 11:00-12:00; Duffield Lecture Theatre

Dan Padnos

"Kelvin-Helmholtz Instability of Cold Streams in Galaxies"

Massive galaxies at high redshift are fed gas from the cosmic web by narrow, dense streams of cold gas, penetrating supersonically through hot, dilute halos. This source of gas is the main driver for star formation in massive galaxies and also affects the conditions in the intergalactic medium (IGM). The interaction of the cold stream with the hot background of the halo may result in heating, dissipation and fragmentation of the stream, potentially affecting clumping, accretion and SF, as well as producing direct observational signatures for cold streams and their breakup. As part of a larger campaign to study these effects, we consider the supersonic Kelvin-Helmholtz Instability (KHI) at the stream-background boundary. Linear analysis of the problem ( reveals that cosmic cold streams can be susceptible to both surface and body modes of KHI and that perturbations are likely to become nonlinear before the stream reaches the galaxy at the halo center. Hence, we turn our attention to the nonlinear stage of KHI. In the nonlinear stage, surface modes evolve into a turbulent shear layer. Using numeric simulations in RAMSES and simple analytic models, we study the growth of this perturbed layer under typical conditions for a supersonic, cold and dense stream flowing through a hot, dilute background. Our preliminary results indicate that sufficiently narrow streams, with a diameter smaller than a few percent of the virial radius, can be completely disrupted by KHI before reaching the galaxy. In this talk, I will describe the linear and nonlinear stages for the different modes of KHI, and relate these general results to the specific scenario of cold streams feeding galaxies.