Lecturer: Mr. Dan Padnos Affiliation: Racah Institute of Physics, The Hebrew University of Jerusalem Abstract: 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. The interaction of the cold stream with the hot halo may result in heating, dissipation and fragmentation of the stream, potentially affecting accretion and star formation rates, generating clumps and producing observational signatures for cold streams. As part of a larger campaign to study these effects, we consider the supersonic Kelvin-Helmholtz Instability (KHI) at the stream-background boundary. This talk 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. Linear analysis of the problem https://arxiv.org/abs/1606.06289 reveals that cosmic cold streams can be susceptible to both surface and body modes of KHI. Furthermore, we find that perturbations are likely to reach nonlinear amplitudes during the stream infall. Using numeric simulations in RAMSES as well as simple analytic models, we study the nonlinear growth of surface modes under typical conditions for cosmic cold streams. 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. Additional details of the upcoming Astrophysics' seminars can be found on the following link. ‏האירוע הזה כולל שיחת וידאו ב-Google Hangouts. הצטרף: https://plus.google.com/hangouts/_/mail.huji.ac.il/astrophysics?hceid=bWFpbC5odWppLmFjLmlsX2c0czhydDlpcmhwZzRvdGNybWIzZGFqcjdvQGdyb3VwLmNhbGVuZGFyLmdvb2dsZS5jb20.pclq093j80aav4ceghke7i9hmc&hs=121