Stability of Cold Flows
Map of gas density in a cosmological zoom in simulation run using the RAMSES code. The streams feeding the galaxy are visible. The box encompasses a sphere of twice the virial radius. See Danovich et al 2012 for additional details. While some cosmological simulations suggest that the streams remain cold and coherent until they hit the galaxy, it is not at all clear that this should indeed be so.
2D RAMSES simulations of Kelvin Helmholtz instability in a confined planar slab. The slab is 10 (100) times denser than the background and its velocity is 0.5 (1.5) times the sound speed in the background in the left (right) movie. The growth rate in the super sonic case is slower and the instability is dominated by different modes, in agreement with the analytic calculation.
There are many other instabilities affecting these cold flows: thermal, gravitational and magnetic. In future work, I intend to extend my models to account for these additional effects, one by one. This includes both analytic work, building upon analysis I have already done, and running numerical simulations of idealized setups. These will build a physical understanding of the problem from the bottom up, allowing us to better understand and improve the physical models included in cosmological simulations.
Our current understanding is that in the early Universe, massive star forming galaxies were fed by dense filaments of cold gas. While this general picture appears robust in simulations and has some observational evidence, different computational methods wildly disagree on whether these streams remain cold and coherent or heat up and fragment on their way from the large scale towards the central galaxy. This has profound implications for the structure and angular momentum of disk galaxies. Since even the best cosmological simulations only mildly resolve the streams, they cannot directly address the problem in any meaningful way. Currently, the fundamental physical question of stability of a supersonic, dense stream in a hot atmosphere remains open and has not been systematically studied in the literature.
Together with my collaborators, I have taken the first step towards answering this question by studying from first principles the stability of such a stream to purely hydrodynamic instabilities, analytically calculating the growth rates of linear perturbations and confirming the results with numerical simulations using the RAMSES code. We find that the physical parameters of these cold streams are near a phase transition, where small changes in the assumed parameters cause large changes in the growth rates. Thus, making concrete predictions regarding stream survival is difficult, and more detailed modelling is needed.