Are Cosmological Gas Accretion Streams Multiphase and Turbulent?

Abstract

Simulations of cosmological filamentary accretion reveal flows ("streams") of warm gas, ~$10^4$ K, which are efficient in bringing gas into galaxies. We present a phenomenological scenario where gas in such flows -- if it is shocked as it enters the halo as we assume -- become biphasic and, as a result, turbulent. We consider a collimated stream of warm gas that flows into a halo from an over dense filament of the cosmic web. The post-shock streaming gas expands because it has a higher pressure than the ambient halo gas, and fragments as it cools. The fragmented stream forms a two phase medium: a warm cloudy phase embedded in hot post-shock gas. We argue that the hot phase sustains the accretion shock. A fraction of the initial kinetic energy of the infalling gas is converted into turbulence among and within the warm clouds. The thermodynamic evolution of the post-shock gas is largely determined by the relative timescales of several processes -- the cooling, the expansion of the post-shock gas, the amount of turbulence in the clouds, and the halo dynamics. We expect the gas to become multiphase when the cooling and dynamical times are of the same order-of-magnitude. In this framework, we show that this occurs in the important mass range of ~$10^{11}$ to $10^{13}$ M$_\odot$ , where the bulk of stars have formed in galaxies. Gas accreting along cosmic web filaments may eventually lose coherence and mix with the ambient halo gas. Through both the phase separation and "disruption" of the stream, the accretion efficiency onto a galaxy in a halo dynamical time is lowered. De-collimating flows make the direct interaction between galaxy feedback and accretion streams more likely, thereby further reducing the overall accretion efficiency. Moderating the gas accretion efficiency through these mechanisms may help to alleviate a number of significant challenges in theoretical galaxy formation.