Are Cosmological Gas Accretion Streams Multiphase and Turbulent?
  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.
