Abstract
Simulations of cosmological filamentary accretion streams into galactic halos
reveal that such flows are warm at T\$\sim\$10\$^4\$K, laminar, and provide high
gas accretion efficiency onto galaxies. We present a phenomenological scenario
which suggests that accretion flows are shocked, become thermally unstable,
biphasic, and are, as a result, turbulent. We consider a collimated stream of
warm gas over denser than the hot, virialized halo gas. The post-shock
streaming gas has a higher pressure than the ambient halo gas, expands, and is
thermally unstable and fragments, forming a two phase medium -- a hot phase
with an embedded warm cloudy phase. The thermodynamic evolution of the
post-shock gas is largely determined by the relative timescales of several
processes, namely the cooling, the expansion of the hot phase and turbulent
warm clouds, and the amount of turbulence in clouds, and the halo dynamics. The
cooling is moderated by mixing with the ambient halo gas and heating due to
turbulent dissipation. We consider the evolution of a stream for a single halo
mass, 10\$^13\$ M\$\_ødot\$, and redshift, 2. We find that the gas becomes
thermally unstable and fragments into a two-phase medium where the cooler phase
is highly turbulent and has a lower bulk velocity than the initial stream. The
turbulent stream loses coherence in less than a halo dynamical time. Both the
phase separation and "disruption" of the stream imply that the accretion
efficiency onto a galaxy in a dynamical time may be less than in simulations
having laminar isothermal flows. 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. abridged
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