Accretion Does Not Drive the Turbulence in Galactic Disks
  Rapid accretion of cold gas plays a crucial role in getting gas into
galaxies. It has been suggested that this accretion proceeds along narrow
streams that might also directly drive the turbulence in galactic gas,
dynamical disturbances, and bulge formation. In cosmological simulations,
however, it is impossible to isolate and hence disentangle the effect of
accretion from internal instabilities and mergers. Moreover, in most
cosmological simulations, the phase structure and turbulence in the ISM arising
from stellar feedback are treated in a sub-grid manner, so that feedback cannot
generate ISM turbulence. In this paper we therefore test the effects of cold
streams in extremely high-resolution simulations of otherwise isolated galaxy
disks using detailed models for star formation and feedback; we then include or
exclude mock cold flows falling onto the galaxies with accretion rates,
velocities and geometry set to maximize their effect on the disk. We find: (1)
Turbulent velocity dispersions in gas disks are identical with or without the
cold flow; the energy injected by the flow is dissipated where it meets the
disk. (2) In runs without stellar feedback, the presence of a cold flow has
essentially no effect on runaway local collapse, resulting in star formation
rates (SFRs) that are far too large. (3) Disks in runs with feedback and cold
flows have higher SFRs, but only insofar as they have more gas. (4) Because
flows are extended relative to the disk, they do not trigger strong resonant
responses and so induce weak morphological perturbation (bulge formation via
instabilities is not accelerated). (5) However, flows can thicken the disk by
direct contribution of out-of-plane streams. We conclude that while inflows are
critical over cosmological timescales to determine the supply and angular
momentum of gas disks, they have weak instantaneous dynamical effects on
galaxies.
