Is Turbulence in the Interstellar Medium Driven by Feedback or Gravity?
  An Observational Test
  Galaxies' interstellar media (ISM) are observed to be
supersonically-turbulent, but the ultimate power source that drives turbulent
motion remains uncertain. The two dominant models are that the turbulence is
driven by star formation feedback and/or that it is produced by gravitational
instability in the gas. Here we show that, while both models predict that the
galaxies' ISM velocity dispersions will be positively correlated with their
star formation rates, the forms of the correlation predicted by these two
models are subtly but measurably different. A feedback-driven origin for the
turbulence predicts a velocity dispersion that rises more sharply with star
formation rate, and that does not depend on the gas fraction (i.e. $\dot{M}_*
\propto \sigma^2$), while a gravity-driven model yields a shallower rise and a
strong dependence on gas fraction (i.e. $\dot{M}_* \propto f_g^2 \sigma$). We
compare the models to a collection of data on local and high-redshift galaxies
culled from the literature, and show that the correlation expected for
gravity-driven turbulence is a better match to the observations than a
feedback-driven model. This suggests that gravity is the ultimate source of ISM
turbulence, at least in the rapidly-star-forming, high velocity dispersion
galaxies for which our test is most effective. We conclude by discussing the
limitations of the present data set, and the prospects for future measurements
to enable a more definitive test of the two models.
