  Two competing models, gravitational instability-driven transport and stellar
feedback, have been proposed to interpret the high velocity dispersions
observed in high-redshift galaxies. We study the major mechanisms to drive the
turbulence in star-forming galaxies using a sample of galaxies from the xCOLD
GASS survey, selected based on their star-formation rate (SFR) and gas fraction
to be in the regime that can best distinguish between the proposed models. We
perform Wide Field Spectrograph (WiFeS) integral field spectroscopic (IFS)
observations to measure the intrinsic gas velocity dispersions, circular
velocities and orbital periods in these galaxies. Comparing the relation
between the SFR, velocity dispersion, and gas fraction with predictions of
these two theoretical models, we find that our results are most consistent with
a model that includes both transport and feedback as drivers of turbulence in
the interstellar medium. By contrast, a model where stellar feedback alone
drives turbulence under-predicts the observed velocity dispersion in our
galaxies, and does not reproduce the observed trend with gas fraction. These
observations therefore support the idea that gravitational instability makes a
substantial contribution to turbulence in high redshift and high SFR galaxies.
