Is Turbulence in the Interstellar Medium Driven by Feedback or Gravity? An Observational Test

TL;DR

This study compares feedback-driven and gravity-driven models of turbulence in the interstellar medium, finding observational evidence favoring gravity as the primary driver in high star formation galaxies.

Contribution

It provides an observational test distinguishing between feedback and gravity-driven turbulence models based on galaxy data, favoring gravity in high star formation environments.

Findings

Gravity-driven model better matches observed correlations.

Feedback model predicts a sharper increase in velocity dispersion.

Results suggest gravity as the main turbulence source in active galaxies.

Abstract

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.