The Star Formation Efficiency per Free Fall Time in Nearby Galaxies

TL;DR

This study measures the star formation efficiency per free fall time in nearby galaxies using high-resolution CO imaging, revealing a median efficiency of about 0.7% and its dependence on scale and galaxy properties.

Contribution

It provides the first direct measurements of $ m extit{epsilon}_{ff}$ in multiple nearby galaxies at GMC scales, using new high-resolution observations and a consistent methodology.

Findings

Median $ m extit{epsilon}_{ff}$ is 0.7% across the sample.

Higher $ m extit{epsilon}_{ff}$ observed at coarser resolutions.

Lowest mass galaxies show the highest $ m extit{epsilon}_{ff}$ values.

Abstract

We estimate the star formation efficiency per gravitational free fall time, $\epsilon_{\rm ff}$, from observations of nearby galaxies with resolution matched to the typical size of a Giant Molecular Cloud. This quantity, $\epsilon_{\rm ff}$, is theoretically important but so far has only been measured for Milky Way clouds or inferred indirectly in a few other galaxies. Using new, high resolution CO imaging from the PHANGS-ALMA survey, we estimate the gravitational free-fall time at 60 to 120 pc resolution, and contrast this with the local molecular gas depletion time to estimate $\epsilon_{\rm ff}$. Assuming a constant thickness of the molecular gas layer ($H = 100$ pc) across the whole sample, the median value of $\epsilon_{\rm ff}$ in our sample is $0.7\%$. We find a mild scale-dependence, with higher $\epsilon_{\rm ff}$ measured at coarser resolution. Individual galaxies show different values of $\epsilon_{\rm ff}$, with the median $\epsilon_{\rm ff}$ ranging from $0.3\%$ to $2.6\%$. We find the highest $\epsilon_{\rm ff}$ in our lowest mass targets, reflecting both long free-fall times and short depletion times, though we caution that both measurements are subject to biases in low mass galaxies. We estimate the key systematic uncertainties, and show the dominant uncertainty to be the estimated line-of-sight depth through the molecular gas layer and the choice of star formation tracers.