Star Formation Efficiency per Free-fall Time in nearby Galaxies

Dyas Utomo, Jiayi Sun, Adam K. Leroy, J. M.Diederik Kruijssen, Eva Schinnerer, Andreas Schruba, Frank Bigiel, Guillermo A. Blanc, Mélanie Chevance, Eric Emsellem, Cinthya Herrera, Alexander P.S. Hygate, Kathryn Kreckel, Eve Charis Ostriker, Jerome Pety, Miguel Querejeta, Erik Rosolowsky, Karin M. Sandstrom, Antonio Usero

Research output: Contribution to journalArticlepeer-review

100 Scopus citations


We estimate the star formation efficiency per gravitational free-fall time, ϵ ff , from observations of nearby galaxies with resolution matched to the typical size of a giant molecular cloud. This quantity, ϵ 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 Physics at High Angular Resolution in nearby Galaxies-Atacama Large Millimeter Array (PHANGS-ALMA) survey, we estimate the gravitational free-fall time at 60-120 pc resolution, and contrast this with the local molecular gas depletion time in order to estimateϵ ff . Assuming a constant thickness of the molecular gas layer (H = 100 pc) across the whole sample, the median value of ϵ ff in our sample is 0.7%. We find a mild scale dependence, with higherϵ ff measured at coarser resolution. Individual galaxies show different values of ϵ ff , with the median ϵ ff ranging from 0.3% to 2.6%. We find the highest ϵ 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 (LOS) depth through the molecular gas layer and the choice of star formation tracers.

Original languageEnglish (US)
Article numberL18
JournalAstrophysical Journal Letters
Issue number2
StatePublished - Jul 10 2018

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • ISM: molecules
  • galaxies: ISM
  • galaxies: spiral
  • galaxies: star formation


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