Journal
ASTROPHYSICAL JOURNAL LETTERS
Volume 861, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/2041-8213/aacf8f
Keywords
galaxies: ISM; galaxies: spiral; galaxies: star formation; ISM: molecules
Categories
Funding
- National Science Foundation [1615105, 1615109, 1653300]
- German Research Foundation (DFG) [KR4801/1-1]
- European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG [714907]
- European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program [694343]
- CONICYT/FONDECYT, Programa de Iniciacion, Folio [11150220]
- European Union's Horizon 2020 research and innovation program [726384-EMPIRE]
- DFG Priority Program [KR 4598/1-2, 1573]
- NSF [1713949]
- Natural Sciences and Engineering Research Council of Canada (NSERC) [RGPIN-2017-03987]
- Spanish MINECO [ESP2015-68964, AYA2016-79006]
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [1713949] Funding Source: National Science Foundation
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We estimate the star formation efficiency per gravitational free-fall time, epsilon(ff), from observations of nearby galaxies with resolution matched to the typical size of a giant molecular cloud. This quantity, epsilon(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 estimate epsilon(ff). Assuming a constant thickness of the molecular gas layer (H = 100 pc) across the whole sample, the median value of epsilon(ff) in our sample is 0.7%. We find a mild scale dependence, with higher epsilon(ff) measured at coarser resolution. Individual galaxies show different values of epsilon(ff), with the median epsilon(ff) ranging from 0.3% to 2.6%. We find the highest epsilon(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.
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