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Star Formation Efficiency per Free-fall Time in nearby Galaxies

Author(s): Utomo, Dyas; Sun, Jiayi; Leroy, Adam K; Kruijssen, JM Diederik; Schinnerer, Eva; et al

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Abstract: 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.
Publication Date: 10-Jul-2018
Electronic Publication Date: 11-Jul-2018
Citation: Utomo, Dyas, Sun, Jiayi, Leroy, Adam K, Kruijssen, JM Diederik, Schinnerer, Eva, Schruba, Andreas, Bigiel, Frank, Blanc, Guillermo A, Chevance, Melanie, Emsellem, Eric, Herrera, Cinthya, Hygate, Alexander PS, Kreckel, Kathryn, Ostriker, Eve C, Pety, Jerome, Querejeta, Miguel, Rosolowsky, Erik, Sandstrom, Karin M, Usero, Antonio. (2018). Star Formation Efficiency per Free-fall Time in nearby Galaxies. ASTROPHYSICAL JOURNAL LETTERS, 861 (10.3847/2041-8213/aacf8f
DOI: doi:10.3847/2041-8213/aacf8f
ISSN: 2041-8205
EISSN: 2041-8213
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Type of Material: Journal Article
Version: Final published version. Article is made available in OAR by the publisher's permission or policy.

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