Star Formation Efficiency per Free-fall Time in nearby Galaxies

• Published in: Astrophysical journal. Letters Vol. 861; no. 2; p. L18
• Main Authors: Utomo, Dyas, Sun, Jiayi, Leroy, Adam K., Kruijssen, J. M. Diederik, Schinnerer, Eva, Schruba, Andreas, Bigiel, Frank, Blanc, Guillermo A., Chevance, Mélanie, Emsellem, Eric, Herrera, Cinthya, Hygate, Alexander P. S., Kreckel, Kathryn, Ostriker, Eve C., Pety, Jerome, Querejeta, Miguel, Rosolowsky, Erik, Sandstrom, Karin M., Usero, Antonio
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 10.07.2018; IOP Publishing; Bristol : IOP Publishing
• Subjects: Angular resolution; Astrophysics; Depletion; Galaxies; galaxies: ISM; galaxies: spiral; galaxies: star formation; Gravitation; Image resolution; ISM: molecules; Line of sight; Milky Way; Molecular clouds; Molecular gases; Physics; Radio telescopes; Star & galaxy formation; Star formation; Stars; Tracers; Uncertainty
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/aacf8f

We estimate the star formation efficiency per gravitational free-fall time, , from observations of nearby galaxies with resolution matched to the typical size of a giant molecular cloud. This quantity, , 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 . Assuming a constant thickness of the molecular gas layer (H = 100 pc) across the whole sample, the median value of in our sample is 0.7%. We find a mild scale dependence, with higher measured at coarser resolution. Individual galaxies show different values of , with the median ranging from 0.3% to 2.6%. We find the highest 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.