Reconciling extragalactic star formation efficiencies with theory: Insights from PHANGS

¹ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium
² Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str 2, D-69120 Heidelberg, Germany
³ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁵ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA
⁶ Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
⁷ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
⁸ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 43 SE-221 00 Lund, Sweden
⁹ European Southern Observatory, Karl-Schwarzschild Straße 2, D-85748 Garching bei München, Germany
¹⁰ Univ Lyon, Univ Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, F-69230 Saint-Genis-Laval, France
¹¹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹² Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹³ Department of Physics, University of Alberta, Edmonton AB T6G 2E1, Canada
¹⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
¹⁵ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁶ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹⁷ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
¹⁸ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁹ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
²⁰ CNRS, IRAP, 9 Av. du Colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France
²¹ Université de Toulouse, UPS-OMP, IRAP, F-31028 Toulouse Cedex 4, France
²² NRAO National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville VA 22903, USA
²³ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, E-28014 Madrid, Spain
²⁴ SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
²⁵ Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

^⋆ Corresponding author.

Received: 20 December 2024
Accepted: 23 May 2025

Abstract

New extragalactic measurements of the cloud population-averaged star formation efficiency per free-fall time, ϵ_ff, from PHANGS show little sign of a theoretically predicted dependence on the gas virial level and weak variation with cloud-scale gas velocity dispersion. We explore ways to bring theory into consistency with the observations, particularly by highlighting systematic variations in internal density structure that must accompany an increase in virial parameter typically found toward denser galaxy centers. To introduce these variations into conventional turbulence-regulated star formation models, we adopted three adjustments, all motivated by the expectation that the background host galaxy has an influence on the cloud scale: (1) We incorporate self-gravity and an internal density distribution that contains a broad power-law (PL) component and resembles the structure observed in local resolved clouds; (2) We allow the internal gas kinematics to include motion in the background potential and let this regulate the onset of self-gravitation; (3) We assume that the distribution of gas densities is in a steady state for only a fraction of a cloud free-fall time. In practice, these changes significantly reduce the efficiencies predicted in multi-free-fall (MFF) scenarios compared to purely lognormal probability density functions (PDFs) and tie efficiency variations to variations in the slope of the PL α. We fit the model to PHANGS measurements of ϵ_ff to identify the PL slopes that yield an optimal match. These slopes vary systematically with galactic environment in the sense that gas that sits furthest from virial balance contains fractionally more gas at high density. We relate this to the equilibrium response of gas in the presence of the galactic gravitational potential, which forces more gas to high density than characteristic of fully self-gravitating clouds. Viewing the efficiency variations as originating with time evolution in the PL slope, our findings would alternatively imply coordination of the cloud evolutionary stage within environment. With this “galaxy regulation” behavior included, our preferred “self-gravitating” multi-freefall sgMFF models function similarly to the original, roughly “virialized cloud” single-free-fall models. However, outside the environment of disks with their characteristic regulation, the flexible MFF models may be better suited.