Cosmic evolution of the star formation efficiency in Milky Way-like galaxies

Journal article, 2025

Current star formation models are based on the structure of the interstellar medium (ISM), yet the details on how local physics propagates to galactic-scale properties are still debated. To investigate this, we use VINTERGATAN, a high-resolution cosmological zoom-in simulation of a Milky Way-like galaxy. We study how the velocity dispersion and density structure of the cold neutral ISM on 50-100 pc scales evolve with redshift and quantify their impact on the star formation efficiency per free-fall time-scale, is an element of(ff). During starbursts velocity dispersions can reach similar to 50 km s( -1), especially throughout last major merger events (1 . 3 < z < 1 . 5). After a merger-dominated phase (1 < z < 5), VINTERGATAN transitions into evolving secularly, featuring velocity dispersion levels of similar to 10 km s( -1) . Despite strongly evolving density and turbulence distributions over cosmic time, Eff at the resolution limit is found to change by only a factor of a few: from median efficiencies of 0.8 per cent at z > 1 to 0.3 per cent at z < 1. The mass-weighted average shows a universal (is an element of(ff)) approximate to 1 per cent , caused by an almost invariant virial parameter distribution in star-forming clouds. Changes in their density and turbulence levels are coupled, so the kinetic-to-gravitational energy ratio remains close to constant. We show that a theoretically motivated Eff is intrinsically different from its observational estimates adopting tracers of star formation, e.g. H alpha. Since the physics underlying star formation can be lost on short time-scales (similar to 10 Myr), caution must be taken when constraining star formation models from observational estimates of is an element of(ff).