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

ABSTRACT 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, $\epsilon _{\rm ff}$ . During starbursts velocity dispersions can reach $\sim$50 $\mathrm{ km\ s^{-1}}$, especially throughout last major merger events ($1.3\lt z\lt 1.5$). After a merger-dominated phase ($1\lt z\lt 5$), VINTERGATAN transitions into evolving secularly, featuring velocity dispersion levels of $\sim$10 $\mathrm{ km\ s^{-1}}$. Despite strongly evolving density and turbulence distributions over cosmic time, $\epsilon _{\rm ff}$ 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 $z\lt 1$. The mass-weighted average shows a universal $\langle \epsilon _{\rm ff} \rangle \approx 1~{{\ \rm per\ cent}}$, 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 $\epsilon _{\rm ff}$ 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 ($\sim$10 Myr), caution must be taken when constraining star formation models from observational estimates of $\epsilon _{\rm ff}$ .