A Turbulent Framework for Star Formation in High-redshift Galaxies

Abstract Observations of distant galaxies suggest that the physics of galaxy formation at high redshifts differs significantly from later times. In contrast to large, steady disk galaxies like the Milky Way, high-redshift galaxies are often characterized by clumpy, disturbed morphologies and bursty star formation histories. These differences between low-mass, bursty galaxies and higher-mass, steady star-forming galaxies have recently been studied in galaxy formation simulations with resolved multiphase interstellar medium (ISM). These simulation studies indicate that while steady disk galaxies can be well-modeled as “equilibrium disks” embedded in a distinct, hot circumgalactic medium (CGM), bursty galaxies are much more dynamic and their star formation occurs in a dispersion-dominated medium that extends to halo scales, with no clear boundary between the ISM and the CGM. We develop an analytic framework to model star formation in bursty galaxies that are not adequately modeled as equilibrium disks. The framework approximates the gas in low-mass halos as a continuous, supersonically turbulent medium with large density fluctuations. Star formation occurs locally in the high-density tail of a roughly log-normal density distribution. This is analogous to turbulent models of star formation in molecular clouds, but here applied on inner CGM scales. By comparing with galaxy formation simulations from the Feedback in Realistic Environments project, we show that this framework can be used to understand star formation efficiencies (SFEs) and radial profiles in halos. The turbulent framework shows explicitly how the instantaneous galaxy-averaged SFE can be relatively low even if the local efficiency in dense gas approaches unity.