How Galaxies Form Stars: The Connection between Local and Global Star Formation in Galaxy Simulations

Abstract

Using a suite of isolated L galaxy simulations, we show that global depletion times and star-forming gas mass fractions in simulated galaxies exhibit systematic and well-defined trends as a function of the local star formation efficiency per freefall time, ff , strength of stellar feedback, and star formation threshold. We demonstrate that these trends can be reproduced and explained by a simple physical model of global star formation in galaxies. Our model is based on mass conservation and the idea of gas cycling between star-forming and non-star-forming states on certain characteristic time scales under the influence of dynamical and feedback processes. Both the simulation results and our model predictions exhibit two limiting regimes with rather different dependencies of global galactic properties on the local parameters. When ff is small and feedback is inefficient, the total star-forming mass fraction, f sf , is independent of ff and the global depletion time, τ dep , scales inversely with ff . When ff is large or feedback is very efficient, these trends are reversed: f sf ∝ −1 ff and τ dep is independent of ff but scales linearly with the feedback strength. We also compare our results with the observed depletion times and mass fractions of star-forming and molecular gas and show that they provide complementary constraints on ff and the feedback strength. We show that useful constraints on ff can also be obtained using measurements of the depletion time and its scatter on different spatial scales.

The [C ii]–SFR correlation in dwarf galaxies across cosmic time

“…Motivated by Semenov et al (2018) and Lupi (2019), we also include an additional fiducial boost to the radial momentum of a factor of two.…”

A simple non-equilibrium feedback model for galaxy-scale star formation: delayed feedback and SFR scatter

“…After all, feedback is not instantaneous with star formation, as ionizing radiation is injected for upwards of 10 Myr (Leitherer et al 1999), supernova feedback is not felt for the first ∼ 5 Myr, and then continues stochastically for ∼ 30 Myr (Agertz et al 2013). The timescales for feedback injection are not coincidentally on the order of the lifetimes of star forming regions themselves in the feedback regulated model (Oklopčić et al 2017;Semenov et al 2018;Grudić et al 2018). Star formation equilibrium need not be expected, even at the 10 6 M giant molecular cloud (GMC) scale.…”

“…steady state) in some averaged sense, that it is locally in some dynamical equilibrium where the ISM is in a constant cycle of collapse, star formation, and cloud destruction/feedback. It is thus never instantaneously in local equilibrium, and is constantly oscillating between those phases (Benincasa et al 2016;Semenov et al 2017Semenov et al , 2018.…”

Forming early-type galaxies without AGN feedback: a combination of merger-driven outflows and inefficient star formation

“…Star formation on large scales could be entirely selfregulated by stellar feedback, as advocated by Ostriker et al (2010) and Hopkins et al (2014). In a seminal albeit recent paper, Semenov et al (2018) have shown that once the star formation efficiency at small scale is large enough (typically larger than 1%), the global depletion time converges to a fixed value set by stellar feedback only (therefore independent of ff ).…”