The Diversity and Variability of Star Formation Histories in Models of Galaxy Evolution

Understanding the variability of galaxy star formation histories (SFHs) across a range of timescales provides insight into the underlying physical processes that regulate star formation within galaxies. We compile the SFHs of galaxies at z = 0 from an exten-sive set of models, ranging from cosmological hydrodynamical simulations (Illustris, IllustrisTNG, Mufasa, Simba, EAGLE), zoom simulations (FIRE-2, g14, and Mar-vel/Justice League), semi-analytic models (Santa Cruz SAM) and empirical models (UniverseMachine), and quantify the variability of these SFHs on dierent timescales using the power spectral density (PSD) formalism. We nd that the PSDs are well described by broken power-laws, and variability on long timescales (& 1 Gyr) accounts for most of the power in galaxy SFHs. Most hydrodynamical models show increased variability on shorter timescales (. 300 Myr) with decreasing stellar mass. Quenching can induce ~ 0:4 - 1 dex of additional power on timescales > 1 Gyr. The dark matter accretion histories of galaxies have remarkably self-similar PSDs and are coherent
with the in-situ star formation on timescales > 3 Gyr. There is considerable diversity among the dierent models in their (i) power due to SFR variability at a given timescale, (ii) amount of correlation with adjacent timescales (PSD slope), (iii) evolution of median PSDs with stellar mass, and (iv) presence and locations of breaks in the PSDs. The PSD framework is a useful space to study the SFHs of galaxies since model predictions vary widely. Observational constraints in this space will help constrain the relative strengths of the physical processes responsible for this variability.