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Understanding the variability of galaxy star formation histories (SFHs)across a range of timescales provides insight into the underlying physicalprocesses that regulate star formation within galaxies. We compile the SFHs ofgalaxies at $z=0$ from an extensive set of models, ranging from cosmologicalhydrodynamical simulations (Illustris, IllustrisTNG, Mufasa, Simba, EAGLE),zoom simulations (FIRE-2, g14, and Marvel/Justice League), semi-analytic models(Santa Cruz SAM) and empirical models (UniverseMachine), and quantify thevariability of these SFHs on different timescales using the power spectraldensity (PSD) formalism. We find that the PSDs are well described by brokenpower-laws, and variability on long timescales ($\gtrsim1$ Gyr) accounts formost of the power in galaxy SFHs. Most hydrodynamical models show increasedvariability on shorter timescales ($\lesssim300$ Myr) with decreasing stellarmass. Quenching can induce $\sim0.4-1$ dex of additional power on timescales$>1$ Gyr. The dark matter accretion histories of galaxies have remarkablyself-similar PSDs and are coherent with the in-situ star formation ontimescales $>3$ Gyr. There is considerable diversity among the different modelsin their (i) power due to SFR variability at a given timescale, (ii) amount ofcorrelation with adjacent timescales (PSD slope), (iii) evolution of medianPSDs 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 modelpredictions vary widely. Observational constraints in this space will helpconstrain the relative strengths of the physical processes responsible for thisvariability.

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