tokens: times. No physical prescriptions govern these behaviors, only the universe’s observed SFH and the end-states of a set of real objects. In this sense, the G13 “model” is not a model at all, but a realization of possible lognormal SFHs for a sample of galaxies. As such, all physics must be inferred from the resulting distribution, or read into the form of the SFH. Sections 3.2 , 3.3 , and 4 revisit this point. Note: Given its mass limit, G13 does not describe the SFHs of local dwarfs (though neither does it contain any). Unlike giant systems (Rodighiero et al. 2011 ), these are known to grow via stochastic bursts (Weisz et al. 2014 ) which a single lognormal cannot capture. Hence, the following discussion is likely not applicable to them. Our principal interest lies in understanding what shapes the longer-term growth modes of galaxies that have consistently dominated the universe’s mass and budget (Section 2.1 ). 2.3. Previous Descriptive Successes In G13 , we showed that the model naturally reproduced the population of galaxies O13 (and subsequently Dressler et al. 2016 ) found to have both late-peaking and narrow SFHs, something impossible to do using τ or delayed- τ SFH parameterizations (Figures 2 and 9 ; O13 ). This encouraged us that the G13 model represented not only an improved description, but perhaps a physically meaningful worldview. More encouragement has come as the model continues to reproduce a range of observations it was neither constrained by nor intended to describe. We explore most of these results below, but review here what we have demonstrated to date: 1. Constrained only by the cosmic SFRD and input galaxies, G13 (Figure 7 ) showed that the model faithfully reproduced distributions at four epochs at . As O13 did for τ /delayed- τ models, G13 showed that Gaussian SFHs failed this test badly. 2. Turning the distributions into modeling constraints and refitting for each galaxy’s , G13 (Figure 13 ) showed that the model reproduced the – relation (the “Star Formation Main Sequence,” or “SFMS”) from Daddi et al. ( 2007 ). 9 9 The agreement with distributions achieved by the initial model suggests that the SFMS could have been reproduced without turning these into constraints. We have not performed this test, however. 3. Similarly constrained at , Abramson et al. ( 2015 ) showed that the model reproduced the evolution of the stellar mass functions of star-forming and non-star-forming galaxies at . This was despite the fact that no mass-sensitive constraints were ever imposed except via the intrinsic mass distribution of the input data. These achievements by no means make the G13 model uniquely successful: others are equally capable (e.g., Sparre et al. 2015 ). However , as G13 entails no explicit treatment of any physical process, we are spurred to investigate what makes this simple mathematical description an accurate expression of complex, poorly understood underlying physics. 2.4. New Challenges Here, we test the G13
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