Log-normal Star Formation Histories in Simulated and Observed Galaxies

Abstract

Gladders et al. have recently suggested that the star formation histories (SFHs) of individual galaxies are characterized by a log-normal function in time, implying a slow decline rather than rapid quenching. We test their conjecture on theoretical SFHs from the cosmological simulation Illustris and on observationally inferred SFHs. While the log-normal form necessarily ignores short-lived features such as starbursts, it fits the overall shape of the majority of SFHs very well. In particular, 85% of the cumulative SFHs are fitted to within a maximum error of 5% of the total stellar mass formed, and 99% to within 10%. The log-normal performs systematically better than the commonly used delayed-τ model, and is superseded only by functions with more than three free parameters. Poor fits are mostly found in galaxies that were rapidly quenched after becoming satellites. We explore the log-normal parameter space of normalization, peak time, and full width at half maximum, and find that the simulated and observed samples occupy similar regions, though Illustris predicts wider, later-forming SFHs on average. The ensemble of log-normal fits correctly reproduces complex metrics such as the evolution of Illustris galaxies across the star formation main sequence, but overpredicts their quenching timescales. SFHs in Illustris are a diverse population not determined by any one physical property of galaxies, but follow a tight relation, where µ ( ) width peak time 3 2 . We show that such a relation can be explained qualitatively (though not quantitatively) by a close connection between the growth of dark matter halos and their galaxies.

Recalibrating the cosmic star formation history

“…A number of explanations for this discrepancy have been suggested in the literature, such as incorrect stel-lar mass measurements, dust corrections, a variable IMF, or incorrect inferred metallicities (Yu & Wang 2016). The inferred CSFRD is also in tension with that produced by recent numerical simulations, which tend to underestimate the normalisation at cosmic noon (Davé 2008;Furlong et al 2015;Diemer et al 2017). We here explore the impact on this discrepancy of updated SFR calibrations.…”

The Variability of the Star Formation Rate in Galaxies. I. Star Formation Histories Traced by EW(Hα) and EW(HδA)

“…In total, there are 29,203 galaxies in the Illustris simulation with stellar mass greater than 10 9 M ⊙ (Diemer et al 2017).…”

“…The change in SFR, and thus in SFR79 due to maintaining a constant ∆SFR 800Myr , is negligible. Right panel: the relation between the SFR79-SFR9/M * in logarithmic space for the best-fit log-normal SFHs of the SF galaxies (sSFR7 > 10 −2 [Gyr −1 ]) selected from Illustris from Diemer et al (2017). Using the best-fit log-normal SFHs smooths out all short-term variations of the SFHs.…”

Stellar Mass Function of Active and Quiescent Galaxies via the Continuity Equation

“…All these shapes can be useful to describe the star-formation history averaged over the entire population of a galaxy survey; however, chemical and photometric data require to differentiate between disc-like and spheroid-like star-formation histories, making the parametric models for each class (e.g., see Fig. 2 in Diemer et al 2017) essentially indistinguishable from our simple adopted shapes. For example, to describe the history of a starforming disc the timescale of the early rise has to be much faster than that of the late decline, to mirror the exponential model of Eq.…”

“…A classic way involves the so-called 'delayed exponential' model M ⋆ (τ ) ∝ τ κ e −τ /τ⋆ , with two parameters κ and τ ⋆ controlling the early powerlaw rise and the late exponential decline. More recently, analogy with the behavior of the cosmic SFR density (see Gladders et al 2013) and indications from numerical simulations (see Diemer et al 2017) have suggested a lognormal shapeṀ…”