Log-normal star formation histories
The star formation histories (SFHs) of galaxies are one of the most fundamental aspects of galaxy formation, but they are hard to measure observationally because we cannot reliably connect different galaxy populations across time. It is, however, well established now that the global SFH of all galaxies in the universe follows, roughly speaking, a log-normal shape in time with a peak around z = 2. Motivated by this finding, Gladders et al. 2013 asserted that perhaps the SFHs of individual galaxies could also be well described by the log-normal functional form which would indicate a slow decline of star formation rather than rapid quenching.
We tested this assumption using the Illustris simulation and found that the log-normal captures the simulated SFHs surprisingly well. The figure below shows examples of a few types of SFHs in dark blue and the best-fit log-normals in light blue. The SFHs are noisy due to the bursty nature of star formation, but the cumulative SFHs (bottom panels) allow for a more meaningful comparison.
In detail, we find that 85% of the Illustris cumulative SFHs are fit to within 5% or less at all times, and 99% to 10% or less. Importantly, the log-normal outperforms the commonly used delayed-tau model. Poor fits typically correspond to rapidly quenched satellites.
We show that there is a simple, tight relation between the peak time and width of the SFHs, in agreement with the “downsizing” picture of star formation. This relation can qualitatively be explained by the formation histories of the dark matter halos that galaxies inhabit.