Decoding the variability in the star formation histories of <i>z</i> ∼ 0.8 galaxies

ABSTRACT The scatter of the star-forming main sequence (SFMS) holds a wealth of information about how galaxies evolve. The time-scales encoded in this scatter can provide valuable insight into the relative importance of the physical processes regulating star formation. In this paper, we present a detailed observational analysis of the time-scales imprinted in galaxy star formation history (SFH) fluctuations by using the stochastic SFH model to fit 1928 massive, $z\sim 0.8$ galaxies in the Large Early Galaxy Astrophysics Census survey. We find that the total intrinsic scatter of the SFMS is ${\sim} 0.3$ dex in galaxies with stellar masses ${\gtrsim} 10^{10}\,\rm {M}_\odot$. This scatter decreases as the time-scale over which star formation rates (SFRs) are averaged increases, declining to a non-negligible ${\sim} 0.15{\!-\!}0.25$ dex at 2 Gyr, underscoring the importance of long-time-scale SFH diversity to the SFMS scatter. Furthermore, galaxies currently above (below) the SFMS tend to have been above (below) the SFMS for at least ${\sim} 1$ Gyr, providing evidence that individual galaxies may follow different median tracks through SFR${\!-\!}\mathrm{\mathit{ M}}_*$ space. On shorter time-scales (${\sim} 30{\!-\!}100$ Myr), galaxies’ SFRs also vary of the order of ${\sim} 0.1{\!-\!}0.2$ dex. Our work supports the idea that the SFMS emerges from a population average of the pathways that individual galaxies trace through the SFR${\!-\!}\mathrm{\mathit{ M}}_*$ plane. The scatter reflects the long-term heterogeneity of these paths likely set by the evolutionary time-scales of halo growth and cooling, accentuated by short-term variations reflecting the dynamical time-scale of the galaxy and its interstellar medium. Our results emphasize the dynamic nature of the SFMS and the importance of understanding the diverse processes governing star formation.