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

Abstract To investigate the variability of the star formation rate (SFR) of galaxies, we define a star formation change parameter, SFR5 Myr/SFR800 Myr, which is the ratio of the SFR averaged within the last 5 Myr to the SFR averaged within the last 800 Myr. We show that this parameter can be determined from a combination of Hα emission and Hδ absorption, plus the 4000 Å break, with an uncertainty of ∼0.07 dex for star-forming galaxies. We then apply this estimator to MaNGA galaxies, both globally within R e and within radial annuli. We find that the global SFR5 Myr/SFR800 Myr, which indicates by how much a galaxy has changed its specific SFR (sSFR), is nearly independent of its sSFR, i.e., of its position relative to the star formation main sequence (SFMS) as defined by SFR800 Myr. Also, at any sSFR, there are as many galaxies increasing their sSFR as decreasing it, as required if the dispersion in the SFMS is to stay the same. The SFR5 Myr/SFR800 Myr of the overall galaxy population is very close to that expected for the evolving main sequence. Both of these provide a reassuring check on the validity of our calibration of the estimator. We find that galaxies with higher global SFR5 Myr/SFR800 Myr appear to have higher SFR5 Myr/SFR800 Myr at all galactic radii, i.e., that galaxies with a recent temporal enhancement in overall SFR have enhanced star formation at all galactic radii. The dispersion of the SFR5 Myr/SFR800 Myr at a given relative galactic radius and a given stellar mass decreases with the (indirectly inferred) gas depletion time: locations with short gas depletion time appear to undergo bigger variations in their star formation rates on Gyr or less timescales. In Wang et al., we showed that the dispersion in star formation rate surface densities ΣSFR in the galaxy population appears to be inversely correlated with the inferred gas depletion timescale and interpreted this in terms of the dynamical response of a gas-regulator system to changes in the gas inflow rate. In this paper, we can now prove directly with SFR5 Myr/SFR800 Myr that these effects are indeed due to genuine temporal variations in the SFR of individual galaxies on timescales between 107 and 109 yr rather than possibly reflecting intrinsic, non-temporal, differences between different galaxies.