The Variability of the Star Formation Rate in Galaxies: I. Star Formation Histories Traced by EW(H$\alpha$) and EW(H${\delta }_A$)

Abstract

To investigate the variability of the star formation rate (SFR) of galaxies, we define a star formation change parameter, SFR${}_{5Myr}$/SFR${}_{800Myr}$ 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 H$\alpha$ emission and H$\delta$ absorption, plus the 4000 A break, with an uncertainty of $\sim$0.07 dex for star-forming galaxies. We then apply this estimator to MaNGA galaxies, both globally within Re and within radial annuli. We find that galaxies with higher global SFR${}_{5Myr}$/SFR${}_{800Myr}$ appear to have higher SFR${}_{5Myr}$/SFR${}_{800Myr}$ 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 SFR${}_{5Myr}$/SFR${}_{800Myr}$ 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. (2019) we showed that the dispersion in star-formation rate surface densities ${\Sigma }_{\mathrm{SFR}}$ 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 SFR${}_{5Myr}$/SFR${}_{800Myr}$ that these effects are indeed due to genuine temporal variations in the SFR of individual galaxies on timescales between $10^7$ and $10^9$ years rather than possibly reflecting intrinsic, non-temporal, differences between different galaxies.

29 pages, 17 figures; ApJ accepted

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