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

TL;DR

This study introduces a new method to measure recent star formation variability in galaxies using spectral features, revealing that galaxies with rapid recent star formation changes also show larger variations across their structures.

Contribution

The paper presents a novel estimator for recent star formation rate variability based on spectral indices, applied to MaNGA data, confirming genuine temporal SFR variations on 10-1000 million year timescales.

Findings

Galaxies with recent SFR enhancements show increased star formation at all radii.

Variations in SFR are linked to gas depletion timescales, with shorter timescales showing larger fluctuations.

The estimator accurately measures SFR variability with an uncertainty of about 0.07 dex.

Abstract

To investigate the variability of the star formation rate (SFR) of galaxies, we define a star formation change parameter, SFR$_{\rm 5Myr}$/SFR$_{\rm 800Myr}$ 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$0.07 dex for star-forming galaxies. We then apply this estimator to MaNGA galaxies, both globally within Re and within radial annuli. We find that galaxies with higher global SFR$_{\rm 5Myr}$/SFR$_{\rm 800Myr}$ appear to have higher SFR$_{\rm 5Myr}$/SFR$_{\rm 800Myr}$ 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$_{\rm 5Myr}$/SFR$_{\rm 800Myr}$ 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 $\Sigma_{\rm SFR}$ 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$_{\rm 5Myr}$/SFR$_{\rm 800Myr}$ 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.