The Variability of the Star Formation Rate in Galaxies. I. Star Formation Histories Traced by EW(Hα) and EW(Hδ_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 Hα emission and Hδ absorption, plus the 4000Å 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 SFR 5Myr /SFR 800Myr , which indicates by how much a galaxy has changed its specific SFR (sSFR) is nearly independent of its sSFR, i.e. of its the position relative to the star formation main sequence (SFMS) as defined by SFR 800Myr . 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 SFR 5Myr /SFR 800Myr 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 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 Σ 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.

The diversity and variability of star formation histories in models of galaxy evolution

“…The rest-UV portion of the SED contains contributions from young stars that probe the SFR out to ∼ 30 − 100 Myr, with a similar timescale probed by the rest-FIR portion of the SED, which contains the re-emitted light from the young stellar light absorbed by dust. In addition to this, features like the strength of Hδ absorption and the 4000Å break are sensitive to SFR within the last ∼ 1 Gyr and to the light-weighted age within ∼ 2 Gyr, respectively (Kauffmann et al 2006;Wang & Lilly 2020b).…”

“…tional constraints in this space (Caplar & Tacchella 2019;Wang & Lilly 2020b) would provide strong constraints on modelling galaxy physics.…”

“…The term 'burstiness' is loosely used to denote variability in SFR across a range of timescales in the literature(Weisz et al 2011b;Guo et al 2016;Matthee & Schaye 2019;Wang & Lilly 2020b). With this in mind, we preface the term with an appropriate timescale range whenever used.MNRAS 000, 1-35 (2020)…”

H-alpha emission in local galaxies: star formation, time variability and the diffuse ionized gas

“…Understanding the star-formation timescale that H𝛼 traces is of great importance since by comparing H𝛼-based SFRs to SFRs obtained with other indicators (for example from the UV continuum), we are able to learn about the variability of star formation (e.g., Sparre et al 2017;Caplar & Tacchella 2019;Emami et al 2019). The variability of star formation (sometimes also called "burstiness") is directly related to how star formation is regulated and it informs us as to what timescales are important for the evolution of galaxies (Iyer et al 2020;Tacchella et al 2020;Wang & Lilly 2020).…”

Surrogate modelling the Baryonic Universe II: on forward modelling the colours of individual and populations of galaxies

“…Although short-term star formation fluctuations have been detected in observations (Weisz et al 2012;Guo et al 2016;Caplar & Tacchella 2019;Broussard et al 2019;Wang & Lilly 2020a;Wang & Lilly 2020b) and are a ubiquitous prediction of galaxy formation models (Sparre et al 2017;Iyer et al 2020), such features are not captured by many of the widely-used SFH models considered by SPS codes. Motivated by this, we study how time variability in galaxy properties affects the synthesis of broad-band optical and near-infrared colours.…”