The recent star formation history of NGC 628 on resolved scales

TL;DR

This study introduces a simple, broadly applicable proxy for recent star formation rate changes in galaxies, calibrated using models and applied to NGC 628, revealing a declining overall star formation activity with localized variations.

Contribution

The paper develops a new, model-independent diagnostic for recent SFR changes based on Hα-FUV color, applicable without detailed spectral energy distribution modeling.

Findings

NGC 628's star formation has been declining overall.

Spiral arms maintain higher recent star formation activity.

Recent star formation correlates with disk pressure and molecular gas content.

Abstract

Star formation histories (SFHs) are integral to our understanding of galaxy evolution. We can study recent SFHs by comparing the star formation rate (SFR) calculated using different tracers, as each probes a different timescale. We aim to calibrate a proxy for the present-day rate of change in SFR, dSFR/dt, which does not require full spectral energy distribution (SED) modeling and depends on as few observables as possible, to guarantee its broad applicability. To achieve this, we create a set of models in CIGALE and define an SFR change diagnostic as the ratio of the SFR averaged over the past 5 and 200 Myr, <SFR5>/<SFR200>, probed by the H$\alpha$-FUV colour. We apply <SFR5>/<SFR200> to the nearby spiral NGC 628 and find that its star formation activity has overall been declining in the recent past, with the spiral arms, however, maintaining a higher level of activity. The impact of the spiral arm structure is observed to be stronger on <SFR5>/<SFR200> than on the star formation efficiency (SFE$_\text{H$_2$}$). In addition, increasing disk pressure tends to increase recent star formation, and consequently <SFR5>/<SFR200>. We conclude that <SFR5>/<SFR200> is sensitive to the molecular gas content, spiral arm structure, and disk pressure. The <SFR5>/<SFR200> indicator is general and can be used to reconstruct the recent SFH of any star-forming galaxy for which H$\alpha$, FUV, and either mid- or far-IR photometry is available, without the need of detailed modeling.