Identification of variability in recent star formation histories of local galaxies based on H$\alpha$/UV ratio

TL;DR

This study examines the variability in recent star formation histories of local galaxies by analyzing Hα/UV flux ratios, revealing how recent star formation activity influences the scatter in the star-forming main sequence.

Contribution

It provides the first large-scale analysis of Hα/UV ratios for local galaxies, linking recent SFHs to main sequence scatter and highlighting the impact of recent star formation variability.

Findings

Galaxies above/below the main sequence have higher/lower Hα/UV ratios.

The Hα/UV ratio correlates with deviations from the main sequence.

Recent SFH diversity contributes to the observed scatter in galaxy main sequence.

Abstract

Because the timescale of H$\alpha$ emission (several tens of Myr) following star formation is significantly shorter than that of UV radiation (a few hundred Myr), the H$\alpha$/UV flux ratio provides insight on the star formation histories (SFHs) of galaxies on timescales shorter than $\sim100$ Myr. We present H$\alpha$/UV ratios for galaxies at $z=$ 0.02--0.1 on the familiar star-forming main sequence based on the AKARI-GALEX-SDSS archive dataset. The data provide us with robust measurements of dust-corrected SFRs in both H$\alpha$ and UV for 1,050 galaxies. The results show a correlation between the H$\alpha$/UV ratio and the deviation from the main sequence in the sense that galaxies above/below the main sequence tend to have higher/lower H$\alpha$/UV ratios. This trend increases the dispersion of the main sequence by 0.04 dex (a small fraction of the total scatter of 0.36 dex), suggesting that diversity of recent SFHs of galaxies has a direct impact on the observed main sequence scatter. We caution that the results suffer from incompleteness and a selection bias which may lead us to miss many sources with high H$\alpha$/UV ratios, this could further increase the scatter from SFHs in the star-forming main sequence.