KMTNet Nearby Galaxy Survey III. Deficient H$\alpha$ flux in the Extended Disks of Spiral Galaxies

TL;DR

This study reveals that Hα fluxes are deficient in the extended disks of spiral galaxies, especially during recent starbursts, indicating a discrepancy between Hα and FUV-derived star formation rates linked to rapid starburst suppression.

Contribution

It provides the first spatially resolved analysis showing Hα flux deficiency correlates with recent starbursts in galaxy outskirts, highlighting the impact of rapid star formation changes.

Findings

Hα flux deficiency is tightly linked to recent starbursts.

Discrepancy between SFR indicators increases at lower SFRs.

Recent starbursts in extended disks are likely triggered by internal processes.

Abstract

We perform a deep wide-field imaging survey of nearby galaxies using H$\alpha$ and broadband filters to investigate the characteristics of star formation in galaxies. Motivated by the finding that star formation rates (SFRs) derived from H$\alpha$ fluxes in dwarf galaxies are systematically lower than those inferred from far-ultraviolet (FUV) fluxes, we attempt to determine whether the same trend exists in the extended disks of two star-forming galaxies. We perform spatially resolved photometry using grid-shaped apertures to measure the FUV and H$\alpha$ fluxes of star-forming regions. We also perform spectral energy distribution (SED) fittings using 11 photometric data (FUV-to-MIR) including data from the literature to estimate the local properties such as internal attenuation of individual star-forming clumps. Comparing SFR$_\mathrm{FUV}$ and SFR$_\mathrm{H\alpha}$, which are converted from the H$\alpha$ and FUV fluxes corrected for the local properties, we find that SFR$_\mathrm{H\alpha}$/SFR$_\mathrm{FUV}$ tends to decrease as the SFR decreases. We evaluate possible causes of this discrepancy between the two SFRs by restricting parameters in the SED fitting and conclude that deficient H$\alpha$ fluxes in the extended disks of galaxies are tightly correlated with recent starbursts. The strong and short starburst which is being rapidly suppressed over the last 10 Myr seems to induce a significant discrepancy between the SFR$_\mathrm{H\alpha}$ and SFR$_\mathrm{FUV}$. In addition, the recent bursts in the extended disks of galaxies appear to have occurred azimuth-symmetrically, implying that these were likely triggered by gas accretion or internal processes rather than external perturbation.