KMTNet Nearby Galaxy Survey. III. Deficient Hα Flux in the Extended Disks of Spiral Galaxies

Abstract We perform a deep, wide-field imaging survey of nearby galaxies using Hα and broadband filters to investigate the characteristics of star formation in galaxies. Motivated by the finding that star formation rates (SFRs) derived from Hα 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 Hα fluxes of star-forming regions. We also perform spectral energy distribution (SED) fittings using 11 photometric data points (FUV-to-MIR), including data from the literature, to estimate the local properties such as internal attenuation of individual star-forming clumps. Comparing SFRFUV and SFRHα , which are converted from the Hα and FUV fluxes corrected for the local properties, we find that SFRHα /SFRFUV tend 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 Hα fluxes in the extended disks of galaxies are tightly correlated with recent starbursts. The strong and short starburst, which has been rapidly suppressed over the last 10 Myr, seems to induce a significant discrepancy between the SFRHα and SFRFUV. 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.