DIISC-V: Variations in H$\alpha$-to-FUV Star Formation Rate Ratios Across Star-forming Regions in Nearby Galaxies

Abstract

We present the variations in far-ultraviolet (FUV) and H$\alpha$ star formation rates (SFR), SFR$_{UV}$ and SFR$_{H\alpha}$, respectively, at sub-kpc scales in 11 galaxies as part of the Deciphering the Interplay between the Interstellar Medium, Stars, and the Circumgalactic medium (DIISC) survey. Using archival GALEX FUV imagery and H$\alpha$+[NII] narrowband images obtained with the Vatican Advanced Technology Telescope, we detect a total of 1335 (FUV-selected) and 1474 (H$\alpha$-selected) regions of recent high-mass star formation, respectively. We find the H$\alpha$-to-FUV SFR ratios tend to be lower primarily for FUV-selected regions, where SFR$_{H\alpha}$ generally underestimates the SFR by an average factor of 2-3, for SFR $<$ 10$^{-4}$ M$_{\odot}$ yr$^{-1}$. In contrast, the SFRs are generally observed to be consistent for H$\alpha$-selected regions. This discrepancy arises from morphological differences between the two indicators. Extended FUV morphologies and larger areas covered by FUV-only regions, along with decreasing overlap between FUV clumps and compact HII regions with $R/R_{25}$ suggest that stochastic sampling of the initial mass function may be more pronounced in the outer regions of galaxies. Our observed H$\alpha$-to-FUV SFR ratios are also consistent with stochastic star formation model predictions. However, using larger apertures that include diffuse FUV emission results in an offset of 1 dex between SFR$_{H\alpha}$ and SFR$_{UV}$. This suggests that the observed low H$\alpha$-to-FUV SFR ratios in galaxies are likely caused by diffuse FUV emission, which can contribute $\sim$60-90\% to the total FUV flux.