A Deeper Look at Faint H$\alpha$ Emission in Nearby Dwarf Galaxies

TL;DR

This study uses deep Hα imaging to detect faint ionized emission in nearby dwarf galaxies, finding that previous observations missed only a small fraction of flux, thus not explaining the declining Hα/FUV ratio with galaxy properties.

Contribution

It provides the deepest Hα observations of dwarf galaxies to date, revealing extended emission that minimally impacts the Hα/FUV ratio trends.

Findings

Detected extended Hα emission up to 2.5 times larger than previous observations.

Found only about 5% additional Hα flux compared to standard surveys.

Confirmed that missing faint emission does not explain the Hα/FUV ratio decline.

Abstract

We present deep H$\alpha$ imaging of three nearby dwarf galaxies, carefully selected to optimize observations with the Maryland-Magellan Tunable Filter (MMTF) on the Magellan 6.5m telescope. An effective bandpass of $\sim$13\AA\ is used, and the images reach 3$\sigma$ flux limits of $\sim$8$\times10^{-18}$ ergs s$^{-1}$ cm$^{-2}$, which is about an order of magnitude lower than standard narrowband observations obtained by the most recent generation of local H$\alpha$ galaxy surveys. The observations were originally motivated by the finding that the H$\alpha$/FUV flux ratio of galaxies systematically declines as global galactic properties such as the star formation rate and stellar mass decrease. The three dwarf galaxies selected for study have star formation rates, that when calculated from their H$\alpha$ luminosities using standard conversion recipes, are $\sim$50\% of those based on the FUV. Follow-up studies of many of the potential causes for the trends in the H$\alpha$/FUV flux ratio have been performed, but the possibility that previous observations have missed a non-negligible fraction of faint ionized emission in dwarf galaxies has not been investigated. The MMTF observations reveal both diffuse and structured H$\alpha$ emission (filaments, shells, possible single-star HII regions) spanning extents up to 2.5 times larger relative to previous observations. However, only up to an additional $\sim5$\% of H$\alpha$ flux is captured, which does not account for the trends in the H$\alpha$/FUV ratio. Beyond investigation of the H$\alpha$/FUV ratio, the impact of the newly detected extended flux on our understanding of star formation, the properties of HII regions, and the propagation of ionizing photons warrant further investigation.