A DEEPER LOOK AT FAINT Hα EMISSION IN NEARBY DWARF GALAXIES

We present deep Hα imaging of three nearby dwarf galaxies, carefully selected to optimize observations with the Maryland-Magellan Tunable Filter (MMTF) on the Magellan 6.5 m telescope. An effective bandpass of ∼13 Å is used, and the images reach 3σ flux limits of ∼8 × 10−18 erg s−1 cm−2, which is about an order of magnitude lower than standard narrowband observations obtained by the most recent generation of local Hα galaxy surveys. The observations were originally motivated by the finding that the Hα/FUV flux ratio of galaxies systematically declines as global galactic properties such as the star formation rate (SFR) and stellar mass decrease. The three dwarf galaxies selected for study have SFRs that, when calculated from their Hα luminosities using standard conversion recipes, are ∼50% of those based on the FUV. Follow-up studies of many of the potential causes for the trends in the Hα/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 Hα emission (filaments, shells, possible single-star H ii regions) spanning extents up to 2.5 times larger relative to previous observations. However, only up to an additional ∼5% of Hα flux is captured, which does not account for the trends in the Hα/FUV ratio. Beyond investigation of the Hα/FUV ratio, the impact of the newly detected extended flux on our understanding of star formation, the properties of H ii regions, and the propagation of ionizing photons warrant further investigation.