Testing the multiple supernovae versus gamma-ray burst scenarios for giant H I supershells

The energy source of the giant H I supershells in nearby galaxies and in our own is still an unsettled issue. Proposed scenarios for production of near-complete ringlike supershells are multiple supernovae (SNe) and γ-ray bursts (GRBs). In the late phase of evolution it is difficult to tell these models apart. We show that, if a supershell has been powered by multiple SNe, peculiar metal abundances should be found in the medium within the bubble. We present line diagnostics that could detect such unusual abundances, such as unusually high ratios of [O]/[Fe] and [Ne]/[Fe]. Among ions of the same element, a higher enhancement is expected in lines from a high-ionization state than in lines from a low-ionization state. Identification of the energy source of H I supershells would set strong constraints on the rates and energetics of GRBs, as well as on their location within a galaxy.