Testing the Multiple Supernovae versus Gamma‐Ray Burst Scenarios for Giant HiSupershells

The energy source of the giant HI supershells in nearby galaxies and in ourown is still an unsettled issue. Proposed scenarios for production ofnear-complete ringlike supershells are multiple supernovae (SNe) and gamma-raybursts (GRBs). In the late phase of evolution it is difficult to tell thesemodels 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. Wepresent line diagnostics that could detect such unusual abundances, such asunusually high ratios of [O]/[Fe] and [Ne]/[Fe]. Among ions of the sameelement, a higher enhancement is expected in lines from a high-ionization statethan in lines from a low-ionization state. Identification of the energy sourceof HI supershells would set strong constraints on the rates and energetics ofGRBs, as well as on their location within a galaxy.Comment: 10 pages, 3 figure, accepted for publication to Ap