Numerical simulations of expanding supershells in dwarf irregular galaxies

We perform numerical hydrodynamic modeling of various physical processes that can form an HI ring as is observed in Holmberg I (Ho I). Three energetic mechanisms are considered: multiple supernova explosions (SNe), a hypernova explosion associated with a gamma ray burst (GRB), and the vertical impact of a high velocity cloud (HVC). The total released energy has an upper limit of ~1054 erg. We find that multiple SNe are in general more effective in producing shells that break out of the disk than a hypernova explosion of the same total energy. As a consequence, multiple SNe form rings with a high ring-to-center contrast ${\cal K}\la 100$ in the HI column density, whereas single hypernova explosions form rings with ${\cal K}\la 10$. Only multiple SNe can reproduce both the size (diameter ~$ 1.7$ kpc) and the ring-to-center contrast (${\cal K}\sim 15{-}20$) of the HI ring in Ho I. High velocity clouds create HI rings that are much smaller in size ($\la$$ 0.8$ kpc) and contrast (${\cal K}\la 4.5$) than seen in Ho I. We construct model position–velocity (pV) diagrams and find that they can be used to distinguish among different HI ring formation mechanisms. The observed pV-diagrams of Ho I (Ott et al. [CITE]) are best reproduced by multiple SNe. We conclude that the giant HI ring in Ho I is most probably formed by multiple SNe. We also find that the appearance of the SNe-driven shell in the integrated HI image depends on the inclination angle of the galaxy. In nearly face-on galaxies, the integrated HI image shows a ring of roughly constant HI column density surrounding a deep central depression, whereas in considerably inclined galaxies ($i>45^\circ$) the HI image is characterized by two kidney-shaped density enhancements and a mild central depression