Numerical simulations of expanding supershells in dwarf irregular galaxies - II. Formation of giant HI rings

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 similar to 10(54) 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 K less than or similar to 100 in the HI column density, whereas single hypernova explosions form rings with K less than or similar to 10. Only multiple SNe can reproduce both the size (diameter similar to 1.7 kpc) and the ring-to-center contrast (K similar to 15- 20) of the HI ring in Ho I. High velocity clouds create HI rings that are much smaller in size (less than or similar to 0.8 kpc) and contrast ( K less than or similar to 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. 2001) 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 > 45degrees) the HI image is characterized by two kidney-shaped density enhancements and a mild central depression.