2005A&A...431..451V

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 ∼10⁵⁴erg. 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≲100 in the HI column density, whereas single hypernova explosions form rings with K≲10. Only multiple SNe can reproduce both the size (diameter ∼1.7kpc) and the ring-to-center contrast (K∼15-20) of the HI ring in Ho I. High velocity clouds create HI rings that are much smaller in size (≲0.8kpc) and contrast (K≲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., 2001AJ....122.3070O) 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°) the HI image is characterized by two kidney-shaped density enhancements and a mild central depression.