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تسجيل مستخدم جديدNumerical simulations of expanding supershells in dwarf irregular galaxies. II. Formation of giant HI rings
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We perform numerical hydrodynamic modeling of various physical processes that can form an HI ring as is observed in Holmberg 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 10^54 ergs. 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 ~1.7 kpc) and the ring-to-center contrast (K ~ 15-20) of the HI ring in Hoolmberg I. High velocity clouds create HI rings that are much smaller in size (< 0.8 kpc) and contrast (K < 4.5) than seen in Holmberg 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 Holmberg I are best reproduced by multiple SNe. We conclude that the giant HI ring in Holmberg 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 deg) the HI image is characterized by two kidney-shaped density enhancements and a mild central depression.
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