Numerical simulations of expanding supershells in dwarf irregular galaxies

II. Formation of giant HI rings

¹ Institute of Physics, Stachki 194, Rostov-on-Don, Russia e-mail: eduard_vorobev@mail.ru
² Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada e-mail: basu@astro.uwo.ca

Received: 19 May 2004
Accepted: 14 October 2004

Abstract

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 in the HI column density, whereas single hypernova explosions form rings with . Only multiple SNe can reproduce both the size (diameter ~ kpc) and the ring-to-center contrast () of the HI ring in Ho I. High velocity clouds create HI rings that are much smaller in size ( kpc) and contrast () 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 () the HI image is characterized by two kidney-shaped density enhancements and a mild central depression.