Numerical simulations of expanding supershells in dwarf irregular galaxies

Free access article

Issue		A&A Volume 431, Number 2, February IV 2005


Page(s)		451 - 464
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20041324

A&A 431, 451-464 (2005)
DOI: 10.1051/0004-6361:20041324

II. Formation of giant HI rings

E. I. Vorobyov^{1, 2} and Shantanu Basu²

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 ${\cal K}\la 100$ 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. 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 ( $i>45^\circ$ ) the HI image is characterized by two kidney-shaped density enhancements and a mild central depression.

Key words: galaxies: dwarf -- ISM: bubbles

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