2004A&A...416..499V

Multiple SN explosions in disk galaxies efficiently evacuate gas and form cavities with the sizes and shapes of the surrounding envelopes determined by the total amount of injected energy and by the initial gas distribution. Such cavities are seen as HI holes when observed in face-on galaxies. Gas hydrodynamics simulations are performed to obtain the quantitative characteristics of HI holes that could serve for the determination of the gas vertical scale height and the corresponding dark matter content and its distribution. Among these characteristics is the ratio of the maximum column density in the HI ring surrounding the hole to the background HI column density and the vertical expansion velocity of gas in the HI ring. We show that in some cases the extragalactic background ionizing radiation may produce HI holes in the outer regions of galaxies, and can account for the existence of HI holes in nearby face-on galaxies with the apparent lack of an underlying stellar population.