Bloated Dwarfs: The Thickness of the HI Disks in Irregular Galaxies

Somewhat counterintuitively, the gas disks of (dwarf) irregular galaxies, such as the LMC, are thicker than those of normal disk galaxies, both in relative as well as in absolute terms. In retrospect, this is easily understood. The velocity dispersion of the gas throughout galaxy disks (regulated by star forming activity) is similar (6-9 kms-1) in dwarf galaxies and in spirals. The gravitational potential of irregulars is considerably smaller, though, with as a result a thicker or puffed-up gas disk. We will explain several methods which can be used to derive the scaleheight of the gas in irregular galaxies. The larger scaleheight we encounter has consequences for the escape of metals to the halo as a result of supernova explosions within OB associations. It also increases the cross section of irregular galaxies, increasing the probability for intercepting lines of sight towards QSOs.