Superassociations in the Arms of Normal and Active Galaxies

Abstract

The properties of violent star formation regions in the arms of spiral galaxies are reviewed with the aim of providing the foundations on which models of nuclear starbursts can be built. It is argued that the classical examples of extranuclear starbursts, giant HII regions and Superassociations, are closely related but fundamentally different classes of objects; their properties are reviewed and discussed in detail.It is shown that giant HII regions are a homogeneous class of gravitationally bound objects ionized by starburst clusters and that the initial mass functions of these clusters change according to their chemical composition being flatter for metal poor systems. Superassociations are loose associations of associations which generally contain one or more giant HII regions. Star formation in superassociations is self-sustaining and therefore these structures may last much longer than individual giant HII regions.Isolated superassociations or HII galaxies are shown to have the same global properties as giant HII regions in late type galaxies. In particular, the correlations between Hβ luminosity and emission line velocity width are similar for both classes. Since HII galaxies can be observed out to large distances this correlation provides a potentially powerful method to calibrate the extragalactic distance scale. A preliminary calibration gives HO = 95 ± 9 km/sec/Mpc.The relations between the properties of giant HII regions and Superassociations with those of their parent galaxies are briefly discussed.