Introduction

Dark Matter (DM) is a ubiquitous constituent of the universe; galaxies of all morphologies are now known to contain it. However, most studies of the distribution of DM about galaxies are weighted towards spiral, irregular, and dwarf irregular (dI) galaxies. This is because these often have extended rotating HI disks which act as excellent dynamical tracers out to the large radii where DM dominates. There is one type of gas rich galaxy that has been largely ignored in DM studies: the blue compact dwarf (BCD) class. In the optical, these galaxies are characterised by blue colours, a high surface brightness, and sharp narrow emission lines: the hallmarks of intense massive star formation. In contrast, dI galaxies have similar luminosities, colours and gas content, but much lower surface brightnesses and larger sizes. Although DM has been inferred from the dynamics of BCDs (Viallefond & Thuan, 1983), the study I describe presents the first detailed mass model decomposition of a BCD rotation curve. Whatever the reasons for neglecting BCDs in the past, I hope this study demonstrates that BCDs are well worthy of further attention, and that they may tell us something new about DM halo properties of galaxies.

The BCD galaxy NGC 2915 has a low surface brightness exponential radial surface brightness profile at large radii, and a high surface brightness blue core at its center (Meurer et al. 1994). Although its appearance is somewhat peculiar (Sérsic et al. 1977), its optically determined properties are not particularly exciting compared to other spectacular examples of the BCD class (e.g. IZw18, IIZw40, NGC 1569, NGC 1705). Becker et al. (1988), in a study of bright southern galaxies, showed that NGC 2915 is extended in HI, has a double horned velocity profile, and a large for its luminosity. It is these properties that made us decide to observe the galaxy with the Australia Telescope Compact Array (ATCA); extended HI distributions invariably reveal the DM halo of galaxies. Some important properties of NGC 2915 are summarized in table 1. Details of our HI study of NGC 2915 can be found in Meurer et al. (1996). Here I will present the main results and their implications.

  
Table 1: Properties of NGC 2915

© Copyright Astronomical Society of Australia 1997