A comparative study of star formation thresholds in gas-rich low surface brightness dwarf galaxies

We present the results of H I synthesis imaging of six Low Surface Brightness Dwarf Galaxies (LSBDGs) and a comparison sample of four ''normal'' gas-rich dwarf galaxies in order to investigate why the star formation process has been inhibited in the LSBDGs. The LSBDGs are defined by unusually high values of M-H/L-B and the presence of extensive low surface brightness structure. The ''normal'' dwarfs were selected to have similar optical properties as the LSBDGs, but had more typical values of M-H/L-B. Both the LSBDGs and normal dwarfs are rotation dominated systems, with V-rot/sigma>5. The HI synthesis maps, in conjunction with H alpha imaging, were used to investigate the locations of the star formation sites with respect to the KI gas and the gravitational potential, Remarkably, despite differences in the star formation efficiencies, as implied by the mean surface brightness and M-H/L-B Of the two samples, no clear distinction is seen in the current star formation activity of LSBDGs and normal dwarfs. The global gas density in all systems is significantly lower than the Toomre instability criterion, suggesting that the star formation process is inefficient in both the normals and LSBDGs. These systems have a moderate amount of H I, but it is spread over a large area, thus reducing the gas surface density. However, local peaks in the H I surface density approach the instability criterion and are correlated with sites of active star formation. Thus, star formation appears to be inhibited globally, but occurs locally in gas-rich dwarf galaxies. (C) 1997 American Astronomical Society.