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 Bri ghtness Dwarf Galaxies (LSBDGs) and a comparison sample of four ''norm al'' gas-rich dwarf galaxies in order to investigate why the star form ation 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 va lues of M-H/L-B. Both the LSBDGs and normal dwarfs are rotation domina ted systems, with V-rot/sigma>5. The HI synthesis maps, in conjunction with H alpha imaging, were used to investigate the locations of the s tar formation sites with respect to the KI gas and the gravitational p otential, Remarkably, despite differences in the star formation effici encies, as implied by the mean surface brightness and M-H/L-B Of the t wo samples, no clear distinction is seen in the current star formation activity of LSBDGs and normal dwarfs. The global gas density in all s ystems is significantly lower than the Toomre instability criterion, s uggesting that the star formation process is inefficient in both the n ormals 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. H owever, local peaks in the H I surface density approach the instabilit y criterion and are correlated with sites of active star formation. Th us, star formation appears to be inhibited globally, but occurs locall y in gas-rich dwarf galaxies. (C) 1997 American Astronomical Society.