STAR-FORMATION THRESHOLDS IN H-II GALAXIES WITH H-I COMPANIONS

We present high resolution VLA 21 cm line observations of five H II ga laxies combined with previous lower resolution data from Taylor et al [AJ, 105, 128 (1993)] and optical broadband R and Halpha CCD images of the systems. Following Kennicutt [ApJ, 344, 685 (1989)] we have calcu lated the threshold H I surface density for star formation for the H I I galaxies and compared the location and shape of this predicted thres hold density contour with the optical shape of the galaxies. We find g enerally a good correlation between these two, although a constant den sity contour of 10(21) CM-2 fits the images of the optical galaxies eq ually as well. The H I synthesis observations have revealed that the H II galaxies have sharply peaked H I radial profiles, in contrast to t he relatively flattened profiles of low surface brightness (LSB) galax ies, suggesting that large central concentrations of gas are a necessa ry condition for the occurrence of bursts of massive star formation se en in H II galaxies. These observations are consistent with the hypoth esis that LSB galaxies represent the quiescent phase of H II galaxies, if a suitable mechanism exists (such as galaxy interactions) to cause H I to concentrate at the center of LSB galaxies prior to the onset o f the burst of star formation. However, it is noted that H II galaxies (and dwarf galaxies in general) span a relatively large range in mass . Since many properties correlate with mass (e.g., gas mass fraction), we point out that great care needs to be taken in choosing the proper comparison samples of LSB and H II galaxies.