Kinematic constraints on evolutionary scenarios for blue compact dwarf galaxies. I. Neutral gas dynamics

We present the results of high spatial resolution, H I synthesis observatio ns of six blue compact dwarf (BCD) galaxies. Optically, the selected galaxi es have smooth, symmetric isophotes and thus are the most likely of the BCD class to fade into an object morphologically similar to a dwarf elliptical when the current starburst ends. The neutral gas in all six galaxies appea rs to be rotationally supported, however, indicating that true morphologica l transformation from a BCD to a dE will require significant loss of angula r momentum. Based on the observed neutral gas dynamics of these and other B CDs, it is unlikely that present-day BCDs will evolve directly into dwarf e llipticals after a starburst phase. We discuss alternative evolutionary sce narios for BCDs and place them within the larger context of galaxy formatio n and evolution models. In general, BCDs appear to have steeper rotation cu rves than similar luminosity, low surface brightness dwarf galaxies. BCDs h ave centrally concentrated mass distributions (stars, gas, and dark matter) and have lower specific angular momenta. Based on disk instability analyse s, steeply rising rotation curves result in higher threshold densities for the onset of star formation. These results suggest that angular momentum ma y play a crucial role in the morphological evolution of low-mass galaxies: galaxies with low angular momenta will be able to collapse into small, comp act galaxies, while galaxies with high angular momenta will be more diffuse systems. Furthermore, because the star formation threshold density is high er in low angular momenta systems, star formation will be delayed until an extremely high surface density is reached. Thus, angular momentum may be th e fundamental parameter that determines whether a low-mass galaxy will have centrally concentrated stellar and gaseous distributions and be more susce ptible to a burst mode of star formation.