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 observations of six blue compact dwarf (BCD) galaxies. Optically, the selected galaxies 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 appears to be rotationally supported, however, indicating that true morphological transformation from a BCD to a dE will require significant loss of angular momentum. Based on the observed neutral gas dynamics of these and other BCDs, it is unlikely that present-day BCDs will evolve directly into dwarf ellipticals after a starburst phase. We discuss alternative evolutionary scenarios for BCDs and place them within the larger context of galaxy formation and evolution models. In general, BCDs appear to have steeper rotation curves than similar luminosity, low surface brightness dwarf galaxies. BCDs have centrally concentrated mass distributions (stars, gas, and dark matter) and have lower specific angular momenta. Based on disk instability analyses, steeply rising rotation curves result in higher threshold densities for the onset of star formation. These results suggest that angular momentum may play a crucial role in the morphological evolution of low-mass galaxies: galaxies with low angular momenta will be able to collapse into small, compact galaxies, while galaxies with high angular momenta will be more diffuse systems. Furthermore, because the star formation threshold density is higher in low angular momenta systems, star formation will be delayed until an extremely high surface density is reached. Thus, angular momentum may be the fundamental parameter that determines whether a low-mass galaxy will have centrally concentrated stellar and gaseous distributions and be more susceptible to a burst mode of star formation.