In order to determine the nature of the atomic gas content of early-type disk galaxies, we have conducted a sensitive search for 21 cm emission from "normal" S0, S0a, and Sa galaxies. By "normal", we mean that the sample was chosen to have luminosities close to the mean for this morphological population. As such, the galaxies observed are representative of 60 percent of the early-type disk galaxies. Consistent integrated flux limits allow the range of the mean gas surface densities to be assessed and compared with that found for a massive (high luminosity) sample previously observed.
For the massive galaxy sample, over 50 percent of the S0's and S0a's were detected. A third of the detected galaxies had estimated hybrid atomic gas surface densities comparable to those found in Sa galaxies (hereafter gas-rich S0's). Another third would probably have gone undetected by less sensitive surveys. Follow-up optical imaging of the gas-rich S0's revealed anemic spiral structures and some star formation. However, this sample represented only 8 percent of the total S0 population. For the "normal" sample, we find similar detection statistics for the S0's and the S0a's (45 and 58 percent); but for Sa's, which were almost all detected for the massive sample, we detected only 53 percent for the "normal" sample. In fact, the distribution of the hybrid gas surface densities of the S0's is very similar to that of the Sa's. We strongly suspect that many of the gas-rich S0's would reveal spiral structure with deep imaging and may therefore be misclassified spirals.
Wednesday program listing