The Missing Interstellar Medium of NGC 147 and M32

We present the results of a search for CO 1 ? 0 emission from NGC 147 and M32, two of the four dwarf elliptical companions of M31. Return of gas from evolved stars to the interstellar medium of these galaxies should have resulted in detections, but we find instead upper limits of 4100 and 5100 M? of H2 for NGC 147 and M32, respectively. Including and earlier H I limit, we find that the gaseous interstellar medium (ISM) of NGC 147 comprises less than 2% of what is expected. The large published H I mass limit for M32 prevents us from reaching a similarly extreme conclusion for this galaxy. These results stand in stark contrast to what is seen in NGC 185 and NGC 205, where the observed gas is approximately what is expected from stellar mass return, although some of the gas in NGC 205 probably had an external origin. There are no obvious differences in masses or luminosities that would explain the results. The proposal that differences may be related to the recent interaction histories of the galaxies with M31 is rendered moot by the lack of orbital information but does not seem to be viable for NGC 147 and NGC 185. We can offer no convincing explanation for these puzzling results, although they may point to a fundamental gap in our understanding of galaxy evolution.