1995A&A...298..743H

Using the 15m SEST and 30m IRAM telescope we have observed 16 ring galaxies in the 2CO10 transition and detected 14. Six ring galaxies have been detected in the ¹²CO(2-1) line. The Cartwheel, often considered as the prototype of ring galaxies, has not been detected. We suggest that the weak CO emission of this galaxy is due to its low metallicity. The observations do not exclude the possibility that the Cartwheel may be H₂rich and actively forming stars. We also present new HI detections of six rings. We have compared the CO emission of the ring galaxies with that of the distance-limited sample of Sage (1993A&A...272..123S, 1993A&AS..100..537S) and found that ring galaxies are bright in CO compared to normal galaxies. If the standard conversion factor from CO emissivities into H₂column densities holds for ring galaxies, this suggests that a large amount of molecular gas is available for star formation and that ring galaxies are actively forming stars. This result is in agreement with the high far-infrared luminosities of ring galaxies. We have also observed two Hoag-type objects in CO but have detected neither of them.