THE CO EMISSION OF RING GALAXIES

Using the 15m SEST and 30m IRAM telescope we have observed 16 ring gal axies in the (CO)-C-12(1-0) transition and detected 14. Six ring galax ies have been detected in the (CO)-C-12(2-1) line. The Cartwheel, ofte n 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 H-2 rich and actively forming stars. We also present new HI detections of six rings. We have compared the CO emission of t he ring galaxies with that of the distance-limited sample of Sage (199 3a,b) and found that ring galaxies are bright in CO compared to normal galaxies. If the standard conversion factor from CO emissivities into H-2 column densities holds for ring galaxies, this suggests that a la rge amount of molecular gas is available for star formation and that r ing galaxies are actively forming stars. This result is in agreement w ith the high far-infrared luminosities of ring galaxies. We have also observed two Hoag-type objects in CO but have detected neither of them .