THE MOLECULAR CLOUD CONTENT OF EARLY-TYPE GALAXIES .5. CO IN ELLIPTICGALAXIES

A survey of CO emission in 29 far-IR selected elliptical galaxies resu lted in 16 detections, of which 3 remain tentative. The molecular gas masses range from 2x10(6) M(.) to 1x10(9) M(.), and appear to be unrel ated to the underlying stellar population. This suggests an external o rigin of the gas. Most of the elliptical galaxies with a molecular gas component have a gas-to-dust mass ratio of similar to 700, where dust masses are derived from the IRAS fluxes, but some appear to have a ra tio as low as 50. A small apparent gas-to-dust mass ratio is also foun d for some late-type galaxies, and is correlated with a low dust tempe rature. We suggest that a large part of the far-infrared emission from these galaxies (both early- and late-types) comes from dust associate d with the atomic gas component rather than star forming regions assoc iated with the molecular gas, and that they contain a cold dust compon ent. Low excitation temperatures for CO transitions in galaxies with c old dust could lead to an underestimate of the molecular gas mass by a factor of 5. The average M(H2)/M(HI) ratio for the elliptical galaxie s is 2-5 times lower than for normal spiral galaxies. Field elliptical s appear more likely to contain an observable molecular gas component than those ellipticals residing in groups and clusters.