, F. Combes
, C. Henkel
Onsala Space Observatory, S-43992 Onsala, Sweden
DEMIRM, Observatoire de Paris, 61 Av. de l'Observatoire,
F-75014 Paris, France
Max Planck Institut für Radioastronomie, Auf dem Hügel 69,
D-53121 Bonn, Germany
to 
,
and appear to be unrelated to the underlying stellar
population. This suggests an external origin of the gas. Most of the
elliptical galaxies with a molecular gas component have a
gas-to-dust mass ratio of 700, where dust masses are derived from
the IRAS fluxes, but some appear to have a ratio as low as 50. A
small apparent gas-to-dust mass ratio is also found for some
late-type galaxies, and is correlated with a low dust
temperature. We suggest that a large part of the far-infrared
emission from these galaxies (both early and late types) comes from
dust associated with the atomic gas component rather than star
forming regions associated with the molecular gas, and that they
contain a cold dust component. Low excitation temperatures for CO
transitions in galaxies with cold dust could lead to an
underestimate of the molecular gas mass by a factor of 5. The
average 
ratio for the elliptical galaxies is 2-5 times
lower than for normal spiral galaxies. Field ellipticals appear more
likely to contain an observable molecular gas component than those
ellipticals residing in groups and clusters.