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The molecular cloud content of early-type galaxies. V. CO in elliptical galaxies

T. Wiklind, 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
Abstract: A survey of CO emission in 29 far-IR selected elliptical galaxies resulted in 16 detections, of which 3 remain tentative. The molecular gas masses range from 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.
Accepted by Astronomy and Astrophysics

Robert Lucas
Thu Mar 9 12:42:24 MET 1995