Molecular gas in elliptical galaxies: CO observations of an IRAS flux-limited sample

A search for CO(2-1) emission from 42 early-type galaxies with 100 mu m flux densities greater than 1 Jy detected (tentatively in some cases) 11 galaxies and tentatively found absorption against the active nucleus in two. Data from this survey and from the literature show that the overall detection rate of CO emission or absorption in elliptical galaxies is 45% and that the detection rate increases with increasing 100 mu m flux density. The data suggest that all elliptical galaxies contain a small amount of cold interstellar matter. The CO and far-infrared fluxes are well correlated, though the relationship shows a lot of scatter, which does not appear to be caused by variations in the temperature of the interstellar dust and probably indicates differences in the physical state of the interstellar medium from galaxy to galaxy. The CO flux densities are completely uncorrelated with the galactic starlight. The detection of CO emission shows that many elliptical galaxies have dense cold gas in their inner regions. The four galaxies which are known to have CO absorption, including the two from the present paper, have relatively narrow absorption components which are at the galaxy's systemic velocity and/or redshifted with respect to this velocity, consistent with infall to the center of the galaxy.