5.21. Molecular gas in the center of the elliptical galaxy NGC759

Abstract

Elliptical galaxies are traditionally defined as gas–free, inert stellar systems. Observations of continuum emission in the far–infrared (FIR) and sub–mm wavelength bands have, however, shown that a large fraction of all ellipticals, ~50% (Jura et al. 1987), contain a dust component. The infrared emission is due to warm dust, in many cases associated with star formation and/or weak AGN activity, while cold dust dominates the long wavelength continuum emission (e.g. Wiklind & Henkel 1995). Some elliptical galaxies also contain a molecular gas component, as seen through CO emission (Lees et al. 1991; Wiklind et al. 1995; Knapp & Rupen 1996). The dust and molecular gas are believed to be associated with each other, but it is not clear what powers the emission: star formation activity and/or AGN activity. Both the molecular gas mass and the FIR luminosity are on average lower in ellipticals than in spiral galaxies of similar luminosities. However, the LFIR/MH2 ratio is larger for the elliptical galaxies. If this ratio is a measure of the star formation efficiency, this suggests that gas is being used up more efficiently in elliptical galaxies than in normal spirals (e.g. Wiklind et al. 1995). Other possibilities is that the FIR dust emission is not only powered by UV–light from young stars, but from other sources as well, or that some dust is not spatially associated with the molecular gas.