Observations of CO isotopic emission and the far-infrared continuum of Centaurus A

Abstract

We present maps of the ^12^CO(1-0) line and the 100 and 50 micron far-infrared continuum emission of Centaurus A, as well as measurements of the ^12^CO(2-1), ^13^CO, and the C^18^O lines at selected positions. The observations were made with the Swedish-ESO Submillimeter Telescope (SEST) and the CPC instrument on board IRAS. The millimeter data show that the bulk molecular material is closely associated with the dust lane and contained in a disk of about 180" diameter with a total molecular mass of about 2 x 10^8^ M_sun_. Additional molecular gas (less than 30%) is located in the bulge. The molecular gas in the nucleus is warm and dense, with a beam-averaged kinetic temperature of the order of 15 K and a number density of 10^3^-3 x 10^4^ cm^-3^. Absorption features in the ^12^CO and ^13^CO lines observed against the nuclear continuum emission indicate that the properties of giant molecular clouds are comparable to those in the Galaxy. The far-infrared data show that to a good approximation the dust temperature has a value of about 42 K across the dust lane and decreases toward the edges. The globally averaged ratio between the far-infrared luminosity and the total molecular mass is about 24 L_sun_/M_sun_, which is consistent with values found for galaxies with starburst activities but not as large as expected for mergers. A comparison of the ^12^CO(1-0) and the far-infrared data as a function of position indicates that a considerable amount (about 50%) of the 100 micron far-infrared mission is not intimately associated with massive star formation. This emission is larger in extent than the molecular disk and can probably be associated with diffuse "cirrus" clouds in Centaurus A.