2014A&A...562A..96I

This paper presents emission line intensities of CO and C° from the compact circumnuclear disk in the center of NGC 5128 (Centaurus A) obtained with the Herschel Space Observatory in the 400-1000GHz range as well as previously unpublished measurements obtained with the ground-based observatories SEST, JCMT and APEX in the 90-800GHz range. The results show that the Cen A center has an emission ladder of CO transitions quite different from those of either star-burst galaxies or (Seyfert) AGNs. In addition, the neutral carbon ([CI]) emission lines from the Cen A center are much stronger relative to the adjacent CO lines than in any other galaxy. The CO surface brightness of the compact circumnuclear disk (CND) is significantly higher than that of the much more extended thin disk (ETD) in the same line of sight. LVG analysis of the CO line profiles decomposed into the constituent contributions show that the ETD is relatively cool and of low excitation, wheres the brighter CND is hotter and more highly excited. Our PDR/XDR models suggest that most of the CND gas is relatively cool (temperatures 25K-80K) and not very dense (≃300cm^–3) if it is primarily heated by UV photons. A small fraction of the gas in both the CND and the ETD has a much higher density (typically 30000cm^–3). A more highly excited, high-density phase is present in the CND, either in the form of an extreme PDR or more likely in the form of an XDR. Such a phase does not occur in the part of the ETD sampled. We have determined, for the first time, the molecular mass parameters of the CND. The total gas mass of the CND is M_CND=8.4x10⁷M_☉, uncertain by a factor of two. The CO-^h2 conversion factor (X_CND) is 4x10²⁰(K.km/s)^–1 also within a factor of two.