The multi-phase environment in the centre of Centaurus A

Abstract

We study the multi-phase medium in the innermost regions of the Centaurus A (Cen A) galaxy. Combined high-resolution observations with the ALMA and Chandra observatories indicate that the hot X-ray emitting plasma coexists with the warm and cold medium in Cen A. This complex environment is a source of CO lines with great impact for its diagnostics. We show the images from the two mentioned instruments covering the nuclear region (diameter of 10 or ∼ 184 pc), and we study the conditions for plasma thermal equilibrium and possible coexistence of cool clouds embedded within the hot X-ray emitting gas. Further, we demonstrate that the multi-phase medium originates naturally by the thermal instability arising due to the interaction of the high-energy radiation field from the nucleus with the ambient gas and dust. We show that cold gas clouds can coexist in the mutual contact with hot plasma, but even colder dusty molecular clouds have to be distanced by several hundred pc from the hot region. Finally, we propose a 3-D model of the appearance of the hot plasma and the CO line-emitting regions consistent with the Chandra image and we derive the integrated emissivity in specific molecular lines observed by ALMA from this model. To reproduce the observed images and the CO line luminosity the dusty shell has to be 419 pc thick and located at 1020 pc from the centre.