  We study the multi-phase medium in the vicinity of the active galactic
nucleus Centaurus A (Cen A). Combined high-resolution observations with the
ALMA and Chandra observatories indicate that the hot X-ray emitting plasma
coexists with the warm and cold media in Cen A. This complex environment is a
source of CO lines with great impact for its diagnostics. We present the images
from the two above-mentioned instruments covering the nuclear region (diameter
of 10'' i.e., ~180 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 (TI) arising due to the
interaction of the high-energy radiation field from the nucleus with the
ambient gas and dust. We demonstrate 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 ~420 pc
thick and located at ~1000 pc from the centre.
