The multi-phase environment in the centre of Centaurus A

TL;DR

This study investigates the multi-phase environment around Centaurus A's nucleus, revealing coexistence of hot, warm, and cold media driven by thermal instability, with a new 3D model explaining observed emissions.

Contribution

It presents high-resolution ALMA and Chandra observations combined with a 3D model of the multi-phase medium in Centaurus A, highlighting the role of thermal instability in its formation.

Findings

Cold gas clouds coexist with hot plasma within ~180 pc.

Dusty molecular clouds are located several hundred parsecs from hot regions.

A 3D model reproduces observed CO line emissions and plasma structure.

Abstract

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.