2024MNRAS.527..531L

Studies of spatial distribution, kinematics, and physical structure of dense molecular gas in the centres of galaxies is crucial in understanding the nature of physical processes, especially those related to active galactic nuclei (AGNs) and starbursts, in the vicinity of central massive black holes. However, our current understanding of dense gas in the circumnuclear discs (CND) remains limited, particularly those around AGNs. Here-, we report high-resolution deep observations of the CS (3-2), HC₃N (15-14), H₂CO (2-1), CH₃CCN (8-7), CO (1-0), and (3-2) lines, toward the central region (R ∼ 0.4 kpc) of the nearest Seyfert-2 galaxy, the Circinus galaxy, using the Atacama Large Millimeter/submillimeter Array. The dense molecular gas traced by CS (3-2) and HC₃N (15-14) exhibit an asymmetric and cloud structure, which is in sharp contrast against the symmetric low-density structure traced by CO (1-0). Four molecular cloud structures surrounding the AGN, which have a typical size of ∼ 20 pc, are detected with the optically thin HC₃N (15-14) line. HC₃N (15-14)/CO (1-0) line ratio in the CND is found to be higher than that in the nuclear region, indicating that the bulk of dense gas accumulates in the CND, instead of the nucleus. HC₃N (15-14) line and optically thick tracers in four molecular clouds, provide a good comparison between density layers and show evidence for a density stratification of the CND.