In section 4. Extragalactic astronomy
ALMA observations of AGN fuelling. The case of PKS B1718–649
Active galaxy nuclei (AGN) are fueled by cold gas falling into the center, and the energy released by their activity can produce some feedback to eject gas, and moderate accretion and growth of the black hole. These complex processes can be traced through observations of the cold gas, under the atomic (HI) or molecular form of either the warm through near-infrared H2 lines or cold through the millimetric CO lines. The authors present ALMA observations of the CO (2–1) line in the spiral galaxy NGC 6328, which hosts a radio AGN, PKSB1718–649. The CO line gas in the disk of the galaxy (15kpc) is warped, a possible sign of interaction. While the outer parts follow the north-south orientation of the galaxy major axis, in the inner parts the gas abruptly changes orientation and forms a circumnuclear disk of radius 700pc, whose major axis is perpendicular to that of the outer disk. This structure is characteristic of a kinematically decoupled core (KDC). Together with the centered CO emission, ALMA reveals a CO absorption in front of the continuum source, at red-shifted velocities by 365km/s with respect to the systemic velocity. This absorbing CO gas could trace molecular clouds falling onto the central super-massive black hole. The physical conditions of molecular clouds in the a circumnuclear disk are in good agreement with the predictions for the conditions of the gas when cold chaotic accretion triggers an active galactic nucleus.