2019A&A...632A..33A

We report ALMA observations of CO(3-2) emission in the Seyfert/nuclear starburst galaxy NGC 613, at a spatial resolution of 17pc, as part of our NUclei of GAlaxies (NUGA) sample. Our aim is to investigate the morphology and dynamics of the gas inside the central kiloparsec, and to probe nuclear fueling and feedback phenomena. The morphology of CO(3-2) line emission reveals a two-arm trailing nuclear spiral at r≤100pc and a circumnuclear ring at a radius of ∼350pc that is coincident with the star-forming ring seen in the optical images. Also, we find evidence for a filamentary structure connecting the ring and the nuclear spiral. The ring reveals two breaks into two winding spiral arms corresponding to the dust lanes in the optical images. The molecular gas in the galaxy disk is in a remarkably regular rotation, however the kinematics in the nuclear region are very skewed. The nuclear spectrum of CO and dense gas tracers HCN(4-3), HCO⁺(4-3), and CS(7-6) show broad wings up to ±300km/s, associated with a molecular outflow emanating from the nucleus (r∼25pc). We derive a molecular outflow mass M_out=2x10⁶M_☉ and a mass outflow rate of M_out=27M_☉/yr. The molecular outflow energetics exceed the values predicted by AGN feedback models: the kinetic power of the outflow corresponds to P_K,out=20%L_AGN and the momentum rate is M_outv∼400L_AGN/c. The outflow is mainly boosted by the AGN through entrainment by the radio jet, but given the weak nuclear activity of NGC 613, we might be witnessing a fossil outflow resulting from a previously strong AGN that has now faded. Furthermore, the nuclear trailing spiral observed in CO emission is inside the inner Lindblad resonance ring of the bar. We compute the gravitational torques exerted in the gas to estimate the efficiency of the angular momentum exchange. The gravity torques are negative from 25 to 100 pc and the gas loses its angular momentum in a rotation period, providing evidence for a highly efficient inflow towards the center. This phenomenon shows that the massive central black hole has significant dynamical influence on the gas, triggering the inflowing of molecular gas to feed the black hole.