Abstract
We report ALMA observations of CO(3–2) emission in the Seyfert/nuclear starburst galaxy NGC 613, at a spatial resolution of 17 pc, 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 ≲ 100 pc and a circumnuclear ring at a radius of ∼350 pc 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 ±300 km s−1, associated with a molecular outflow emanating from the nucleus (r ∼ 25 pc). We derive a molecular outflow mass Mout = 2 × 106 M⊙ and a mass outflow rate of Ṁout = 27 M⊙ yr−1. The molecular outflow energetics exceed the values predicted by AGN feedback models: the kinetic power of the outflow corresponds to PK, out = 20%LAGN and the momentum rate is Ṁoutv ∼ 400LAGN/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.
Highlights
The energy of active galactic nuclei (AGN) is well understood as being due to gas accretion onto the supermassive black hole (SMBH; Antonucci 1993)
We find that the circumnuclear disk (CND) region lies in the AGN-dominated part of the diagram, while the clump in the star-forming ring of NGC 613 is dominated by star formation
We study the morphology and the kinematics of the molecular gas in the central 1 kpc and our main findings are summarized below:
Summary
The energy of active galactic nuclei (AGN) is well understood as being due to gas accretion onto the supermassive black hole (SMBH; Antonucci 1993). Gas inflows into the center of galaxies can fuel the SMBH and the energy input by the AGN can trig-. The feedback can in turn regulate the SMBH growth and suppress star formation (e.g., Croton et al 2006; Sijacki et al 2007). Feeding and feedback are key processes in the co-evolution of black holes (BH) and their host galaxies, which is well established by the tight M-σ relation (e.g., Magorrian et al 1998; Gültekin et al 2009; McConnell & Ma 2013). It is important to study the efficiency of angular momentum transport in galaxy disks in order to understand how the star formation and nuclear activity are fueled and to elucidate.
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