Black hole feeding and star formation in NGC 1808

A Audibert,S Martín,P Van Der Werf,A Eckart,S Viti,V Casasola,K Dasyra,L Hunt,S Muller,F Combes,F Boone,S García-Burillo,S Aalto,M Krips

doi:10.1051/0004-6361/202039886

Abstract

We report on Atacama Large Millimeter Array (ALMA) observations of CO(3−2) emission in the Seyfert2/starburst galaxy NGC 1808, at a spatial resolution of 4 pc. Our aim is to investigate the morphology and dynamics of the gas inside the central 0.5 kpc and to probe the nuclear feeding and feedback phenomena. We discovered a nuclear spiral of radius 1″ = 45 pc. Within it, we found a decoupled circumnuclear disk or molecular torus of a radius of 0.13″ = 6 pc. The HCN(4−3) and HCO+(4−3) and CS(7−6) dense gas line tracers were simultaneously mapped and detected in the nuclear spiral and they present the same misalignment in the molecular torus. At the nucleus, the HCN/HCO+and HCN/CS ratios indicate the presence of an active galactic nucleus (AGN). The molecular gas shows regular rotation, within a radius of 400 pc, except for the misaligned disk inside the nuclear spiral arms. The computations of the torques exerted on the gas by the barred stellar potential reveal that the gas within a radius of 100 pc is feeding the nucleus on a timescale of five rotations or on an average timescale of ∼60 Myr. Some non-circular motions are observed towards the center, corresponding to the nuclear spiral arms. We cannot rule out that small extra kinematic perturbations could be interpreted as a weak outflow attributed to AGN feedback. The molecular outflow detected at ≥250 pc in the NE direction is likely due to supernovae feedback and it is connected to the kpc-scale superwind.

Highlights

The principal issue in understanding how supermassive black holes (SMBHs) are growing at the center of spiral galaxies is the ability to unveil the processes feeding the nuclear activity and – when gas is present in the circumnuclear environment – the transferring of the angular momentum outward to allow the gas to inflow
In order to investigate the physical phenomena of active galactic nucleus (AGN) feeding and feedback in this paper, we study the kinematics of the molecular gas in the nearby Starburst/Seyfert 2 galaxy NGC 1808
Atacama Large Millimeter Array (ALMA) band 3 observations at ∼93 GHz and 2.6 resolution found a spectral index αnuc ∼ −1 (Salak et al 2016, 2017), typical of synchrotron emission. They suggested that the nuclear region is dominated by a high-energy source, namely, supernovae explosions or a lowluminosity AGN (LLAGN)

Summary

Introduction

The principal issue in understanding how supermassive black holes (SMBHs) are growing at the center of spiral galaxies is the ability to unveil the processes feeding the nuclear activity and – when gas is present in the circumnuclear environment – the transferring of the angular momentum outward to allow the gas to inflow. These processes are complex and take place.

Objectives

Results

Discussion

Conclusion