ALMA images the many faces of the NGC1068 torus and its surroundings (1909.00675v2)

Abstract: We investigate the fueling and the feedback of nuclear activity in the Seyfert 2 galaxy NGC1068, by studying the distribution and kinematics of molecular gas in the torus and its connections. We use ALMA to image the emission of a set of molecular gas tracers in the circumnuclear disk (CND) and the torus of the galaxy using the CO(2-1), CO(3-2) and HCO+(4-3) lines with spatial resolutions ~0.03"-0.09"(2-6pc). ALMA resolves the CND as an asymmetric ringed disk of D~400pc-size and mass of ~1.4x10^8 Msun. The inner edge of the ring is associated with edge-brightened arcs of NIR polarized emission identified with the working surface of the AGN ionized wind. ALMA proves the existence of a molecular torus of M_torus ~ 3x10^5Msun, which extends over a large range of spatial scales D=10-30pc around the central engine. The new observations evidence the density radial stratification of the torus: the HCO+(4-3) torus, with a full size D=11pc, is a factor of 2-3 smaller than its CO(2-1) and CO(3-2) counterparts, which have full-sizes D=26pc and D=28pc, respectively. The torus is connected to the CND through a network of gas streamers. The kinematics of molecular gas show strong departures from circular motions in the torus, the gas streamers, and the CND. These velocity distortions are interconnected and are part of a 3D outflow that reflects the effects of AGN feedback on the kinematics of molecular gas across a wide range of spatial scales. We conclude that a wide-angle AGN wind launched from the accretion disk is impacting a sizeable fraction of the gas inside the torus (~0.4-0.6 x M_torus). However, a large gas reservoir (~1.2-1.8 x 10^5Msun) close to the equatorial plane of the torus remains unaffected by the AGN wind and can continue fueling the AGN for ~1-4Myr. AGN fueling seems nevertheless thwarted on intermediate scales (15pc < r < 50pc).

• The paper reveals a radially stratified molecular torus (10–30 pc) and asymmetric circumnuclear dynamics in NGC1068 using ALMA data.
• The paper employs CO(2-1), CO(3-2), and HCO⁺(4-3) tracers to detect non-circular motions up to 200 km/s, indicating strong AGN-driven outflows.
• The study identifies intricate gas streamers linking the torus and CND, providing evidence that AGN feedback significantly shapes host galaxy evolution.

Imaging NGC 1068: Insights from ALMA Observations

The paper presents a comprehensive paper using the Atacama Large Millimeter Array (ALMA) to analyze the molecular torus and its surroundings in the Seyfert 2 galaxy NGC 1068. This investigation aims to unravel the complex feeding and feedback mechanisms of the active galactic nucleus (AGN) by examining the distribution and dynamics of various molecular gas tracers in the circumnuclear disk (CND) and the torus.

Methodology

The paper employs observations from ALMA, focusing on molecular tracers such as CO(2-1), CO(3-2), and HCO$^+$(4-3) across the CND and torus with an unprecedented spatial resolution of 2-6 pc. The paper leverages the different sensitivity of these tracers to various gas densities ($n$(H$_2$)~$\subset10^3-10^7$ cm$^{-3}$) to infer the density stratification and complex gas dynamics around the central SMBH of NGC 1068.

Key Findings

1. CND Morphology and Dynamics:
   • The CND is characterized by an asymmetric ring extending over $\simeq400$ pc in diameter, with a clear deficit of molecular gas within the central 130 pc. ALMA data reveals this deficit aligning with an AGN-driven ionized outflow.
   • Kinematic analysis indicates a significant radial expansion within the disk, with non-circular motions spanning up to 200 km/s, indicative of feedback from the AGN wind affecting gas dynamics on large scales.
2. Molecular Torus Characteristics:
   • The analysis supports a radially stratified molecular torus approximately 10-30 pc in size, comprising $\simeq3 \times 10^5 M_{\sun}$ of molecular gas. The gas distribution within this torus is notably asymmetrical, with variations in size depending on the molecular tracer used, highlighting a stratification in terms of gas density and temperature.
   • The torus shows strong kinematic asymmetries, with a notable presence of high-velocity gas components potentially linked to outflow activities.
3. Torus-CND Connections:
   • ALMA captures intricate gas streamers linking the torus and CND, suggesting outflow processes potentially driven by the AGN. This connection aligns with observed ionized wind impacts and is reinforced by the degree of polarization in near-infrared (NIR) emissions.
4. Evidence for an Outflowing Torus:
   • The paper postulates that significant fractions of the torus' gas are entrained by AGN winds, described within a framework of a 3D outflow model. This model aligns with observations of velocity offsets and line splitting in the molecular lines, challenging conventional rotating disk models.

Implications

The findings assert a complex interplay between inflow and outflow mechanisms in AGN environments, providing empirical backing to models of AGN-driven feedback shaping host galaxy evolution. This paper expands the understanding of AGN feeding processes by illustrating how ionized winds can impact molecular gas distribution and dynamics, potentially contributing to self-regulation in SMBH growth.

Future Prospects

The detailed kinematic maps and models suggest avenues for further exploration, particularly in resolving the dynamics at smaller scales and understanding the transition between inflow and outflow processes in AGN host galaxies. Future observations with enhanced resolution and sensitivity could refine these models and elucidate the processes governing AGN-host interactions comprehensively. This research underlines the critical role of high-resolution observations in dissecting the mechanisms of AGN feedback in driving galaxy evolution.