ALMA Reveals an Inhomogeneous Compact Rotating Dense Molecular Torus at the NGC 1068 Nucleus

Abstract: We present the results of our ALMA Cycle 4 high-spatial-resolution (0.04-0.07") observations, at HCN J=3-2 and HCO+ J=3-2 lines, of the nucleus of NGC 1068, the nearby prototypical type 2 active galactic nucleus (AGN). Our previous ALMA observations identified the compact emission of these lines at the putative location of the torus around a mass-accreting supermassive black hole. We now report that we have detected the rotation of this compact emission, with the eastern and western sides being redshifted and blueshifted, respectively. Unlike the previously reported CO J=6-5 emission, both the morphological and dynamical alignments of the HCN J=3-2 and HCO+ J=3-2 emission are roughly aligned along the east-west direction (i.e., the expected torus direction), suggesting that these molecular lines are better probes of a rotating dense molecular gas component in the torus. The western part of the torus exhibits larger velocity dispersion and stronger emission in the HCN J=3-2 and HCO+ J=3-2 lines than the eastern part, revealing a highly inhomogeneous molecular torus. The dense molecular gas in the torus and that of the host galaxy at 0.5-2.0" from the AGN along the torus direction are found to be counter-rotating, suggesting an external process happened in the past at the NGC 1068 nucleus.

Inhomogeneous Compact Rotating Dense Molecular Torus in the NGC 1068 AGN

The research paper titled "ALMA Reveals an Inhomogeneous Compact Rotating Dense Molecular Torus at the NGC 1068 Nucleus" presents significant observations made using the Atacama Large Millimeter/submillimeter Array (ALMA) of the active galactic nucleus (AGN) of NGC 1068. This nearby prototypical type 2 AGN serves as an important subject for understanding the unified model of AGNs, where type 1 and type 2 AGNs are differentiated by the inclination of their obscuring torus relative to the observer. The study addresses key issues related to the dense molecular torus that surrounds the mass-accreting supermassive black hole (SMBH) in NGC 1068.

Observations and Methodology

Utilizing ALMA's high spatial resolution capabilities (0$\farcs$04--0$\farcs$07), the researchers focused on the HCN J=3--2 and HCO$^{+}$ J=3--2 molecular lines within the nucleus of NGC 1068. The observations revealed a compact emission region indicative of the dense molecular torus associated with the AGN. Through enhanced spatial resolution, they identified rotational motion within this compact emission, with redshifted and blueshifted positions evident along the east-west axis—a signature of rotating dense molecular gas.

Notably, the results contradict previous reports of CO J=6--5 emissions, which appeared misaligned and turbulent, possibly due to outflow effects. The present observations suggest that HCN J=3--2 and HCO$^{+}$ J=3--2 lines are more reliable in probing the dense molecular alignment and dynamics of the torus.

Key Findings

1. Morphological and Dynamic Alignment: The alignment of dense molecular emissions and dynamics along the east-west direction confirms the spatial coexistence of dense molecular gas with the dust in the torus.

2. Torus Inhomogeneity: The molecular emissions show significant inhomogeneity. Enhanced emission and turbulent dispersion in the western region compared to the eastern part signify variable molecular turbulence and possibly asymmetric star formation effects within the torus.

3. Rotational Dynamics: The derived rotational motion indicates velocities inconsistent with pure Keplerian dynamics predicted by the SMBH mass, suggesting complex non-Keplerian dynamics within the torus.

4. Counter-Rotating Components: Observations revealed that the dense molecular gas in the torus and the host galaxy exhibit counter-rotating patterns, likely resulting from external influences, such as minor mergers.

Implications and Future Prospects

The study provides insights into the intricate dynamics and structural composition of AGN tori, crucial for refining current models of AGN behavior and classification. The detection of inhomogeneity and non-Keplerian dynamics implies complex physical processes within AGN tori, challenging simpler axisymmetric and uniform models.

The counter-rotating nature of torus and host galaxy components suggests that external processes affecting tori need further investigation, potentially involving interactions such as minor mergers. Additionally, potential mass inflow from host galaxies to AGN tori, as suggested by bridging emissions, opens avenues for exploring the mechanics of mass accretion in AGNs.

Future studies employing multi-line observations at high spatial resolutions could elucidate ongoing dynamic processes and validate complex models in AGN physics. As technology progresses, spectral line data beyond the current molecular tracers may pave the way for deeper understanding and novel discoveries in AGN phenomena.

The paper successfully leverages ALMA's capabilities to advance knowledge of AGN torus structures, providing a foundation for theoretical models and observational strategies in active galaxy research.