AGN Feeding and Feedback: Resolving the Torus

The gist The torus plays a critical role in AGN fueling and feedback by regulating star formation within host galaxies, and high-resolution observations with ALMA2040 are necessary to probe this feeding mechanism across various AGN types.

Scientific context and open questions

AGN play a key role in galaxy evolution by regulating star formation within their host galaxies through radiative feedback, radio jets, and massive outflows driven by the large amounts of energy released during accretion in the active galactic nucleus (AGN) phase<ref:2610.21678#pg2>. It is now well established that SMBH can regulate the formation of stars in their host galaxies through radiative feedback, radio jets, and massive outflows driven by the large amounts of energy released during accretion in the active galactic nucleus (AGN) phase<ref:2610.21678#pg3>. The central element of the Unified Model for AGN is the torus of dust and gas around the SMBH (e.g., Krolik & Begelman 1988; Antonucci 1993)<ref:2610.21678#pg3>. This key structure not only explains the observational distinction between type 1 AGN, where both broad and narrow line regions (BLR and NLR) are visible, and type 2 AGN, where the torus obscures the BLR, but, more importantly, it plays a critical role in the final stages of AGN fueling as the main reservoir of material around the SMBH<ref:2610.21678#pg3>.

How does the material in the torus feed the AGN?

The process by which material feeds into the SMBH involves several physical steps that must occur for gas to reach the accretion disk<ref:2610.21678#pg4>. This sequence includes:

1. The molecular gas in the torus must lose angular momentum<ref:2610.21678#pg4>.

2. It must pass through the dust sublimation radius<ref:2610.21678#pg4>.

3. This involves a potential molecular dust-free neutral BLR, the atomic BLR, and ultimately being funnelled into the accretion disk<ref:2610.21678#pg4>.

How does feeding/feedback operate in the most luminous AGN?

For high-luminosity AGN (Lbol>1045−47 erg s−1), such as quasars (QSOs), the tori and their surroundings remain unresolved due to greater distances, making it essential to resolve these inner regions<ref:2610.21678#pg3>. The torus geometry, winds, and outflows depend on the AGN luminosity<ref:2610.21678#pg4>.

Resolving the neutral molecular BLR and resolved torus

Around the SMBH at the center of AGN, accretion disks of ionized gas are expected to be fueled by a BLR<ref:2610.21678#pg5>. Between the molecular torus and the accretion disk, it is not yet clear in what form the matter exists<ref:2610.21678#pg5>. Dust can survive up to temperatures of the order of 1500 K, and inside the sublimation radius, there might exist neutral molecular gas that progressively gains high rotational velocity<ref:2610.21678#pg5>. This could represent a BLR0 region, i.e., a broad line region of dustless neutral gas<ref:2610.21678#pg5>. ALMA2040 could detect this BLR0 through the wide velocity width of molecular lines, analogous to the detection of BLRs in the UV/optical using broad atomic lines<ref:2610.21678#pg6>.

Torus of high-luminosity AGN - local QSO

Studies of resolved torus regions have been restricted to nearby Seyfert-like AGN because current facilities lack the sensitivity and angular resolution required to study the tori of more luminous AGN, such as QSOs<ref:2610.21678#pg8>. Most luminous local Seyferts studied so far present nuclear cold molecular gas deficits with respect to circum-nuclear scales of a few hundreds of parsecs, which is interpreted as an imprint left by AGN feedback when energetic winds clear molecular gas from the torus-scale regions<ref:2610.21678#pg8>.

Technical Requirements

The proposed science cases require the detection of molecular gas in a wide range of physical conditions, typically in dust-obscured regions, and at high spatial resolution (7–10 mas for local Seyferts and QSOs). The 100–350 GHz range provides multiple rotational transitions of CO, HCN, and HCO+, which probe from cold and dense to warm molecular gas, and vibrationally excited CO lines sensitive to the potential hot molecular gas in the dense BLR0. Addressing these key questions requires a next-generation sub-mm interferometer with a 5× increase in instantaneous spectral line sensitivity at ∼300 GHz, compared to current ALMA (∼4× compared to anticipated ALMA-WSU), and a ∼10× smaller PSF area through ∼3× longer baselines than ALMA-WSU.

Synergies with current and future facilities

High-resolution integral-field spectroscopy with HARMONI and imaging with METIS will enable the isolation of the dusty torus emission and help disentangle its contribution from that arising in the surrounding CND. These observations will enable the spatially resolved characterization of the ionized gas kinematics in nuclear regions on scales matched to ALMA2040 torus molecular maps, allowing for feedback characterization on the same scale as feeding. The Square Kilometre Array (SKA) will provide a complementary probe of the neutral atomic phase of AGN feeding and feedback through the H I 21 cm line.

Conclusion

Together with the companion ALMA2040 case for direct dynamical BH mass measurements, these synergies exploit a key strength of ALMA2040: solving the cold gas on the scales where BH growth, gas inflows, and AGN feedback are physically connected.

Acknowledgements

We thank the anonymous referees for their helpful comments and suggestions, which have improved this manuscript. This work was developed as part of the broader ALMA2040 community effort. MPS acknowledges support from grants RYC2021-033094-I and PID2023-146667NB-I00 funded by MCIN/AEI/10.

1. Improved AGN fueling models can now incorporate spatially resolved dynamics of material feeding an AGN down to the accretion disk in nearby galaxies, as proposed by ALMA2040, enabling a more detailed understanding of angular momentum loss mechanisms.

2. The improved system can resolve the structure and kinematics of the circum-nuclear medium, specifically distinguishing between a molecular torus and a potential BLR0 region, which allows for direct dynamical measurement of SMBH mass from gas kinematics via the velocity field provides a dust-free, molecular SMBH mass.

3. The AI can perform spatially resolved characterization of ionized gas kinematics on scales matched to the molecular torus maps, allowing for the measurement of the ionized wind energetics in the same aperture as the molecular torus and BLR0, thereby characterizing feedback on scales where BH growth, gas inflows, and AGN feedback are physically connected.

4. The system can better predict species abundances in dense regions by utilizing CLOUDY model calculations with specific parameters like log(U) = −5 to estimate column densities up to NH2 ∼1025−26 cm−2 for the molecular gas within the BLR0.

5. The improved system can enable the detection of optically thick molecular lines, such as the CO v=1 lines, by leveraging the high sensitivity of ALMA2040 and its bandwidth to detect flux even when the v=1 line will also be optically thick.

Active Galactic Nuclei (AGN) play a key role in galaxy evolution by regulating star formation within their host galaxies. It is therefore crucial to understand how the central supermassive black holes (SMBH) are fueled by material from the galactic disk, and how AGN feedback impacts the surrounding gas. Most information so far comes from very nearby active galaxies (z < 0.008; D < 35 Mpc), providing the best spatial resolution and revealing randomly oriented dusty molecular tori that fuel SMBHs. A facility such as ALMA2040, offering significantly higher sensitivity and better angular resolution than current ALMA, would enable progress in two fundamental areas that remain beyond the capabilities of all existing and planned observatories: (i) probing the dynamics of the material feeding the AGN down to the accretion disk in nearby galaxies; and (ii) spatially resolving the tori and circum-nuclear regions of more energetic and cosmologically significant AGN, such as local quasars (z 0.1 and Lbol > 10 45 erg/s).