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Destruction of the central black hole gas reservoir through head-on galaxy collisions


A massive black hole exists in almost every galaxy. Black holes occasionally radiate a vast amount of light by releasing gravitational energy of accreting gas with a cumulative active period of only a few 108 yr, the so-called duty cycle of the active galactic nuclei. Many galaxies today host a starving massive black hole. Although galaxy collisions have been thought to enhance nucleus activity1,2, the origin of the duty cycle, especially the shutdown process, is still a critical issue3. Here, we show that galaxy collisions are also capable of suppressing black hole fuelling, by using an analytic model and three-dimensional hydrodynamic simulations and by applying the well-determined parameter sets for the galactic collision in the Andromeda galaxy4,5. Our models demonstrate that a central collision of galaxies can strip the torus-shaped gas surrounding the massive black hole, the putative fuelling source. The derived condition for switching off the black hole fuelling indicates that a notable fraction of currently bright nuclei can become inactive, which is reminiscent of the fading or dying active nucleus phenomena6,7,8,9 that are associated with galaxy merging events. Galaxy collisions may therefore be responsible for both switching off and turning on the nucleus activity, depending on the collision orbit (head-on or far-off-centre).

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Fig. 1: Time evolution of the torus gas in the meridian plane of the torus frame.
Fig. 2: Mass stripped fraction fstrip.
Fig. 3: Relation between the torus mass and the host galaxy of AGN.
Fig. 4: Distribution of orbital periods and pericentres for satellite galaxies.

Data availability

Source data are provided with this paper. The data that support the findings of this study are available from the corresponding author upon reasonable request, although the requester will be responsible for providing the very considerable resources needed for transferring and storing these data.

Code availability

The parent code MAGI has been made publicly available at It is expected that most of the extensions and modifications made to meet the specific requirements for this project will be made available in the future release; those interested can contact the corresponding author for further information.


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We are grateful to A. Wagner, M. Umemura and K. Ohsuga for careful reading of the manuscript and for the comments that improved the manuscript. Numerical computations were performed with computational resources provided by the Multidisciplinary Cooperative Research Program of the Center for Computational Sciences at the University of Tsukuba, the Oakforest-PACS operated by the Joint Center for Advanced High-Performance Computing (JCAHPC), and resources in the Information Technology Center at the University of Tokyo. This work was supported in part by Grants-in-Aid for Specially Promoted Research by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) (16002003), and by Grants-in-Aid for Scientific Research by the Japan Society for the Promotion of Sciences (JSPS) ((S) 20224002; (A) 21244013; (C) JP17K05389) and JSPS KAKENHI grant nos. JP20K14517 and JP20K04022.

Author information




Y.M. contributed to modelling, code development, numerical simulations, analysis, discussion and manuscript preparation. M.M. contributed to modelling, connection to recent Galactic observations, discussion and manuscript preparation. T.K. contributed to modelling, generalization to actual AGN environments, discussion and manuscript preparation.

Corresponding author

Correspondence to Yohei Miki.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Jorge Moreno and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Simulation parameters.

Parameters of three-dimensional hydrodynamic simulations are summarized.

Extended Data Fig. 2 Radial profile of the ratio of the gas column-densities of the infalling satellite galaxy to that of the central torus, Σsatellitetorus.

The solid and dotted curves show the gas column-densities ratio for η = 100 and 10, respectively.

Source data

Extended Data Fig. 3 Orbital parameters of the Milky Way satellites.

Pericentres, rperi, apocentres, rapo, orbital eccentricities, e, and orbital periods, Tr, are tabulated (median values with 84.1 and 15.9 percentiles at the 1σ confidence level).

Source data

Extended Data Fig. 4 A series of snapshots of a clump exposed to the fast gas travelling from the left side of the simulation box.

From a to e, each panel shows the volume-density distribution ρ(z, x) in the y = 0 plane at 0, 1, 2, 3, 4 kyr after the simulation starts.

Extended Data Fig. 5 Time evolution of various clumps exposed to fast infalling gas.

Different colours and linetypes indicate various clump masses Mclump and clump radii (rclump): Mclump = 1.4M (blue), 14 M (red) and 140 M (black); rclump = 10−3 pc (solid), 10−2 pc (dotted) and 0.1 pc (dashed).

Extended Data Fig. 6 Time evolution of various clumps examined in Extended Data Fig. 5, presented with the normalized quantities in both axes.

The crossing time of a clump tcross is defined as rclump divided by the infalling gas flow velocity of 850 km s−1: tcross = 1.2 yr, 12 yr and 120 yr for rclump = 10−3 pc, 10−2 pc and 0.1 pc, respectively. All nine curves are totally overlapping in the normalised axes.

Source data

Supplementary information

Supplementary Video 1

Time evolution of the torus gas in the torus frame (corresponding to Fig. 1a–d).

Supplementary Video 2

Time evolution of the torus gas in the torus frame (corresponding to Fig. 1e–h).

Source data

Source Data Fig. 1

Numerical data used to generate the figure.

Source Data Fig. 2

Numerical data used to plot curves in the figure.

Source Data Fig. 3

Numerical data used to plot points in the figure.

Source Data Fig. 4

Numerical data used to generate graphs in the figure.

Source Data Extended Data Fig. 2

Numerical data used to generate graphs in the figure.

Source Data Extended Data Fig. 3

Numerical data used to generate table (data are identical to those for Fig. 4).

Source Data Extended Data Fig. 6

Numerical data used to generate graphs in the figure (data are identical to those for Extended Data Fig. 5).

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Miki, Y., Mori, M. & Kawaguchi, T. Destruction of the central black hole gas reservoir through head-on galaxy collisions. Nat Astron 5, 478–484 (2021).

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