Review Article | Published:

Nuclear obscuration in active galactic nuclei


The material surrounding accreting supermassive black holes connects the active galactic nucleus with its host galaxy and, besides being responsible for feeding the black hole, provides important information on the feedback that nuclear activity produces on the galaxy. In this Review, we summarize our current understanding of the close environment of accreting supermassive black holes obtained from studies of local active galactic nuclei carried out in the infrared and X-ray regimes. The structure of this circumnuclear material is complex, clumpy and dynamic, and its covering factor depends on the accretion properties of the active galactic nucleus. In the infrared, this obscuring material is a transition zone between the broad- and narrow-line regions, and, at least in some galaxies, it consists of two structures: an equatorial disk/torus and a polar component. In the X-ray regime, the obscuration is produced by multiple absorbers across various spatial scales, mostly associated with the torus and the broad-line region. In the coming decade, the new generation of infrared and X-ray facilities will greatly contribute to our understanding of the structure and physical properties of nuclear obscuration in active galactic nuclei.

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The authors acknowledge A. Alonso-Herrero, P. Gandhi, N. A. Levenson and M. Stalevski for useful comments that helped to improve this Review. C.R.A. acknowledges the Ramón y Cajal Program of the Spanish Ministry of Economy and Competitiveness through project RYC-2014-15779 and the Spanish Plan Nacional de Astronomía y Astrofísica under grant AYA2016-76682-C3-2-P. C.R. acknowledges financial support from the China-CONICYT fellowship programme, FONDECYT 1141218 and Basal-CATA PFB–06/2007. This work is sponsored by the Chinese Academy of Sciences (CAS), through a grant to the CAS South America Center for Astronomy (CASSACA) in Santiago, Chile.

Author information

The authors’ order is purely alphabetical as they both contributed equally to this work. They decided on the concept of the Review and provided/adapted the figures. C.R. and C.R.A. wrote the X-ray and infrared parts of the text, respectively, and worked together to put them in common.

Competing interests

The authors declare no competing financial interests.

Correspondence to Cristina Ramos Almeida.

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Fig. 1: Sketch of the main AGN structures seen along the equatorial and polar directions.
Fig. 2: ALMA maps of the dust continuum and molecular gas in the nucleus of NGC 106815.
Fig. 3: Sketches of two clumpy tori with different covering factors, based on the clumpy torus scheme
Fig. 4: Structure of the obscuring material in local AGNs.
Fig. 5: Evolution of the obscuring material’s covering factor with luminosity.
Fig. 6: Examples of absorption variability in the X-ray spectrum of nearby AGNs.
Fig. 7: Interferometry results from VLTI/MIDI observations of two nearby AGNs.