The close environments of accreting massive black holes are shaped by radiative feedback


The majority of the accreting supermassive black holes in the Universe are obscured by large columns of gas and dust1,2,3. The location and evolution of this obscuring material have been the subject of intense research in the past decades4,5, and are still debated. A decrease in the covering factor of the circumnuclear material with increasing accretion rates has been found by studies across the electromagnetic spectrum1,6,7,8. The origin of this trend may be driven by the increase in the inner radius of the obscuring material with incident luminosity, which arises from the sublimation of dust9; by the gravitational potential of the black hole10; by radiative feedback11,12,13,14; or by the interplay between outflows and inflows15. However, the lack of a large, unbiased and complete sample of accreting black holes, with reliable information on gas column density, luminosity and mass, has left the main physical mechanism that regulates obscuration unclear. Here we report a systematic multi-wavelength survey of hard-X-ray-selected black holes that reveals that radiative feedback on dusty gas is the main physical mechanism that regulates the distribution of the circumnuclear material. Our results imply that the bulk of the obscuring dust and gas is located within a few to tens of parsecs of the accreting supermassive black hole (within the sphere of influence of the black hole), and that it can be swept away even at low radiative output rates. The main physical driver of the differences between obscured and unobscured accreting black holes is therefore their mass-normalized accretion rate.

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Figure 1: Relation between the fraction of obscured AGN and the Eddington ratio.
Figure 2: Relation between the fraction of obscured AGN and the luminosity for different ranges of the Eddington ratio.
Figure 3: Eddington ratio–column density diagram.
Figure 4: Radiation-regulated unification of AGN.


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This work is dedicated to the memory of our friend and collaborator Neil Gehrels. We acknowledge the work done by the Swift/BAT team to make this project possible. We thank M. Kishimoto, C.-S. Chang, D. Asmus, M. Stalevski, P. Gandhi and G. Privon for discussions. We thank N. Secrest for providing us with the stellar masses of the Swift/BAT sample. This paper is part of the Swift/BAT AGN Spectroscopic Survey (BASS, 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. We acknowledge financial support from FONDECYT 1141218 (C.R., F.E.B.), FONDECYT 1160999 (E.T.), Basal-CATA PFB–06/2007 (C.R., E.T., F.E.B.), the China-CONICYT fund (C.R.), the Swiss National Science Foundation (grant PP00P2 138979 and PP00P2 166159, K.S.), the Swiss National Science Foundation (SNSF) through the Ambizione fellowship grant PZ00P2 154799/1 (M.J.K.), the NASA ADAP award NNH16CT03C (M.J.K.), the Chinese Academy of Science grant no. XDB09030102 (L.C.H.), the National Natural Science Foundation of China grant no. 11473002 (L.C.H.), the Ministry of Science and Technology of China grant no. 2016YFA0400702 (L.C.H.), the ERC Advanced Grant Feedback 340442 (A.C.F.), and the Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS (F.E.B.). Part of this work was carried out while C.R. was Fellow of the Japan Society for the Promotion of Science (JSPS) at Kyoto University. This work was partly supported by the Grant-in-Aid for Scientific Research 17K05384 (Y.U.) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). We acknowledge the usage of the HyperLeda database (

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C.R. wrote the manuscript with comments and input from all authors, and performed the analysis. B.T. calculated the bolometric corrections, B.T., M.J.K., K.O. and I.L. analysed the optical spectra and inferred the black hole masses, C.R. carried out the broad-band X-ray spectral analysis and Y.U. calculated the intrinsic column density distribution of AGN for different ranges of the Eddington ratio.

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Correspondence to Claudio Ricci.

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Extended data figures and tables

Extended Data Figure 1 Eddington ratio distribution for different classes of AGN.

ac, Histograms of λEdd for unobscured (NH < 1022 cm−2; a), obscured Compton-thin (1022 cm−2 ≤ NH < 1024 cm−2; b) and Compton-thick (NH ≥ 1024 cm−2; c) AGN. The vertical red dashed lines show the median values for the different subsets of sources.

Extended Data Figure 2 Eddington ratio versus X-ray luminosity and black hole mass.

a, b, Scatter plots of λEdd versus the 2–10-keV intrinsic luminosity (L2–10; a) and the black hole mass (MBH; b) for unobscured (NH ≤ 1022 cm−2; black open diamonds), obscured (1022 cm−2 ≤ NH < 1024 cm−2; red filled circles) and Compton-thick (NH ≥ 1024 cm−2; blue filled squares) AGN. The black dashed lines represent values for constant mass (a) and luminosity (b).

Extended Data Figure 3 Fraction of obscured sources versus luminosity.

Fraction of obscured Compton-thin sources versus the intrinsic 14–150-keV luminosities for the non-blazar AGN of the Swift/BAT 70-month catalogue. The fraction of obscured sources is normalized in the NH = 1020–1024 cm−2 range. The filled area represents the 16th and 84th quantiles of a binomial distribution20.

Extended Data Figure 4 Fraction of obscured sources versus λEdd for two ranges of luminosity and black hole mass.

a, b, Fraction of obscured Compton-thin sources versus Eddington ratio for two bins of the 14–150-keV intrinsic luminosity (a) and of the black hole mass (b). The dashed vertical lines represent the effective Eddington limit for dusty gas with NH = 1022 cm−2 () and NH = 1023 cm−2 (). The plots are normalized to unity in the interval 20≤ log[NH (cm−2)] < 24, and the shaded areas represent the 16th and 84th quantiles of a binomial distribution20. The same trend found for the whole sample is obtained when looking at different bins of L14–150 and MBH, confirming that the Eddington ratio is the main parameter driving obscuration.

Extended Data Figure 5 Relation between the fraction of obscured AGN and the Eddington ratio assuming different bolometric corrections.

a, b, The bolometric corrections used are dependent on the bolometric luminosity (a; blue111 and red108 lines) and on the Eddington ratio (b; blue112 and red108 lines). The shaded areas represent the 16th and 84th quantiles of a binomial distribution20. The figure shows that our results are mostly independent on the choice of the bolometric correction.

Extended Data Figure 6 Median value of the column density versus Eddington ratio for AGN with 20 ≤ log[NH (cm−2)] ≤ 24.

The plot highlights the sharp transition at log(λEdd) ≈ −1.5 between AGN being typically obscured to unobscured. The filled area shows the median absolute deviation. The dashed vertical lines represent the effective Eddington limit for a dusty gas with NH = 1022 cm−2 () and NH = 1023 cm−2 () for standard dust grain composition of the interstellar medium, showing that radiation pressure regulates the median column density of AGN.

Extended Data Figure 7 Median value of the column density versus Eddington ratio for different luminosity and black hole mass ranges.

a, b, Same as Extended Data Fig. 6 but for two different ranges of the intrinsic 14–150-keV luminosity (a; in erg s−1) and black hole mass (b; in M). The filled areas represent the median absolute deviations. The dashed vertical lines represent the effective Eddington limit for dusty gas with NH = 1022 cm−2 () and NH = 1023 cm−2 () for standard dust grain composition of the interstellar medium.

Extended Data Table 1 Intrinsic fraction of sources with a given column density in different ranges of λEdd

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Ricci, C., Trakhtenbrot, B., Koss, M. et al. The close environments of accreting massive black holes are shaped by radiative feedback. Nature 549, 488–491 (2017).

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