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An intermediate-mass black hole in the centre of the globular cluster 47 Tucanae

A Corrigendum to this article was published on 03 May 2017

Abstract

Intermediate-mass black holes should help us to understand the evolutionary connection between stellar-mass and super-massive black holes1. However, the existence of intermediate-mass black holes is still uncertain, and their formation process is therefore unknown2. It has long been suspected that black holes with masses 100 to 10,000 times that of the Sun should form and reside in dense stellar systems3,4,5,6. Therefore, dedicated observational campaigns have targeted globular clusters for many decades, searching for signatures of these elusive objects. All candidate signatures appear radio-dim and do not have the X-ray to radio flux ratios required for accreting black holes7. Based on the lack of an electromagnetic counterpart, upper limits of 2,060 and 470 solar masses have been placed on the mass of a putative black hole in 47 Tucanae (NGC 104) from radio and X-ray observations, respectively8,9. Here we show there is evidence for a central black hole in 47 Tucanae with a mass of solar masses when the dynamical state of the globular cluster is probed with pulsars. The existence of an intermediate-mass black hole in the centre of one of the densest clusters with no detectable electromagnetic counterpart suggests that the black hole is not accreting at a sufficient rate to make it electromagnetically bright and therefore, contrary to expectations, is gas-starved. This intermediate-mass black hole might be a member of an electromagnetically invisible population of black holes that grow into supermassive black holes in galaxies.

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Figure 1: Projected distribution of neutron stars in 47 Tuc.
Figure 2: Kinematic data for 47 Tuc compared with theoretical models.
Figure 3: Comparison of N-body model likelihoods of 47 Tuc.
Figure 4: Inferred masses of the black hole and the globular cluster.

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Acknowledgements

This work was supported in part by the Black Hole Initiative at Harvard University, through the grant from the John Templeton Foundation.

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Authors and Affiliations

Authors

Contributions

B.K. initiated the project, led the collaboration, and wrote the manuscript. H.B. calculated the N-body models. A.L. made contributions to the conceptual definition of the project. All authors contributed to the analysis.

Corresponding author

Correspondence to Bülent Kızıltan.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Cluster surface brightnesses and corresponding neutron star spatial distributions in N-body models for projected half-light radii.

The solid, dashed and dotted lines represent N-body models with an IMBH, with and without primordial binaries, respectively. a, All models have roughly comparable surface brightnesses; in contrast, b, the distribution of neutron stars is notably different for the IMBH model. The neutron star spatial distributions for N-body simulations with and without primordial binaries are similar. Therefore, it is unlikely that primordial binaries play a considerable role in shaping the final segregation profile of clusters with an IMBH.

Extended Data Figure 2 Comparison of the observed and predicted pulsar accelerations.

The observed acceleration (shaded areas) for each pulsar (named at the top of each panel) in 47 Tuc is compared with the integrated acceleration distributions for pulsars with the same line-of-sight distance predicted from N-body simulations (solid line, models with IMBH; dashed line, model without IMBH). The KL divergence method is used to calculate the integrated information entropy between distributions (equation (1)). The shaded areas show the 68%, 95% and 99% range of possible accelerations experienced owing to the gravitational potential of the cluster. Darker shades represent higher probability. The ambiguity is largely due to the unknown intrinsic spin-down of individual pulsars.

Extended Data Figure 3 Predictive power correlates with number of observed pulsars.

Shown are the normalized probabilities of N-body models with different black hole masses, as in Fig. 4a, for different numbers of randomly selected pulsars. The converging inference with increasing number of pulsars is indicative of statistical learning and demonstrates that the information comes from observations. The line thickness scales with the level of ambiguity for each inference.

Extended Data Table 1 Pulsars with timing solutions in 47 Tuc (NGC 104)

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Kızıltan, B., Baumgardt, H. & Loeb, A. An intermediate-mass black hole in the centre of the globular cluster 47 Tucanae. Nature 542, 203–205 (2017). https://doi.org/10.1038/nature21361

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