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A supra-massive population of stellar-mass black holes in the globular cluster Palomar 5


Palomar 5 is one of the sparsest star clusters in the Galactic halo and is best known for its spectacular tidal tails, spanning over 20° across the sky. With N-body simulations, we show that both distinguishing features can result from a stellar-mass black hole population, comprising ~20% of the present-day cluster mass. In this scenario, Palomar 5 formed with a ‘normal’ black hole mass fraction of a few per cent, but stars were lost at a higher rate than black holes, such that the black hole fraction gradually increased. This inflated the cluster, enhancing tidal stripping and tail formation. A billion years from now, the cluster will dissolve as a 100% black hole cluster. Initially denser clusters end up with lower black hole fractions, smaller sizes and no observable tails. Black hole-dominated, extended star clusters are therefore the likely progenitors of the recently discovered thin stellar streams in the Galactic halo.

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Fig. 1: Comparison between the N-body model wBH-1 and observations of Pal 5 and its stream.
Fig. 2: Results for different initial conditions for clusters with BHs.
Fig. 3: Dependence of cluster and stream properties on the BH content.
Fig. 4: Dependence of stream visibility on BH content.
Fig. 5: Mass function of bound stars and remnants of wBH-1 at 11.5 Gyr.
Fig. 6: Surface density and velocity dispersion profiles of models with and without BHs.

Data availability

A snapshot of the wBH-1 model is published on Zenodo ( All N-body data are available upon request from the corresponding author.

Code availability

NBODY6++GPU is available from LIMEPY is available from


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M.G. and E.B. acknowledge financial support from the European Research Council (grant number ERC StG-335936, CLUSTERS) and M.G. acknowledges support from the Spanish Ministry of Science and Innovation through a Europa Excelencia grant (EUR2020-112157). F.A. acknowledges support from a Rutherford fellowship (grant number ST/P00492X/2) from the Science and Technology Facilities Council. E.B. acknowledges financial support from a Vici grant from the Netherlands Organisation for Scientific Research (NWO). M.G. thanks G. Pérez Forcadell for installing the GPU server at the ICCUB on which all the simulations were run. We thank R. Ibata for sharing the data of Pal 5’s surface density profile, Ł. Wyrzykowski for discussions on microlensing and S. Aarseth, K. Nitadori and L. Wang for maintaining NBODY6 and NBODY6++GPU and making the codes publicly available. M.G. and F.A. thank L. Wang and S. Banerjee for discussions on the recent SSE and BSE updates and the implementation in NBODY6++GPU. This research made use of ASTROPY, a community-developed core Python package for astronomy92,93 (

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M.G. ran all N-body simulations, analysed them and was in charge of the writing. D.E. was in charge of stream modelling and deriving the orbit of Pal 5 and the parameters of the MW model. F.A. contributed to the BH physics of the N-body models. E.B. converted stream models to observed quantities and J.P. contributed to the binary properties. All authors assisted in the development, analysis and writing of the paper.

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Correspondence to Mark Gieles.

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Gieles, M., Erkal, D., Antonini, F. et al. A supra-massive population of stellar-mass black holes in the globular cluster Palomar 5. Nat Astron 5, 957–966 (2021).

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