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No massive black holes in the Milky Way halo

Nature volume 632, pages 749–751 (2024)Cite this article

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Abstract

The gravitational wave detectors have shown a population of massive black holes that do not resemble those observed in the Milky Way1,2,3 and whose origin is debated4,5,6. According to a possible explanation, these black holes may have formed from density fluctuations in the early Universe (primordial black holes)7,8,9, and they should comprise several to 100% of dark matter to explain the observed black hole merger rates10,11,12. If these black holes existed in the Milky Way dark matter halo, they would cause long-timescale gravitational microlensing events lasting years13. The previous experiments were not sufficiently sensitive to such events14,15,16,17. Here we present the results of the search for long-timescale microlensing events among the light curves of nearly 80 million stars located in the Large Magellanic Cloud that were monitored for 20 years by the Optical Gravitational Lensing Experiment survey18. We did not find any events with timescales longer than 1 year, whereas all shorter events detected may be explained by known stellar populations. We find that compact objects in the mass range from 1.8 × 10−4M⊙ to 6.3M⊙ cannot make up more than 1% of dark matter, and those in the mass range from 1.3 × 10−5M⊙ to 860 M⊙ cannot make up more than 10% of dark matter. Thus, primordial black holes in this mass range cannot simultaneously explain a substantial fraction of dark matter and gravitational wave events.

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Fig. 1: The 95% upper limits on PBHs (and other compact objects) as constituents of dark matter.

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Data availability

The data used to perform the analysis (event detection efficiencies, source star counts and posterior distributions of event parameters) are publicly available at https://www.astrouw.edu.pl/ogle/ogle4/LMC_OPTICAL_DEPTH/ and Zenodo (https://doi.org/10.5281/zenodo.10879577) (ref. 57).

Code availability

The custom codes for the simulation of microlensing events towards the LMC and the calculation of limits on PBHs as dark matter are available upon request from the corresponding author.

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Acknowledgements

We thank all the OGLE observers for their contribution to the collection of the photometric data over the decades. We thank T. Bulik for his comments on the paper. This research was funded in part by the National Science Centre, Poland, grant OPUS 2021/41/B/ST9/00252 awarded to P.M.

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Authors and Affiliations

  1. Astronomical Observatory, University of Warsaw, Warsaw, Poland

    Przemek Mróz, Andrzej Udalski, Michał K. Szymański, Igor Soszyński, Łukasz Wyrzykowski, Paweł Pietrukowicz, Szymon Kozłowski, Radosław Poleski, Jan Skowron, Dorota Skowron, Krzysztof Ulaczyk, Mariusz Gromadzki, Krzysztof Rybicki, Patryk Iwanek, Marcin Wrona & Milena Ratajczak

  2. Department of Physics, University of Warwick, Coventry, UK

    Krzysztof Ulaczyk

  3. Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot, Israel

    Krzysztof Rybicki

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Contributions

P.M. led the analysis and interpretation of the data and wrote the paper. A.U. is the principal investigator of the OGLE project and was responsible for the data reduction. All authors collected the OGLE photometric observations and reviewed, discussed and commented on the presented results and on the paper.

Corresponding author

Correspondence to Przemek Mróz.

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Extended data figures and tables

Extended Data Fig. 1 Expected number of microlensing events from a PBH dark matter halo.

a, Number of microlensing events as a function of their Einstein timescale that should have been discovered by OGLE assuming that the entire dark matter were composed of PBHs of 0.01 (red), 1 (blue), and 100 M⊙ (green). b, Number of gravitational microlensing events expected to be detected by OGLE if entire dark matter were composed of compact objects of a given mass \({N}_{\exp }(f=1,M)\). Thin solid lines correspond to fields observed during OGLE-III and OGLE-IV phases (from 2001 to 2020), dashed lines – fields observed during OGLE-IV only (from 2010 to 2020). Blue lines mark the contribution from the Milky Way dark matter halo, red lines – the LMC dark matter halo.

Extended Data Fig. 2 95% upper limits on PBHs as constituents of dark matter.

a, Dependence of the limits on the Milky Way halo model. The black solid line marks limits for the Cautun et al. (2020)24 model. The blue dashed line—Jiao et al. (2023)27 model. b, Limits as a function of the limiting magnitude.

Extended Data Fig. 3 Predictions for a multi-peak PBH mass function.

a, Multi-peak mass function of PBHs from ref. 10 (assuming spectral index \({\widetilde{n}}_{{\rm{s}}}=0.960\)). b, Expected distribution of event timescales assuming that the entire dark matter is composed of PBHs with the mass function from ref. 10. In total, we should have detected 513 microlensing events.

Extended Data Fig. 4 Poisson probability of observing 13 events as a function of the expected number of events in the model.

The blue and red dashed lines mark the models by ref. 24 and ref. 25, respectively.

Extended Data Fig. 5 Expected number of microlensing events from known stellar populations in the LMC and Milky Way disk in OGLE-IV fields.

The Milky Way disk model used in calculations is from Han & Gould (2003)25. Black asterisks mark events that are a part of the statistical sample of ref. 20, black dots mark other events.

Extended Data Fig. 6 Expected distributions of timescales and parallaxes of microlensing events from known stellar populations in the LMC and Milky Way disk.

a, b, Expected distributions of Einstein timescales for the Han & Gould (2003)25 and Cautun et al. (2020)24 Milky Way disk models. Vertical blue lines mark the detected events. c, d, Expected distributions of microlensing parallaxes.

Extended Data Fig. 7 Contours of the likelihood function (Equation (1)).

a, Contours for the Han & Gould (2003)25 Milky Way disk model. b, Contours for the Cautun et al. (2020)24 Milky Way disk model. The color codes the difference \(\Delta {\chi }^{2}\equiv 2(\mathrm{ln}{{\mathcal{L}}}_{\max }-\mathrm{ln}{\mathcal{L}})\).

Extended Data Fig. 8 Gaia EDR3 proper motions of stars in the LMC

.

Extended Data Table 1 Expected number of microlensing events from known stellar populations in the Milky Way disk and the LMC
Full size table
Extended Data Table 2 Parameters of the LMC proper motion model
Full size table

Supplementary information

Supplementary Table 1

The 95% upper limits on primordial black holes (and other compact objects) as constituents of dark matter

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Mróz, P., Udalski, A., Szymański, M.K. et al. No massive black holes in the Milky Way halo. Nature 632, 749–751 (2024). https://doi.org/10.1038/s41586-024-07704-6

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