A Spectroscopic Search for Dormant Black Holes in Low-metallicity Binaries
Creators
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1.
California Institute of Technology
- 2. Gemini Observatory/NSF's NOIRLab, Casilla 603, La Serena, Chile
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3.
Carnegie Observatories
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4.
Max Planck Institute for Astronomy
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5.
University of California, Berkeley
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6.
Harvard-Smithsonian Center for Astrophysics
Abstract
The discovery of the massive black hole (BH) system Gaia BH3 in pre-release Gaia DR4 data suggests that wide BH binaries with luminous companions may be significantly overrepresented at low metallicities. Motivated by this finding, we have initiated a spectroscopic survey of low-metallicity stars exhibiting elevated RUWE values in Gaia DR3, using the FEROS and Automated Planet Finder spectrographs. We identify promising BH binary candidates as objects with instantaneously measured radial velocities (RVs) that are very different from their mean RVs reported in Gaia DR3. Thus far, we have observed over 500 targets, including a nearly complete sample of stars with [Fe/H] < −1.5, RUWE > 2, and G < 15. Our search has yielded one promising target exhibiting slow acceleration and an RV more than 98 km s−1 different from its DR3 mean RV, as well as dozens of other candidates with smaller RV discrepancies. We quantify the sensitivity of our search using simulations, demonstrating that it recovers at least half of the BH companions within our selection criteria. We make all the spectra and RVs from our survey publicly available and encourage further follow-up.
Copyright and License
© 2025. The Author(s). Published by IOP Publishing Ltd on behalf of the Astronomical Society of the Pacific (ASP). Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
This research was supported by NSF grant AST-2307232. H.W.R. acknowledges the European Research Council for support from the ERC Advanced grant ERC-2021-ADG-101054731. C.Y.L. acknowledges support from the Harrison and Carnegie Fellowships. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.
Contributions
P.N. was responsible for performing the data analysis. K.E. came up with the initial research concept and obtained the funding. H.R. measured abundances from the high-resolution spectra. C.Y.L., J.D.S., H.I., and J.R.L. assisted in planning and executing the APF observations, while J.M.H., R.S., H.W.R., and V.C. assisted in planning the FEROS observations. J.D.S. obtained and reduced the MIKE spectrum. R.A. measured metallicities from XP spectra. All authors were involved in writing and editing the manuscript.
Facilities
APF - , Max Planck:2.2 m - (FEROS), Magellan:Baade (MIKE) - .
Software References
astropy (Astropy Collaboration et al. 2013,2018, 2022), emcee (Foreman-Mackey et al. 2013).
Files
Nagarajan_2025_PASP_137_094202.pdf
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Additional details
Related works
- Is new version of
- Discussion Paper: arXiv:2507.12532 (arXiv)
Funding
- National Science Foundation
- AST-2307232
- European Research Council
- ERC-2021-ADG-101054731
Dates
- Accepted
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2025-08-27
- Available
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2025-09-15Published online