Hunting Dormant Black Holes in the Galaxy with SDSS-V

Introduction

Stellar evolution models suggest that there ought to be ~10e7 stellar-mass black holes (BHs) in our Milky Way. However, we currently know only of ~20 BHs, and those are in binaries where accretion makes them shine in X-rays. Beyond that, no non-accreting ‘dormant’ BH has ever been robustly identified across the Galactic disk.

Importance of Finding Dormant BHs

Finding dormant BHs in binaries (dBHBs) is fundamental to learning:

1. When which BHs form
2. How massive stars die
3. What the precursors of BH gravitational wave events are

Such BHs cause characteristic time variations in radial velocity (RV), flux, and light-centroid positioning of their luminous companion, providing an avenue for detection and study.

Current Data and Challenges

Spectroscopic, astrometric, and photometric surveys now yield the data needed to search for dBHBs. Yet, recent dBHB candidates have instead turned out to be short-lived evolutionary phases of close binary stars. Thus, any successful search for dBHBs must entail:

• Sifting through vast samples using a combination of these signatures
• Rigorously eliminating ‘false positives’

Proposed Search for dBHBs

I propose an unprecedented search for Galactic dBHBs that should find ~100 of them, drawing crucially on the spectra of ~580,000 massive stars from the SDSS-V survey. As SDSS-V project scientist, I have helped shape it as the only all-sky, multi-epoch spectroscopic survey, systematically focused on stellar physics.

Methodology

Novel analysis of these spectra will be meshed with detailed modeling of TESS light curves and Gaia astrometry. Through guaranteed-time, high-resolution follow-up spectroscopy on the candidates, we will get:

• Detailed RV curves
• Crucial ‘spectroscopic disentangling’ to identify false positives that have two luminous components

Conclusion

Either dBHBs do not exist in any numbers in our Galaxy, or this proposal will find and characterize them. Beyond the ‘risky’ search for dBHBs, this program will break ground in identifying numerous other dark companions to massive stars, such as white dwarfs or neutron stars.