Stellar evolution models suggest that there ought to be 10 million stellar-mass black holes (BHs) in our Milky Way.
At the onset of the project, we kne only of ~20 BHs, and those were exclusively in the rare binaries systems,
where accretion onto the BHs makes them shine in X-rays. Beyond that, no non-accreting ‘dormant’ BH had
ever been robustly identified across the Galactic disk.
Finding dormant BHs in binaries (dBHBs) is fundamental to learning when which BHs form, how massive
stars die, and 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. 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, and rigorously eliminating ‘false positives’.
This proposal set out to execute an unprecedented search for Galactic dBHBs, and should find ~100 of them
if initial model predictions are correct. In this search, the project draws crucially not only on data from ESA's Gaia mission,
but also on the spectra of ~580,000 massive stars from the SDSS-V survey. As SDSS-V project scientist, I had helped
shape this project as the only all-sky, multi-epoch spectroscopic survey, systematically focused on stellar physics. 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,the project. gets detailed RV
curves and crucial ‘spectroscopic disentangling’ to identify false positives that have two luminous components.
Either dBHBs do not exist in any numbers in our Galaxy, or this search will find and characterize them.
Beyond the ‘risky’ search for dBHBs, this program will break ground in identifying numerous other dark
companions to stars, such as white dwarfs or neutron stars. If fully successful, the project will have made
decisive contributions to understanding how many black holes we have in the Milky Way, and
what stars end their lives as black holes.
