Periodic Reporting for period 1 - Stellar-BHs-SDSS-V (Hunting Dormant Black Holes in the Galaxy with SDSS-V)
Periodo di rendicontazione: 2023-08-01 al 2026-01-31
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.
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.
Overall, the first years have been very successful, in four respects:
a) the acquisition and accumulation of the data necessary for the dormant black hole (sBH) searches
b) the build-up of diverse software tools, to find the different dBH signatures
c) the breakthrough discovery of the first dormant black holes in our Galaxy
d) the first population constraints on the incidence of such dBH, derived from large data sets
On a) the necessary data sets: Several team members (esp. Villasenor, Müller-Horn and Rix) have worked on assuring that the SDSS-V survey started fully with its OBA star program. An extensive amount of data vetting and validation is corrently ongoing, to see which rapid radial velocity variations are due to potential dBHs, not data issues. Two team members (El-Badry & Rix) were closely involved in the earliest validation of dBH candidates from the DR3 data release of the Gaia mission; they also assembled the earliest, and arguably best, samples od dBH candidates for spectrosocpic follow-up. A concerted team effort (Müller-Horn, El-Badry, Villasenor) worked on planning and executing the candidate follow-up using the 2.2m FEROS spectrograph (whose operation is in part funded by ERC resources).
On b) As laid out in the proposal, a number of software tools needed to be built up to search for dBHs in different period regimes. A first version of all these software tools is now in place, with necessary improvements still upcoming. Photometric light curve analysis has been led by M. Green. The spectral disentangling code development has been led by R. Seeburger, and is yieding excellent results on identifying dBH "confounders". Team mamber K. El-Badry succeeded in the hard challenge to model the Gaia mission data-taking process well enough to create a code that can predict for any envisioned binary star system whether it will get an orbit solution or not; this has opened up population modelling of Gaia data.
On c) The biggest success of the project so far is the first discovery of dormnant black holes in the Galaxy: ultimately the core goal of this proposal (see next section). This happened through the identification of "astrometric candidates" in the Gaia DR3 data, the elimination of confounders/impostors, and the verification using the 2.2m telescope with the FEROS spectrograph. Note that this follow-up was enabled by the "facility resources" funded as part of this grant.
On d) We have already achieved several important population constraints (beyond the discoveries of of the first dBH). By analyzing TESS data, team-member M. Green could derive the most stringent to date constraint (upper limit) on dBHs in close binaries: they must be lass than one in a million. Team-member Johanna Müller-Horn worked on and just succeeded in the derivation of new candidate catalogs from the statistical information contained in the Gaia catalog.
a) the acquisition and accumulation of the data necessary for the dormant black hole (sBH) searches
b) the build-up of diverse software tools, to find the different dBH signatures
c) the breakthrough discovery of the first dormant black holes in our Galaxy
d) the first population constraints on the incidence of such dBH, derived from large data sets
On a) the necessary data sets: Several team members (esp. Villasenor, Müller-Horn and Rix) have worked on assuring that the SDSS-V survey started fully with its OBA star program. An extensive amount of data vetting and validation is corrently ongoing, to see which rapid radial velocity variations are due to potential dBHs, not data issues. Two team members (El-Badry & Rix) were closely involved in the earliest validation of dBH candidates from the DR3 data release of the Gaia mission; they also assembled the earliest, and arguably best, samples od dBH candidates for spectrosocpic follow-up. A concerted team effort (Müller-Horn, El-Badry, Villasenor) worked on planning and executing the candidate follow-up using the 2.2m FEROS spectrograph (whose operation is in part funded by ERC resources).
On b) As laid out in the proposal, a number of software tools needed to be built up to search for dBHs in different period regimes. A first version of all these software tools is now in place, with necessary improvements still upcoming. Photometric light curve analysis has been led by M. Green. The spectral disentangling code development has been led by R. Seeburger, and is yieding excellent results on identifying dBH "confounders". Team mamber K. El-Badry succeeded in the hard challenge to model the Gaia mission data-taking process well enough to create a code that can predict for any envisioned binary star system whether it will get an orbit solution or not; this has opened up population modelling of Gaia data.
On c) The biggest success of the project so far is the first discovery of dormnant black holes in the Galaxy: ultimately the core goal of this proposal (see next section). This happened through the identification of "astrometric candidates" in the Gaia DR3 data, the elimination of confounders/impostors, and the verification using the 2.2m telescope with the FEROS spectrograph. Note that this follow-up was enabled by the "facility resources" funded as part of this grant.
On d) We have already achieved several important population constraints (beyond the discoveries of of the first dBH). By analyzing TESS data, team-member M. Green could derive the most stringent to date constraint (upper limit) on dBHs in close binaries: they must be lass than one in a million. Team-member Johanna Müller-Horn worked on and just succeeded in the derivation of new candidate catalogs from the statistical information contained in the Gaia catalog.
There. are numerous results beyond the state of the art that have been achieved by the team so far, reflected in ~20 refereed publications.
The most important results (beyond state-of-the-art can be summarized as follows):
1. Discovery of Gaia BH1 and BH2: The first dynamically confirmed dormant stellar-mass black holes identified through astrometry. Gaia BH1 (9.6 M☉ at 480 pc) and Gaia BH2 (8.9 M☉ at 1160 pc) are the nearest known black holes and represent a new population of non-interacting BH binaries that were invisible to X-ray surveys. These discoveries validated the core methodology of this ERC project.
2. Population of Wide-Orbit Neutron Star Candidates: Systematic identification of 21 NS candidates in wide binaries from Gaia DR3, demonstrating that the techniques developed for BH detection extend to the full compact object mass spectrum.
3. Massive Binary Statistics at Low Metallicity: The BLOeM survey revealed that the close binary fraction of massive stars is significantly enhanced at SMC metallicity (Z ~ 0.2 Z☉), with important implications for gravitational wave source formation in the early Universe.
4. Constraints on Short-Period BH Companions: Upper limits on the frequency of BH companions to Sun-like stars at periods <10 days (Green et al. 2025), demonstrating that close BH binaries with solar-type companions are rare (<0.1%).
The most important results (beyond state-of-the-art can be summarized as follows):
1. Discovery of Gaia BH1 and BH2: The first dynamically confirmed dormant stellar-mass black holes identified through astrometry. Gaia BH1 (9.6 M☉ at 480 pc) and Gaia BH2 (8.9 M☉ at 1160 pc) are the nearest known black holes and represent a new population of non-interacting BH binaries that were invisible to X-ray surveys. These discoveries validated the core methodology of this ERC project.
2. Population of Wide-Orbit Neutron Star Candidates: Systematic identification of 21 NS candidates in wide binaries from Gaia DR3, demonstrating that the techniques developed for BH detection extend to the full compact object mass spectrum.
3. Massive Binary Statistics at Low Metallicity: The BLOeM survey revealed that the close binary fraction of massive stars is significantly enhanced at SMC metallicity (Z ~ 0.2 Z☉), with important implications for gravitational wave source formation in the early Universe.
4. Constraints on Short-Period BH Companions: Upper limits on the frequency of BH companions to Sun-like stars at periods <10 days (Green et al. 2025), demonstrating that close BH binaries with solar-type companions are rare (<0.1%).