SDSS releases DR20, pairing 200,000 X-ray sources with black holes
The Sloan Digital Sky Survey (SDSS) announced Data Release 20 (DR20) of its Black Hole Mapper (BHM) program, part of the fifth-generation survey SDSS-V. The release adds the first optical BOSS spectra gathered from the southern hemisphere, at Las Campanas Observatory in Chile, alongside expanded observations from Apache Point Observatory in New Mexico. In total, DR20 contains over 3.3 million optical spectra spanning 500,000 galaxies and 1.5 million stars.
The centerpiece of the black hole work is SPIDERS, a sub-program that pairs SDSS optical spectroscopy with X-ray sky maps from the German-led eROSITA mission. SPIDERS has produced optical identifications and redshift measurements, meaning precise cosmic distances, for about 200,000 X-ray targets, which the team describes as the largest, most uniform spectroscopic follow-up of X-ray sources assembled to date. Most of these X-ray sources are actively accreting supermassive black holes, known as active galactic nuclei (AGN) or quasars.
By building an X-ray luminosity function, essentially a census of how many black holes exist at different power outputs and cosmic eras, the team traced supermassive black hole growth back to redshifts near six, deep into the early universe. The combined SDSS-eROSITA dataset found more giant black holes in that early era and higher densities of the most luminous AGN at high redshift than previously expected, indicating that rapidly growing supermassive black holes were more common early on than researchers had assumed. The survey also uncovered black holes that traditional optical and UV surveys miss, particularly the lower-luminosity, more distant ones. Calculating total accumulated black hole growth over time, the researchers found that black holes selected through soft X-rays account for only a minority of total black hole mass in the local universe, implying that 70% to 90% of all supermassive black hole growth happened behind heavy veils of dust and gas, or during phases when even X-ray emission was suppressed.
Beyond the X-ray work, BHM tracks quasar variability through several time-domain sub-programs. Its Reverberation Mapping program repeatedly observes targeted quasar fields over days to years to measure the time delay between light from the central accretion disk and the surrounding broad-line region, yielding direct geometric black hole mass measurements. The All-Quasar Multi-Epoch Spectroscopy (AQMES) program monitors tens of thousands of quasars across repeated epochs to track outflows, binary black hole candidates and quasars that dramatically change appearance over time. Both hemispheres are served by SDSS-V's Robotic Focal Plane System, automated fiber-positioning robots installed on the 2.5-meter telescopes at Apache Point and Las Campanas that speed up multi-object observations.
SDSS says all DR20 Black Hole Mapper data is open to researchers, educators and the public, accessible through its Science Archive and Catalog Archive servers, the SciServer Compute platform, and the Zora and Valis web interfaces, along with value-added catalogs and Jupyter Notebook tutorials. The dataset builds on the earlier eRASS1 X-ray source catalog (Merloni et al., 2024) and two forthcoming papers on the second eROSITA data release and the resulting AGN luminosity function.
Key facts
- DR20 adds 3.3 million optical spectra across 500,000 galaxies and 1.5 million stars, including the first southern-hemisphere BOSS spectra from Las Campanas Observatory in Chile.
- The SPIDERS sub-program paired SDSS optical spectroscopy with eROSITA X-ray maps to obtain optical identifications and redshifts for about 200,000 X-ray targets, most of them accreting black holes.
- The resulting X-ray luminosity function shows more giant black holes and higher densities of luminous AGN at redshifts near six than previously expected.
- Researchers calculate that 70% to 90% of all supermassive black hole growth in the local universe happened hidden behind dust and gas, or during X-ray-suppressed phases.
- All DR20 data, catalogs and Jupyter tutorials are freely available through SDSS's Science Archive Server, SciServer Compute, and the Zora and Valis web interfaces.
Why it matters
DR20 is the first release to combine SDSS-V's optical spectroscopy at scale with eROSITA's X-ray sky maps across both hemispheres, giving astronomers the most uniform census yet of accreting supermassive black holes. The finding that 70% to 90% of black hole growth happened hidden behind dust, gas, or X-ray-suppressed phases changes how much of the universe's black hole growth astronomers think earlier, optically biased surveys ever saw.
Who it affects
Astronomers and astrophysicists studying black hole demographics and galaxy evolution, particularly researchers working with X-ray and multi-wavelength surveys such as eROSITA. The open-access design also targets educators, students and early-career researchers, who BHM Program Head Scott Anderson specifically credits with leading much of the emerging research.
How to use it
DR20 is free and open to anyone. Spectra, tabular data and eROSITA counterpart catalogs are accessible through SDSS's Science Archive Server and Catalog Archive Server, including SQL queries via SciServer Compute. The Zora and Valis web interfaces let users search target metadata, inspect spectral masks and compare spectra with theoretical models, either in a browser or programmatically in Python. Value-added catalogs and step-by-step Jupyter Notebook tutorials are included. The data is described online at sdss.org/dr20.
How solid is it
This is an official SDSS-V collaboration release, issued jointly with the German eROSITA Consortium after what principal investigator Andrea Merloni describes as more than a decade of joint planning. It cites a published paper (Merloni et al., 2024, Astronomy & Astrophysics) plus two papers currently in press or under submission, and quotes four named scientists directly, including the SDSS-V director and the eROSITA principal investigator.
Risks and caveats
Two of the three cited papers underpinning the analysis, the eROSITA-DE second data release and the AGN X-ray luminosity function study, are listed as in press or submitted rather than peer reviewed and published. The release also notes that some spectra were unusual enough to break the survey's standard automated analysis pipeline and required manual visual inspection, a reminder that parts of this dataset rely on human judgment calls rather than fully automated processing.
“This release marks the culmination of more than a decade of joint planning and scientific exchange between the German eROSITA Consortium and the Sloan Digital Sky Survey”
— Andrea Merloni, eROSITA Principal Investigator and BHM Survey Scientist