SDSS Data Release 20 maps supermassive black holes across cosmic dawn
New data from the Sloan Digital Sky Survey shows that rapidly growing giant black holes were common in the early universe, with the majority of mass growth occurring behind obscuring dust and gas.
The Sloan Digital Sky Survey (SDSS) has released Data Release 20 (DR20), a significant milestone for the Black Hole Mapper (BHM) programme that delivers the first southern hemisphere optical observations coordinated with X-ray data from the eROSITA mission. The release encompasses over 3.3 million optical spectra covering 500,000 galaxies and 1.5 million stars, gathered from Apache Point Observatory in New Mexico and Las Campanas Observatory in Chile. This dataset provides unprecedented insights into the masses, growth, and physics of quasars and active galactic nuclei across cosmic time.
A core component of this release is the SPIDERS project, which pairs SDSS optical spectroscopy with X-ray sky maps to provide optical identifications and precise distance measurements for approximately 200,000 X-ray targets. This represents the largest uniform spectroscopic follow-up of X-ray sources ever assembled. While the survey captures galaxy clusters and energetic star systems, the vast majority of these high-energy beacons are actively accreting supermassive black holes, allowing astronomers to probe the central engines of these giants and trace their growth from the local universe back to the early cosmic dawn.
The combined wide-sky coverage of SDSS and eROSITA has provided the tightest constraints to date on the rarest and most luminous quasars. The survey reveals that rapidly growing giant black holes were surprisingly common in the early universe, with higher space densities of the most luminous active galactic nuclei at high redshifts than previously expected. These findings challenge earlier models by demonstrating that the most powerful black hole engines were more prevalent during the universe’s formative epochs.
Further analysis of the data indicates that traditional optical and ultraviolet surveys miss a substantial fraction of accreting black holes, particularly at lower luminosities and extreme distances. By calculating total accumulated black hole growth over time, researchers found that soft X-ray-selected black holes account for only a minority of total black hole mass in the local universe. This implies that between 70% and 90% of all supermassive black hole growth occurred behind heavy veils of dust and gas, or during phases where even X-ray emissions were suppressed.
The BHM programme leverages the variability of quasars through sub-programs such as Reverberation Mapping and the All-Quasar Multi-Epoch Spectroscopy. These initiatives measure time delays to yield direct geometric measurements of black hole masses and track dynamical changes, including binary black hole candidates and changing-look quasars. The data also includes manually inspected spectra that broke standard automated analysis pipelines, highlighting the wild variety of quasar behaviour that required human verification.
SDSS-V Director Juna Kollmeier noted the excitement of using quasar variations at scale to understand black hole evolution, while Dr. Andrea Merloni highlighted the culmination of more than a decade of joint planning between the eROSITA Consortium and SDSS. The operational leap is enabled by Robotic Focal Plane Systems that accelerate multi-object observation rates. All DR20 data products, including value-added catalogs and Jupyter Notebook tutorials, are openly available to researchers and the public via the Science Archive Server and web interfaces.
