SDSS DR20 maps 500,000 supermassive black holes, exposing hidden early growth

The Sloan Digital Sky Survey has released Data Release 20, the largest expansion yet of its Black Hole Mapper program. The release adds the survey’s first southern-hemisphere optical spectra from Las Campanas Observatory in Chile alongside expanded coverage from Apache Point in New Mexico, totaling more than 3.3 million optical spectra spanning roughly 500,000 galaxies and 1.5 million stars. Robotic fiber-positioning systems on 2.5m telescopes at both sites drove the observing pace and enabled rapid target-of-opportunity follow-up across both hemispheres.

The scientific centerpiece is SPIDERS, which cross-matches SDSS optical spectroscopy against X-ray sources from the German eROSITA mission to deliver optical IDs and redshifts for about 200,000 X-ray targets — the largest uniform spectroscopic follow-up of X-ray sources assembled to date. Most of those sources are actively accreting supermassive black holes. The combined X-ray/optical census pushes the black-hole growth timeline back toward redshift six and shows that luminous, fast-growing black holes were more common in the early universe than models predicted. Notably, soft X-ray-selected black holes account for only a minority of local black-hole mass, implying that 70–90% of supermassive black hole growth happened obscured behind dust and gas. Time-domain programs (reverberation mapping and multi-epoch AQMES monitoring) added direct mass measurements and caught ‘changing-look’ quasars whose spectra were wild enough to break the automated pipelines, forcing manual inspection.

As with SDSS’s 25-year track record, all DR20 products are openly available worldwide through the survey’s Science and Catalog Archive Servers, browser and Python interfaces, value-added AGN catalogs, and Jupyter tutorials. The release caps more than a decade of coordination between SDSS-V and the eROSITA consortium and sets up the survey’s eventual DR22 capstone.