Sloan's 20th Data Release Hands Over Three Million Spectra and Its First Southern Sky

The Sloan Digital Sky Survey has published its twentieth data release, the third from the survey's fifth phase, and the first to include optical spectra taken from the Southern Hemisphere. It is the largest single dump of spectra in the project's 26-year history.

DR20 carries more than three million spectra. The Black Hole Mapper program contributes roughly 500,000 spectra of stellar-mass, intermediate-mass and supermassive black holes. The Milky Way Mapper contributes over three million spectra covering 1.5 million individual stars. The Local Volume Mapper adds spectra of hydrogen clouds, planetary nebulae and nearby galaxies. Layered on top are hundreds of thousands of X-ray-emitting active galactic nuclei, galaxy clusters and active stars identified through SPIDERS, the program that cross-matches SDSS spectroscopy against the eROSITA X-ray all-sky survey — coordinated here with eROSITA's second data release. Eighteen new or updated value-added catalogs round it out.

When the survey started in 2000, it was a single instrument pointed at one half of the sky: the 2.5-meter Sloan Foundation Telescope in New Mexico, imaging and taking spectra of the north. DR20 marks how far that has drifted from the original design. The release contains newly reduced Baryon Oscillation Spectroscopic Survey data from both hemispheres, taken through Feb. 2, 2025, folded together with spectroscopy from every previous SDSS phase, and it delivers the first slice of the all-sky panoptic spectroscopic map that SDSS-V was built to produce. The academic record of the release appeared as a preprint on July 30 in the Astrophysical Journal Supplement Series.

The collaboration is unusually insistent that the data be usable, not merely downloadable. "We want everyone to make use of the SDSS data products, whether they are a professional astronomer doing their research, a student learning how to work with data, a teacher looking for classroom material, or someone who is just interested in astronomy in general," said Anne-Marie Weijmans of the University of St Andrews, the survey's data products coordinator. "That is why we have so many different platforms for people to interact with the data, and we take care to provide numerous examples and tutorials for each of these. The data should not just be publicly available but be publicly usable."

Emily Griffith, an NSF postdoctoral fellow at the University of Colorado Boulder and spokesperson for SDSS-V, called DR20 "an enormous leap in the scale of SDSS data products." She added: "It's the product of years of work by a dedicated, international team of astronomers. We're excited to share DR20 with the world and see what the community discovers with it."

The survey is not slowing down. "But we aren't stopping here," said Juna Kollmeier, a Carnegie Science staff scientist and the director of SDSS-V. "We just spent a week talking about what will go into DR21 and DR22! We're going to need bigger computers!"