The Biggest X-Ray Map of the Sky Ever Made Lists 1.9 Million Sources. Almost Nine in Ten Are Black Holes Feeding in Other Galaxies.

The German eROSITA Consortium has released the most comprehensive catalog of the X-ray universe ever assembled: more than 1.9 million point-like sources and about 64,000 extended ones, drawn from three full scans of the sky by the eROSITA telescope aboard the Russian-German Spektr-RG observatory. The second data release, known as DR2, nearly doubles the count from the mission's first release.

eROSITA, short for extended ROentgen Survey with an Imaging Telescope Array, was built by the Max Planck Institute for Extraterrestrial Physics (MPE) and collaborators to map the soft X-ray sky. Launched in 2019, it has scanned the entire sky every six months, with each pass stacked on the last to reach fainter objects. DR2 combines data from the observatory's first 556 days of observations.

The point sources are mostly stars and actively accreting black holes. The extended sources include galaxy clusters and supernova remnants. What makes the catalog useful, though, is not just the detections but the identifications. The release includes multiwavelength information that matches each X-ray source to its most likely optical and infrared counterpart, and from that the team concludes about 88% of the sources are extragalactic, dominated by supermassive black holes actively feeding at the centers of galaxies beyond the Milky Way.

"X-ray detection is only the first step," said Mara Salvato, the eROSITA spokesperson and chair of the follow-up working group. "By linking X-ray sources to their counterparts at other wavelengths, we can work out what these objects are, where they sit on the cosmic distance ladder, and build clean, well-defined samples on an unprecedented scale."

The release lands alongside the Sloan Digital Sky Survey's 20th data release, SDSS DR20, which includes a large body of optical spectroscopy taken specifically for eROSITA-DE's surveys. The two datasets are the product of almost a decade of collaboration, and together they enable one of the largest and most detailed studies of supermassive black holes to date, letting researchers build three-dimensional maps of active black holes and trace how the population evolved over cosmic time.

One early finding cuts against expectations. "The most luminous black holes at high redshift are like needles in a haystack," said William Roster, lead author of the accompanying study. "Thanks to DR2, we found more needles than expected, suggesting that rapidly growing black holes were more abundant in the early universe than previously thought."

The data are now public. "This is a dataset of unprecedented scale and completeness, now in the hands of the global astronomical community," said eROSITA principal investigator Andrea Merloni. "With nearly 2 million sources, DR2 provides a unique foundation for discoveries ranging from rare objects to large-scale population studies."