Scientists find 2 million active black holes, exploding stars and more, nearly doubling our view of the high-energy universe

Astronomers have nearly doubled the number of known cosmic X-ray sources, offering the most comprehensive public view yet of the universe's hottest and most energetic objects.

The second major public data release from the German-led eROSITA X-ray telescope contains a catalog of nearly 2 million X-ray-emitting objects, including supermassive black holes, galaxy clusters, stars and other extreme cosmic phenomena. The unprecedented dataset — called DR2 — will help astronomers investigate how black holes grow, galaxies evolve and the universe's largest structures formed, according to a statement from the Max Planck Institute for Extraterrestrial Physics.

"DR2 is the best inventory of the X-ray sky we have to date and opens the door to robust statistical studies of cosmic populations," Miriam E. Ramos-Ceja, Ground Segment Manager of the eROSITA instrument and lead author of the DR2 publication said in the statement.

The new release combines observations collected during eROSITA's first three all-sky scans over 556 days, allowing astronomers to detect increasingly faint X-ray sources that previous surveys missed.

Of the nearly 2 million detections, more than 1.9 million are point-like sources, most of them active supermassive black holes feeding at the centers of distant galaxies or stars within the Milky Way. These black holes offer astronomers a window into how galaxies grow and evolve over cosmic time, while the stellar sources help researchers better understand everything from stellar activity to the life cycles of stars.

Another roughly 64,000 sources are extended objects, including massive galaxy clusters, nearby galaxies and the glowing remnants of exploded stars (also known as supernovas). Galaxy clusters are especially valuable because they trace the universe's largest structures, helping scientists investigate the influence of dark matter and dark energy on the evolution of the cosmos.

Unlike visible light, X-rays reveal some of the universe's most energetic environments. Material spiraling toward a black hole can be heated to millions of degrees before crossing the event horizon, while neutron stars, supernova remnants and the enormous reservoirs of hot gas filling galaxy clusters also emit strongly in X-rays.

The size of the catalog gives astronomers a powerful new dataset for studying the high-energy universe. With nearly 2 million sources observed under the same conditions, astronomers can study how supermassive black holes have grown over billions of years, map the distribution of galaxy clusters to better understand the large-scale structure of the universe, and search for rare or unusual objects that may otherwise have gone unnoticed. Large, uniform surveys also make it easier to compare different classes of objects and identify long-term trends that smaller observations cannot reveal.

A full version of the western X-ray sky from eROSITA. (Image credit: MPE, J. Sanders)

This animation shows a representation of the active galactic nuclei (AGN) identified in the DR2 catalogue. Each dot shows a single object, where the distance from the three-dimensional center increases with the source's redshift, or how far away it is from us.  (Image credit: MPE/J. Sanders)

The release also includes optical and infrared counterparts for many of the X-ray sources, helping astronomers determine whether each object is a nearby star, a distant galaxy, an active supermassive black hole or another type of high-energy phenomenon. In the portion of the catalog where counterparts have been identified, about 88% are distant galaxies hosting actively feeding supermassive black holes, according to the statement.

"X-ray detection is only the first step," Mara Salvato, eROSITA spokesperson and chair of the follow-up working group, said in the statement. "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."

Although eROSITA stopped collecting scientific observations in early 2022, the mission completed enough sky scans beforehand to support additional public data releases. Researchers say the next Data Release 3 (DR3) is planned for 2028.

The eROSITA-DE DR2 catalogue is available to view online.

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