Scientists nearly double the universe's high-energy object count

A new survey of X-ray sources from the eROSITA X-ray telescope, a German-led telescope project, has nearly doubled the number of known cosmic X-ray sources, with over 2 million active black holes, galaxy clusters, stars, and other extreme cosmic phenomena detected.

This unprecedented dataset, dubbed the Data Release 2 (DR2) catalog, provides the most comprehensive view yet of the universe's hottest and most energetic objects, allowing astronomers to 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, the DR2 catalog contains a catalog of nearly 2 million X-ray-emitting objects, including supermassive black holes, galaxy clusters, stars, and other extreme cosmic phenomena.

"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," said Miriam E. Ramos-Ceja, Ground Segment Manager of the eROSITA instrument and lead author of the DR2 publication in the statement.

The eROSITA X-ray telescope, launched in June 2019, has been scanning the sky for X-ray sources since its first all-sky scan in 2020. The DR2 catalog combines observations collected during its 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.

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.

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," said Mara Salvato, eROSITA spokesperson and chair of the follow-up working group 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."