Two million black holes, galaxies and more: Groundbreaking discovery nearly doubles known cosmic X-ray sources
Scientists at the Max Planck Institute for Extraterrestrial Physics have released the second data set from the eROSITA X-ray telescope, which has doubled the number of known X-ray sources in the sky. The Institute revealed that this data set (DR2) is the most comprehensive catalogue of the X-ray universe currently available and contains two million sources including supermassive black holes, galaxy clusters, nearby galaxies, and remnants of exploded stars or supernovae.
The observations started in December 2019, when eROSITA started surveying the entire sky every six months. eROSITA is part of the Russian-German "Spectrum-Roentgen-Gamma" (SRG) satellite, which is at the second Earth-Sun Lagrange Point, or L2, approximately 932,000 miles (1.5 million kilometers) away from Earth. Its primary goal, before it stopped operations in 2022, was to study extragalactic X-ray sources and galaxy clusters to determine the role of dark energy in the expansion of the universe. Earth’s atmosphere blocks X-ray radiation, so most telescopes designed to study the universe in this wavelength must be installed in space for uninterrupted observations.
According to the Max Planck Institute, DR2 combines data collected over the mission’s first 556 days, spanning three full sky surveys which were conducted by progressively increasing depth and sensitivity for X-rays. Out of two million objects, more than 1.9 million are point-like sources such as stars and supermassive black holes, along with around 64,000 galaxy clusters, individual galaxies, and supernova remnants. The catalogue also includes nearly 15,000 heavily obscured and intrinsically energetic systems that are often missed at softer X-ray energies.
“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 a statement.
The authors noted that the catalog also includes optical and infrared counterparts of the X-ray sources to help differentiate the nature of the entities. Of the objects for which they found a counterpart in different wavelengths, roughly 88 percent turned out to be extragalactic—dominated by supermassive black holes actively feeding on material from their surroundings. “X-ray detection is only the first step. 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,” said Mara Salvato, eROSITA spokesperson and chair of the follow-up working group.
William Roster, lead author of another study based on a data release by the Sloan Digital Sky Survey (SDSS), said that DR2 suggests that rapidly growing black holes were more abundant in the early universe than previously thought. Notably, the next data release from eROSITA is planned for 2028, and it will further boost the effort to uncover the mysteries of the dark universe.
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