A new map of the entire sky, as seen in X-rays, looks deeper into space than any other of its kind.
The map, released June 19, is based on data from the first full scan of the sky made by the eROSITA X-ray telescope onboard the Russian-German SRG spacecraft, which launched in July 2019. The six-month, all-sky survey, which began in December and wrapped up in June, is only the first of eight total sky surveys that eROSITA will perform over the next few years. But this sweep alone cataloged some 1.1 million X-ray sources across the cosmos — just about doubling the number of known X-ray emitters in the universe.
These hot and energetic objects include Milky Way stars and supermassive black holes at the centers of other galaxies, some of which are billions of light-years away and date back to when the universe was just one-tenth of its current age.
eROSITA’s new map reveals objects about four times as faint as could be seen in the last survey of the whole X-ray sky, conducted by the ROSAT space telescope in the 1990s (SN: 6/29/91). The new images “are just spectacular to look at,” says Harvey Tananbaum, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., not involved in the mission. “You have this tremendous capability of looking at the near and the far … and then, of course, delving in detail to the parts of the images that you’re most interested in.”
Mapping the sky in X-rays
In this map of X-rays emitted by celestial objects across the sky, X-rays are color-coded by energy: Blue signifies the highest energies, followed by green, yellow and red. The red foreground glow comes from hot gas near the solar system, whereas the Milky Way plane appears blue, because gas and dust in the disk absorb all but the most energetic X-rays. The map includes many of the Milky Way’s supernova remnants, including Cassiopeia A and Vela, and a star system called Scorpius X-1, which was the first X-ray source discovered beyond the sun. It also showcases the Milky Way’s star-forming regions, such as the Orion Nebula and the Cygnus Superbubble (SN: 6/29/12), and a mysterious arc of X-rays called the North Polar Spur. Beyond the Milky Way are nearby galaxies, such as the Large Magellanic Cloud, and distant galaxy clusters, such as those contained in the Shapley Supercluster.
Jeremy Sanders, Hermann Brunner and the eSASS team/MPE, Eugene Churazov, Marat Gilfanov (on behalf of IKI) 
Jeremy Sanders, Hermann Brunner and the eSASS team/MPE, Eugene Churazov, Marat Gilfanov (on behalf of IKI)
eROSITA can flag potentially interesting X-ray phenomena, such as flares from stars getting shredded by black holes, which other telescopes with narrower fields of view but better vision can then investigate in detail, Tananbaum says. The new map also allows astronomers to probe enigmatic X-ray features, such as a giant arc of radiation above the plane of the Milky Way called the North Polar Spur.
This X-ray feature may be left over from a nearby supernova explosion, or it might be related to the huge blobs of gas on either side of the Milky Way disk, known as Fermi Bubbles, says eROSITA team member Peter Predehl, an X-ray astronomer at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany (SN:6/8/20). eROSITA observations could help the Russian and German teams involved in the mission figure out what the spur is.
About 20 percent of the marks on eROSITA’s map are stars in the Milky Way with intense magnetic fields and hot coronae. Scattered among these are star systems containing neutron stars, black holes and white dwarfs, and remnants of supernova explosions. eROSITA also caught several fleeting bursts from events like stellar collisions.
A supernova remnant called Vela (center, reddish green) is one of the most prominent X-ray sources in the sky. The supernova exploded about 12,000 years ago, about 800 light-years away, and overlaps with two other known supernova remnants: Vela Junior (faint purple ring at bottom left) and Puppis A (blue cloud at top right). All three explosions left behind neutron stars, but only the stars at the centers of Vela and Vela Junior are visible to eROSITA.Peter Predehl and Werner Becker/MPE, Davide Mella
Beyond the Milky Way, most of the X-ray emitters that eROSITA found are supermassive black holes gobbling up matter at the centers of other galaxies (SN: 6/18/20). Such active galactic nuclei comprise 77 percent of the catalog.
Distant clusters of galaxies made up another 2 percent of eROSITA’s haul. These clusters were visible to the telescope thanks to the piping hot gas that fills the space between galaxies in each cluster, which emits an X-ray glow.
The Shapley Supercluster (pictured at right) is composed of many smaller clumps of galaxies about 650 million light-years away. Each blob of X-rays in this picture — which spans 180 million light-years across — is a galaxy cluster that contains hundreds to thousands of galaxies. Zoomed-in images on the left showcase a few of the most massive clusters in the bunch.Esra Bulbul and Jeremy Sanders/MPE
“At present, we probably know about a little less than 8,000 clusters of galaxies,” says Craig Sarazin, an astronomer at the University of Virginia in Charlottesville not involved in the work. But over its four-year mission, eROSITA is expected to find a total of 50,000 to 100,000 clusters. In the first sweep alone, it picked up about 20,000.
That census could give astronomers a much better sense of the sizes and distributions of galaxy clusters over cosmic history, Sarazin says. And this, in turn, may give new insight into features of the universe that govern cluster formation and evolution. That includes the precise amount of invisible, gravitationally binding dark matter out there, and how fast the universe is expanding.
The Large Magellanic Cloud is a small galaxy that orbits the Milky Way. This little galactic neighbor emits a diffuse X-ray glow from its interstellar material (red) as well as brighter pinpoints of X-rays from supernova remnants (circles) and binary star systems.Frank Haberl and Chandreyee Maitra/MPE
This glowing ring is thought to have originated from a burst of X-rays emitted by the black hole and companion star at its center, which was detected by other telescopes in early 2019. Some of the X-rays in the outburst were scattered into a ring by a dust cloud that the radiation encountered. That scattering delayed the X-rays in the ring on their way to Earth, allowing the eROSITA telescope to detect them during its sky survey in February 2020, one year after the outburst was originally detected.Georg Lamer/Leibniz-Institut für Astrophysik Potsdam, Davide Mella
The Carina Nebula, about 7,500 light-years away is one of the largest diffuse nebulae our galaxy. It’s home to many massive, young stars, as well as a smaller nebula called Homunculus. That plume formed when one of the stars in the Eta Carinae system blew up — a celestial explosion famously observed in 1843 (SN: 1/4/10).Manami Sasaki/Dr. Karl Remeis Observatory/FAU, Davide Mella
Closer to home, observations of supernova remnants could help clear up some confusion about the life cycles of big stars, says eROSITA team member Andrea Merloni, an astronomer also at the Max Planck Institute for Extraterrestrial Physics. Past X-ray surveys have found fewer supernova remnants than theorists expect to see, based on how many massive stars they think have blown up over the course of the galaxy’s history. But eROSITA observations are now revealing plumes of debris that could be previously overlooked stellar graves. “Maybe we’ll start balancing this budget between the expected number of supernovae and the ones that we are detecting,” Merloni says.
eROSITA is now beginning its second six-month, all-sky survey. When combined, the telescope’s eight total maps will be able to reveal objects one-fifth as bright as those that could be seen on a single map. That not only allows astronomers to see more X-ray sources in more detail, but track how objects in the X-ray sky are changing over time.
