The German-Russian, eRosita space telescope mounted on the Spectrum-Roentgen-Gamma (SRG) was launched in July of 2019. Rotating at an observing position some 1.5 million kilometers from Earth, the spacecraft has created a seminal 3D map from 165 gigabytes of data, recording over a million violent sources of X-rays in the cosmos that reveals how the universe accelerates under the mysterious force called dark energy. The X-ray eye is expected to detect up to three million supermassive black holes, many of which will be new to science, including both obscured and unobscured objects, providing a unique view of the evolution of the supermassive objects within the emerging cosmos.
SRG is also mapping a cosmic web of about 100,000 galactic clusters by detecting the X-ray glow from their intergalactic plasma and from the plasma filaments that join them and X-rays from as many as 700,000 stars in the Milky Way.
“That’s actually pretty much the same number as had been detected in the whole history of X-ray astronomy going back 60 years. We’ve basically doubled the known sources in just six months,” said Kirpal Nandra, who heads the high-energy astrophysics group at the Max Planck Institute for Extraterrestrial Physics (MPE). “The data is truly stunning and I think what we’re doing here will revolutionize X-ray astronomy,” he told BBC News.
According to Nature, SRG will map the entire sky eight times, and researchers will compare the maps and look for changes, for example how some of the supermassive black holes at galactic centers become extremely bright when they devour matter at a high rate, and then go back to relative quiescence.
Violence at the Plane of Our Galaxy
The map shown above uses Aitoff projection, which unwraps the sphere of the sky on to an ellipse. The band across the middle is the plane of our Milky Way Galaxy, with the center of the galaxy in the middle of the ellipse dominated by highly energetic sources, including include stars with strong, magnetically active and extremely hot atmospheres. The image has been encoded with color to help describe what’s going on. Blues represent higher energy X-rays (1-2.3 kiloelectron volts, keV); greens are mid-range (0.6-1 keV); and reds are lower energy (0.3-0.6 keV).
Vela Supernova Remnant 800 Light-years from Earth
The greens and yellows that draw a kind of mushroom feature covering a great swathe of the map represent hot gas inside and just outside our galaxy. This material imprints information about the formation and evolution of the Milky Way. The bright yellow patch just above the plane on the far right is a concentration of supernova remnants, dominated by the Vela supernova remnant –an explosion that happened thousands of years ago but a mere 800 light-years from Earth.
The diffuse red glow at the top and bottom of the map is X-ray emission from hot gas beyond the Milky Way along with white signatures of super-massive black holes. About 80% of all the sources contained in the new map are the gargantuan black holes lurking at the center of distant galaxies. Several are seen when the Universe was younger than one billion years old, less than 10% of its present age.
During the next 3.5 years, astronomers hope to capture some clues about the nature of dark energy, the mysterious “force” that appears to be pushing the cosmos apart at an ever accelerating rate.
“Dark energy is incredibly strange, but actually it makes sense to me that it went unnoticed,” said Noble Prize winning physicist Adam Riess in an interview. “I have absolutely no clue what dark energy is. Dark energy appears strong enough to push the entire universe – yet its source is unknown, its location is unknown and its physics are highly speculative.”
Dark energy is the big prize, but it would only come at the end of the mission, explained Nandra. “Eight surveys allows us to go really deep into the distant Universe. Basically, we’re trying to detect all of the clusters of galaxies in the Universe above a certain mass limit. We’ve got a nice sample already – maybe around 10,000. But we’re hoping to get at least 100,000 clusters of galaxies.”
SRG will also investigate the Universe’s distribution of ordinary matter and dark matter, looking for hints of the nature of dark-matter particles by trying to confirm previous signals that showed peaks in X-ray emissions from some galactic centers, which some researchers have suggested may come from the decay of an unknown, heavier relative of the known subatomic particles called neutrinos that could be a major component of dark matter.
“So far, the dark-matter explanation is still on the table” as a potential cause of the X-ray signal, said Esra Bulbul, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics and a lead scientist on the mission in 2019.