Where do the bubbles outside the Milky Way come from?
Back in 2020, astronomers made a striking discovery in the first complete sky map from the eRosita X-ray telescope aboard the SRG observatory: a huge circular structure of hot gas below the Milky Way plane that occupies most of the southern sky. A similar structure in the northern sky, called the “North Polar Spur,” had been known for some time and was thought to have originated from an early supernova explosion. Taken together, the northern and southern structures instead both appear to emerge from the galactic center and are reminiscent of an hourglass in shape. These so-called eRosita bubbles have continued to occupy researchers ever since, and it appears that the bubbles are more complex than previously thought.
Although strikingly similar in shape to Fermi bubbles, the eRosita bubbles are larger and more energetic than their counterparts. Collectively called “galactic bubbles” because of their size and location, the structures offer an exciting opportunity to study the history of star formation and find new clues about the formation of the Milky Way, according to Anjali Gupta, lead author of a new study and professor of astronomy at Columbus State Community College. The eRosita bubbles are located in the gas surrounding the Milky Way, a region known as the circumgalactic medium.
“Our goal was to learn more about the circumgalactic medium, a region that is very important to understanding how our galaxy formed and evolved,” Gupta said. “Many of the regions we studied happened to be in the bubble region, so we wanted to see how the bubbles differed from more distant regions.” Previous studies had assumed that these bubbles were heated by gas blown outward from the galaxy. “We were therefore surprised to find that the temperature in the bubble region and outside the bubble region was the same,” Gupta said. Apparently, the bubbles are bright because they are filled with extremely dense gas, not because they are hotter than the surrounding area.
Gupta and Smita Mathur, co-author of the study and a professor of astronomy at Ohio State University, conducted their analysis using observations from the Suzaku satellite, a joint mission of NASA and the Japan Space Research Agency. By analyzing 230 archival observations made between 2005 and 2014, the researchers were able to characterize the diffuse emission – the electromagnetic radiation from very low-density gas – of the galactic bubbles, as well as the other hot gases that surround them.
Although the origin of these bubbles is controversial in the scientific literature, this study is the first to begin to clarify that question, Mathur said. Because the team found an abundance of non-solar neon and magnesium-oxygen ratios in the bubbles, their results strongly suggest that the galactic bubbles were originally formed by star formation or by the energy input of massive stars and other astrophysical phenomena, rather than by the activities of a supermassive black hole. “Our data support the theory that these bubbles were most likely formed by intense star-forming activity at the galactic center,” Mathur said.