Astronomers have detected a remarkable new feature in the first all-sky survey map produced by the eROSITA X-ray telescope on SRG: a huge circular structure of hot gas below the plane of the Milky Way occupying most of the southern sky. A similar structure in the Northern sky, the “North polar spur,” has been known for a long time and had been thought to be the trace of an old supernova explosion. Taken together, the northern and the southern structures instead are reminiscent of a single hourglass-shaped set of bubbles emerging from the galactic center.
“The sharp boundaries of these bubbles most likely trace shocks caused by the massive injection of energy from the inner part of our galaxy into the galactic halo,” points out Peter Predehl, first author of the study now published in Nature. “Such an explanation has been previously suggested for the Fermi bubbles, and now with eROSITA their full extent and morphology has become evident.”
This discovery will help astronomers to understand the cosmic cycle of matter in and around the Milky Way, and other galaxies. Most of the ordinary (baryonic) matter in the Universe is invisible to our eyes, with all the stars and galaxies that we observe with optical telescopes comprising less than 10% of its total mass. Vast amounts of unobserved baryonic matter are expected to reside in tenuous haloes wrapped like cocoons around the galaxies and the filaments between them in the cosmic web. These haloes are hot, with a temperature of millions of degrees, and thus only visible with telescopes sensitive to high-energy radiation.
The bubbles now seen with eROSITA trace disturbances in this hot gas envelope around our Milky Way, caused either by a burst of star formation or by an outburst from the supermassive black hole at the galactic center. While dormant now, the black hole could well have been active in the past, linking it to active galactic nuclei (AGN) with rapidly growing black holes seen in distant galaxies. In either case, the energy needed to power the formation of these huge bubbles must have been enormous at 10^56 ergs, equivalent to the energy release of 100,000 supernovae, and similar to estimates of AGN outbursts.