An article published in the journal “Nature Astronomy” reports a study of the so-called eROSITA bubbles that reveals magnetized structures that form a galactic halo aligned with them reaching heights of more than 16,000 light-years above the galactic plane. A team of researchers led by the Italian National Institute for Astrophysics conducted an investigation that spans the entire electromagnetic spectrum from radio waves to gamma rays to examine the eROSITA bubbles and map them. The alignment of the magnetized halo with the bubbles suggests a common origin, which could be given by star formation activity.
The image (Courtesy Zhang (INAF) et al. 2024, Nature Astronomy. All rights reserved) shows the eROSITA bubbles seen in X-rays (green) and the magnetic field in the halo (white). The red color shows the polarized intensity of synchrotron radiation. The light blue circles represent the Fermi bubbles seen in gamma rays.
The discovery of the eROSITA bubbles, a giant hourglass-shaped structure of hot gas with lobes extending from the center of the Milky Way, was announced in December 2020. Their name comes from the fact that they were discovered using eROSITA, one of the instruments on the Spektr-RG space telescope. These structures are associated with other structures that were already known, the Fermi bubbles.
That was the kind of discovery that marks the beginning of a new phase of studies, in this case, to understand the nature and origin of the eROSITA bubbles. The authors of this new study used data collected in many astronomical surveys conducted in different bands covering the entire electromagnetic spectrum. This made it possible to obtain the first multi-wavelength study of the eROSITA bubbles since their discovery.
Data analysis and theoretical analysis were combined to study the eROSITA bubbles. The result revealed the magnetized galactic halo and indicates that the bubbles are generated by intense outflows of gas that are in turn generated by the explosive death of stars in supernovae. The filamentary structures of magnetic fields are linked to winds with temperatures of up to 3.5 million Kelvin that are ejected from the Milky Way disk and fed by star-forming regions at the end of the galactic bar, the structure that crosses the galaxy and is composed of stars.
Outflows of this type can be generated by the accretion of disks that surround supermassive black holes at the centers of active galaxies. This had led to the hypothesis that the eROSITA bubbles were generated during a past episode of activity of Sagittarius A*, the supermassive black hole at the center of the Milky Way. The fact that powerful outflows are being ejected today when Sagittarius A* is quiescent suggests that this may be a common phenomenon in other galaxies without active nuclei.
He-Shou Zhang, a researcher at the Italian National Institute for Astrophysics in Milan and the first author of this study, explained that this study opens new frontiers in our understanding of the galactic halo and will contribute to helping our knowledge of the Milky Way’s star-forming ecosystem. This means that it brings progress but the studies will continue because the galactic ecosystem is showing a complexity that was unknown with Sagittarius A*, stars that form and die, the eROSITA bubbles, the Fermi bubbles, and perhaps more.
