Discovery of Magnetic Halo Around Milky Way Provides New Insights into Galactic Evolution

A recent discovery of a magnetic halo surrounding the Milky Way galaxy could significantly alter our understanding of galactic evolution.

Researchers from the National Institute for Astrophysics (INAF) in Italy have uncovered large-scale magnetized structures that extend far beyond the galactic plane, offering new insights into how galaxies like our own evolve over time.

The findings, based on data from over ten all-sky surveys and published in Nature Astronomy, reveal the presence of a vast and highly organized magnetic field that spans more than 16,000 light-years both above and below the Milky Way. This magnetic halo is not only a remarkable discovery in itself but also provides clues about the origin of energetic outflows in the galaxy, some of which may be tied to the explosive death of stars.

The Structure and Scale of the Magnetic Halo

The newly discovered magnetic halo is composed of large filaments, thin magnetic structures that stretch to an immense scale. According to the research, these filaments are related to the eROSITA Bubbles, enormous gas bubbles that were first observed in 2020 by the eROSITA X-ray telescope aboard the Russian-German space mission Spectr-Roentgen-Gamma (SRG). These bubbles extend across the sky and are powered by galactic outflows—streams of hot gas and energy expelled from the galaxy’s core. What makes this discovery particularly striking is the organization of the magnetic fields within these bubbles. The filaments, which extend up to 150 times the width of the full moon, are highly structured, a characteristic that surprised many astronomers.

“These magnetic ridges we observed are not just coincidental structures but are closely related to the star-forming regions in our galaxy,” explained He-Shou Zhang, the study's lead author and researcher at INAF. The data suggest that the magnetic fields in these ridges are shaped by intense outflows of gas and energy, much of which originates from regions of active star formation at the ends of the Galactic Bar, a central structure in the Milky Way where much of the galaxy’s gas, dust, and stars are concentrated. The outflows themselves are likely driven by supernovae—the explosive deaths of massive stars—which propel material into the galactic halo and play a crucial role in fueling the formation of new stars.

Galactic Outflows and Their Role in Evolution

One of the key findings from this study is the role that galactic outflows play in shaping the Milky Way's magnetic halo. These outflows, which consist of hot gas expelled from the galaxy's central regions, contribute to the large-scale magnetic structures observed in the halo. The study marks the first time that these outflows have been directly linked to the star-forming ring at the end of the Galactic Bar, an area rich in stellar nurseries where new stars are born from collapsing clouds of gas. “Our results find that intense star formation at the end of the galactic bar contributes significantly to these expansive, multiphase outflows,” Zhang stated.

This connection between star formation and galactic outflows is a crucial discovery for understanding how galaxies like the Milky Way evolve. The energy from dying stars in supernovae not only triggers the formation of new stars but also drives material out of the galactic disk into the halo, where it interacts with the galaxy’s magnetic field. These interactions, in turn, help shape the structure of the halo and influence the overall dynamics of the galaxy. Gabriele Ponti, a researcher at INAF and co-author of the study, remarked, “It is well established that a small fraction of 'active' galaxies can launch outflows, powered by accretion onto supermassive black holes or starbursts events, which profoundly impact their host galaxy. What is fascinating to me here is that we see that the Milky Way, a quiescent galaxy like many others, can also eject powerful outflows.”

This discovery challenges the traditional view that galactic outflows are primarily the result of extreme events like supermassive black hole activity or starburst events. Instead, the Milky Way—a relatively quiet, or quiescent, galaxy—appears capable of producing similarly powerful outflows, driven by more moderate processes like star formation and supernovae. This finding has broad implications for our understanding of galactic feedback, the processes by which galaxies regulate their own growth through the interaction between stars, gas, and magnetic fields.

A New Perspective on Galactic Feedback and Magnetic Fields

The study’s findings offer a new perspective on the role of magnetic fields in the evolution of galaxies. As Martijn Oei, a radio astronomy and cosmology researcher at Caltech, who was not involved in the study, noted, “What we’re now learning is that the halos of galaxies, or the large-scale surroundings of galaxies, are magnetic, and magnetic fields play an important role in how galaxies evolve.” While magnetic fields have long been known to exist in galaxies, their precise role in shaping galactic structures has remained poorly understood. This new discovery, which provides detailed measurements of the Milky Way’s magnetic halo, offers the first concrete evidence linking these fields to processes of star formation and galactic feedback.

The researchers used a wide range of multi-wavelength surveys—ranging from radio waves to gamma rays—to observe these magnetic structures. By combining data from different parts of the electromagnetic spectrum, they were able to map the complex interactions between galactic outflows and the magnetic field with unprecedented precision. This comprehensive approach allowed the team to confirm the large-scale nature of the magnetic features, providing a clearer picture of how the Milky Way’s magnetic halo is structured.

“This work provides the first detailed measurements of the magnetic fields in the Milky Way’s X-ray emitting halo and uncovers new connections between star-forming activities and galactic outflows,” Zhang emphasized. The research highlights how star-forming regions at the end of the galactic bar contribute to the generation of these outflows, further underscoring the interconnectedness of star formation, supernovae, and magnetic fields in shaping the galaxy’s evolution.

Future Implications and Ongoing Research

The discovery of this magnetized galactic halo opens new frontiers in the study of spiral galaxies like the Milky Way. By providing the first direct link between galactic outflows and star formation, the study offers new insights into the processes that drive the evolution of galaxies over time. As researchers continue to analyze the data, these findings may also shed light on similar structures in other spiral galaxies, helping to place the Milky Way within the broader context of galactic evolution in the universe.

The ongoing research into the eROSITA bubbles and the magnetic structures associated with them will likely yield further breakthroughs in our understanding of galactic dynamics. As Zhang concluded, “Our work is a timely result. It is the first comprehensive multi-wavelength study for the eROSITA Bubbles since their discovery in 2020. The study opens up new frontiers in our understanding of the galactic halo and will help our knowledge of the Milky Way’s complex and impetuous star-forming ecosystem.”

By unraveling the complexities of the Milky Way’s magnetic halo and its relationship with star formation, galactic outflows, and supernovae, this study not only advances our understanding of our own galaxy but also provides a valuable framework for studying the evolution of galaxies throughout the cosmos.