A sweeping new analysis published in Astronomy & Astrophysics has revealed a striking feature in the fabric of our galaxy: a tunnel-like structure of hot plasma connecting our solar system’s Local Hot Bubble to distant interstellar regions. The discovery, based on data from the eROSITA X-ray telescope aboard the Spectrum-Roentgen-Gamma (SRG) observatory, offers the clearest view yet of the complex, dynamic environment in which our solar system is embedded.
Revisiting the Local Hot Bubble
The Local Hot Bubble (LHB) is a massive cavity of low-density, superheated gas that extends roughly 1,000 light-years across. This region, which surrounds the solar system, is thought to have been carved out by a series of supernova explosions about 14 million years ago. These explosions swept away interstellar gas and dust, leaving behind a million-degree Kelvin plasma that continues to glow in soft X-rays detectable by instruments like eROSITA.
Mapping the bubble from within has long been a challenge. “It’s like trying to describe the shape of your fish tank while standing in the middle of it,” said lead researcher Michael Yeung of the Max Planck Institute for Extraterrestrial Physics. To overcome this, Yeung’s team divided the sky into over 2,000 sections and analyzed the soft X-ray light emitted from each, building the most detailed three-dimensional map of the LHB to date.
Their findings showed that the bubble is not a neat sphere but an irregular structure, expanding more perpendicular to the galactic plane than along it. The team also found a clear north-south temperature gradient, with the Southern Hemisphere registering an average temperature of 121.8 ± 0.6 eV, compared with 100.8 ± 0.5 eV in the Northern Hemisphere. This gradient had gone undetected in earlier surveys due to limitations in the instruments used, such as the R1 and R2 band ratios from the ROSAT satellite.
The Unexpected Tunnel Toward Centaurus
One of the most surprising findings was the detection of a tunnel-like corridor of hot plasma extending from the bubble’s edge toward the constellation Centaurus. This structure appears to cut a clear channel through cooler, denser interstellar material, suggesting a physical connection to another low-density cavity beyond the LHB.
“What we didn’t know was the existence of an interstellar tunnel towards Centaurus, which carves a gap in the cooler interstellar medium,” said Max Planck astrophysicist Michael Freyberg, a co-author of the study. The tunnel could lead to neighboring cavities such as the Gum Nebula or other regions mapped in previous surveys, indicating that the LHB may be part of a larger network of interstellar tunnels and bubbles.
This possibility echoes a hypothesis first put forward in 1974, which suggested that the galaxy might be threaded by a web of supernova-blown bubbles connected by corridors of hot, ionized gas. These structures could act as conduits for magnetic fields, cosmic rays, and interstellar gases, shaping how matter and energy move across the Milky Way.
Building a New Picture of the Galactic Neighborhood
Beyond the tunnel toward Centaurus, Yeung’s team identified dust-free cavities filled with plasma throughout the LHB. These cavities seem to form interconnected pathways, further supporting the idea that the Milky Way is not a uniform void but a dynamic, porous structure.
The team also modeled the bubble’s emission measure, a parameter related to the density and size of the plasma, and found it was anti-correlated with local dust column density. In other words, areas with fewer dust grains are associated with stronger X-ray emission, reinforcing the picture of interconnected cavities.
The average thermal pressure of the LHB was measured at 10,100 cm⁻³ K, which is significantly lower than that of typical supernova remnants or stellar wind-blown bubbles. This suggests the LHB may be “open” toward high galactic latitudes, allowing energy and plasma to escape and possibly connect with other bubbles.
