What To Know
- According to a first hypothesis, these structures would be the result of a major energetic event that occurred at the center of the Milky Way.
- In detail, the North Polar Spur is a well-known structure in astronomy, appearing as a band of radiation in the sky, often detected in radio waves and X-rays.
- They found that the NPS and the LPC are at least 1,000 parsecs apart, implying that the eROSITA bubbles are much further away and likely anchored to the galactic center.
The discovery of eROSITA bubbles
The eROSITA bubbles were discovered using the eROSITA X-ray telescope, part of the Spectrum-X-Gamma space observatory launched in 2019. The telescope is designed to map the universe in X-rays, revealing cosmic structures invisible at other wavelengths. When analyzing the data collected by eROSITA, astronomers identified a gigantic hourglass-shaped structure at the center of the Milky Way. This structure, composed of two nearly circular lobes, extends well beyond the galactic plane, forming a pair of bubbles. These eROSITA bubbles resemble another structure discovered a decade earlier, the Fermi bubbles, which are also located near the galactic center. However, the newly identified structures are significantly larger and more energetic. Moreover, while Fermi bubbles are observed by detecting the gamma rays and X-rays they emit, these are detected by soft X-rays, photons that are highly energetic but less powerful than gamma rays.
Hypotheses about their origin
Since their discovery, several hypotheses have been put forward to explain the origin of the eROSITA bubbles. These hypotheses mainly focus on two possible scenarios. According to a first hypothesis, these structures would be the result of a major energetic event that occurred at the center of the Milky Way. This event could be linked to intense activity of the supermassive black hole located at the heart of our galaxy. By expelling large quantities of matter and energy, this black hole would have then created these gigantic bubbles, extending over a scale of 10,000 parsecs (about 32,600 light-years). Another hypothesis suggests that the eROSITA bubbles could be a smaller structure, located at a distance of about 100 parsecs (about 326 light-years) from the Sun. According to this theory, the bubbles would be a local phenomenon, appearing by coincidence in the direction of the galactic center. This structure could be linked to local astrophysical phenomena, such as stellar winds or ejections of matter from neighboring stars. To determine which of these hypotheses is most likely, a team of Chinese and European researchers therefore undertook to map the region of these bubbles in high resolution. By analyzing data from the Milky Way’s dust clouds, the researchers were then able to obtain clues about the distance and nature of these eROSITA bubble structures. 
A visualization of the eRosita (blue) and Fermi (pink) bubbles. Image credit: ESA / Gaia / DPAC / CC BY-SA 3.0 IGO.
What did the researchers discover?
In this work, the researchers focused on two important features of the northern bubble: the North Polar Spur (NPS) and the Lotus Petal Cloud (LPC). In detail, the North Polar Spur is a well-known structure in astronomy, appearing as a band of radiation in the sky, often detected in radio waves and X-rays. It is associated with a broad emission of hot plasma and was identified well before the discovery of the eROSITA bubbles. The NPS is generally located above the Galactic plane and extends towards the Galactic north. It is visible in X-ray and radio observations as a bright band across the sky. The Lotus Petal Cloud is a more recently identified structure, appearing in X-ray data as another distinct feature in the northern bubble of eROSITA. The LPC gets its name from its characteristic shape that resembles lotus petals. This is a region where gas and dust appear to accumulate, forming a complex and dense structure. That said, the results of the analyses revealed that these two features are not independent, but are part of a single three-dimensional structure. In particular, they found that the NPS and the LPC are at least 1,000 parsecs apart, implying that the eROSITA bubbles are much further away and likely anchored to the galactic center. In addition, the researchers observed arcs of radio emission connecting the NPS and the LPC, attributed to the shock wave from the bubble front. This observation helped define the outer edge of the northern eROSITA bubble, supporting the hypothesis of a giant bubble anchored to the galactic center. In contrast, the edge of the southern bubble remains less defined due to weaker X-ray emissions and complex features observed in the radio spectrum.
Implications of the results
The recent discoveries on eROSITA bubbles have important implications for our understanding of the structure and history of the Milky Way. If eROSITA bubbles are indeed giant structures blown out of the galactic center, this suggests that our galaxy has experienced major energetic events in its recent past. These events could be related to eruptions of the supermassive black hole at the center of the Milky Way, releasing vast amounts of energy and matter into interstellar space. Furthermore, confirming the existence of these giant bubbles could help astronomers better understand the mechanisms of formation and evolution of galactic structures. By studying the interactions between eROSITA bubbles and their environment, researchers could notably obtain valuable information on the dynamics of matter and energy within our galaxy. Finally, these results demonstrate the importance of multi-wavelength observations and international collaborations in astrophysics. The discoveries made by the eROSITA telescope and the subsequent analyses carried out by the researchers show how different techniques and perspectives can be combined to reveal hidden aspects of our universe. Source: The Astrophysical Journal Letters
