In the cosmic cacophony of our galaxy, a silent player has long intrigued astronomers: the Local Hot Bubble. This vast region of hot, diffuse gas envelops our solar system and forms part of the complex interstellar medium. But a recent study using data from the eROSITA telescope has unveiled an unexpected twist in this celestial enigma: a surprising north-south temperature divide within the bubble.
The Local Hot Bubble, a shell of hot ionized gas, is thought to have formed from supernova explosions millions of years ago. Its existence has been a topic of interest since the late 20th century, acting as a soft X-ray background in astronomical observations. Yet, its detailed characteristics remained elusive until the eROSITA mission, aboard the Spectrum-Roentgen-Gamma (SRG) satellite, provided an unprecedented view.
The eROSITA telescope, a joint Russian-German initiative, embarked on a mission to survey the entire sky in X-rays, producing the most detailed images of the universe’s energetic processes. Among its many discoveries, the revelation of a temperature disparity in the Local Hot Bubble stands out. According to the study, led by Michael C. H. Yeung and his team at the Max Planck Institute for Extraterrestrial Physics, the bubble’s temperature is not uniform. It displays a clear dichotomy: the southern hemisphere is warmer than its northern counterpart.
Analyzing the eROSITA data, the researchers found that the average temperature in the southern hemisphere of the Local Hot Bubble is about 121.8 electron volts (eV), while the northern hemisphere averages around 100.8 eV. This discovery is particularly intriguing because it challenges the assumption that the bubble’s temperature would be evenly distributed, given its expected origin from supernova remnants that should theoretically spread energy uniformly.
The methodology behind this discovery involved dividing the sky into approximately 2,000 regions, each analyzed for its X-ray spectral characteristics. By focusing on high-latitude regions, where interference from the galactic plane is minimal, the team was able to discern subtle variations in temperature. Their findings were further supported by independent measurements of dust layers in space, which helped validate the temperature readings and their spatial distribution.
But what could cause such a temperature difference? The study suggests several possibilities. One theory is that the initial conditions of the interstellar medium into which the Local Hot Bubble expanded were not uniform. Variations in density and composition could have led to differential heating as the bubble formed. Alternatively, recent astrophysical events, such as additional supernovae or stellar winds, might have injected energy differentially, affecting the bubble’s thermal landscape.
The implications of this discovery are significant for our understanding of the interstellar medium. The Local Hot Bubble is a major component of the galactic ecosystem, influencing cosmic ray propagation, star formation, and the overall energy balance in our corner of the galaxy. By understanding its true nature, astronomers can refine models of galactic evolution and improve predictions about future changes in our cosmic environment.
Reference
The SRG/eROSITA diffuse soft X-ray background – I. The local hot bubble in the western Galactic hemisphere Michael C. H. Yeung, Gabriele Ponti, Michael J. Freyberg, Konrad Dennerl, Teng Liu, Nicola Locatelli, Martin G. F. Mayer, Jeremy S. Sanders, Manami Sasaki, Andy Strong, Yi Zhang, Xueying Zheng and Efrain Gatuzz A&A, 690 (2024) A399 DOI: https://doi.org/10.1051/0004-6361/202451045
