Scientists have recently made significant strides in understanding the mysterious sources of heat that sustain the super-hot gas surrounding the Milky Way galaxy, which has puzzled astronomers for years. This gas, which extends up to 700,000 light-years from the center of the galaxy, was first detected several decades ago. Initially, scientists believed this gaseous matter, at temperatures reaching millions of degrees Kelvin, was primarily affected by the gravitational pull of the galaxy, causing the gas particles to swirl around in an attempt to escape the strong gravitational forces.
However, the discovery of even hotter gas in recent years—reaching temperatures around 10 million degrees Kelvin—has raised new questions. This hot gas was detected through faint X-ray emissions emanating in all directions from the Milky Way, and was also observed in the spectra of distant quasars as an absorbing medium. These observations sparked a research frenzy as astronomers sought to identify the sources of heat that were keeping this gas so hot, as well as the mechanisms responsible for its persistent fiery nature.
A team of scientists from the Raman Research Institute (RRI), along with collaborators from IIT-Palakkad and Ohio State University, has proposed a new model in two related studies published in The Astrophysical Journal. Their research offers fresh insights into the origins of the hot gas and its behavior.
The team confirmed that the X-ray emitting hot gas surrounding the Milky Way is distinct from the gas seen absorbing X-ray signals in the spectra of distant quasars. The X-ray emitting gas is a result of a “puffed-up” region surrounding the stellar disk of the Milky Way, which is heated by the continuous cycle of star formation and subsequent supernova explosions. These supernovae, caused by the death of massive stars, release enormous amounts of energy, heating the surrounding gas to temperatures of millions of degrees.
Mukesh Singh Bisht, a Ph.D. student at RRI and one of the lead researchers, explained that the explosions of these massive stars enrich the surrounding gas with heavy elements created in the cores of stars, such as sulfur, magnesium, and neon. These elements, known as α-elements, are important indicators of nuclear processes occurring within stars. The ongoing stellar explosions continually heat up the gas and enrich it with new elements, causing the gas to swirl around the Milky Way’s disk, sometimes escaping into the surrounding space or cooling and falling back onto the galaxy’s disk.
The study also revealed that the gas detected through absorption studies was enriched with these α-elements, providing a vital clue about the source of the gas. Biman Nath, a faculty member at RRI and one of the study’s co-authors, explained that the presence of these elements indicates nuclear reactions within massive stars. These elements are released into space when stars explode as supernovae, adding to the complex chemistry of the surrounding gas.
In addition to the star formation within the Milky Way, the researchers pointed out the role of runaway stars—stars that are ejected from the galaxy’s disk. When these stars explode as supernovae, they also contribute to the generation of hot gas around them. If the supernovae of these runaway stars align with the direction of distant quasars, the gas surrounding them can absorb and produce shadow-like signals, which is what is observed in the absorption spectra of the quasars.
At the same time, the researchers suggest that the ongoing star formation in the Milky Way’s stellar disk results in a continuous veil of hot gas surrounding the galaxy. This veil explains the persistent X-ray emissions detected around the galaxy, as the hot gas is constantly replenished by new supernovae and stellar activity.
The findings of this study provide new insights into the intricate dynamics of the hot gas surrounding the Milky Way, offering a potential explanation for both the X-ray emissions and the absorption signals seen in distant quasars. The researchers plan to further investigate these findings by studying the signals at different frequencies, which could provide additional clues about the origins of the gas and the processes maintaining its high temperatures.
This research helps to unravel the mysteries of the galactic environment, shedding light on the complex interactions between stars, gas, and the overall structure of the Milky Way. It underscores the importance of continuous stellar activity, such as supernovae, in shaping the galactic gas and its physical properties. As scientists continue to study this hot gas, the results may offer broader insights into the evolution of galaxies and the processes that sustain them across vast cosmic timescales.