Fiery gas surrounds the Milky Way, but what is heating it?

For decades, astronomers have puzzled over the intense heat and persistence of the fiery hot gas surrounding the Milky Way. 

New research by scientists from the Raman Research Institute (RRI), IIT-Palakkad, and Ohio State University sheds light on this mystery. The team has proposed a model to identify the sources responsible for heating and maintaining this extraordinary phenomenon.

The Milky Way’s hidden gas reservoir

The Milky Way contains more gas than stars, and this gas plays a crucial role in sustaining star formation. This massive gas reserve has enabled the galaxy to continue forming stars over billions of years. 

However, the gas is so tenuous that it has long eluded observation, making it difficult for astronomers to measure its true volume.

In the past few decades, scientists confirmed the existence of a vast sphere of hot gas enveloping the Milky Way. This gas extends up to 700,000 light-years and reaches temperatures of several million degrees Kelvin. 

Researchers initially attributed its high temperature to the galaxy’s gravitational forces. The gas swirls constantly, preventing it from falling inward under the pull of gravity.

The discovery of even hotter gas

Recent observations revealed something even more astonishing: a hotter layer of gas surrounding the Milky Way. 

This gas, estimated to be around 10 million degrees Kelvin, was identified through faint X-ray emissions detected in all directions. The same gas also appeared as an absorbing medium in the spectra of three distant quasars.

This discovery posed new questions: What was causing this gas to reach such extreme temperatures, and how was it being sustained? These findings sparked a surge of research aimed at identifying the mysterious heat sources.

Supernovae: The engines heating the Milky Way

The new model proposed by RRI and its collaborators provides a detailed explanation for the heating mechanism. 

The research shows that the X-ray-emitting gas and the absorbing gas are not the same. Instead, the X-ray-emitting gas originates from a puffed-up region surrounding the Milky Way’s stellar disk.

This region is created by the ongoing star formation within the Milky Way. Massive stars in these star-forming regions eventually explode as supernovae, releasing vast amounts of energy. These explosions heat the surrounding gas to extreme temperatures.

“Explosions keep heating up the gas floating around the disk of the Milky Way, and they enrich the gaseous matter with elements synthesized within massive stars,” explained Mukesh Singh Bisht, a PhD student at RRI.

The turbulent gas, propelled by supernovae, either escapes into the surrounding medium or cools and falls back onto the disk. This process sustains the hot gas layer observed in X-rays.

Clues from enriched gas

The absorbing hot gas also provided critical insights into its origins. Researchers found that it was enriched with α-elements such as sulfur, magnesium, and neon. 

These elements, whose nuclei are multiples of helium nuclei, are produced through nuclear reactions within massive stars. They are expelled into space during supernova explosions.

“This fiery gas, at least in a few directions, seems to be enriched with large quantities of α-elements. This is a vital clue of nuclear reactions occurring within the stellar core,” said Biman Nath, an RRI faculty member and co-author of the studies.

Runaway stars – stars ejected from the Milky Way’s disk – also play a role in this process. When these stars explode as supernovae above the disk, they create pockets of α-enriched gas.

Explaining the observed signals

The researchers found that the absorbing gas interacts with light from distant sources, such as quasars. When aligned with these light sources, the gas absorbs specific wavelengths, creating shadow signals that match observations.

Meanwhile, the X-ray-emitting gas forms a veil around the stellar disk, produced by the continuous star formation and supernova activity within the Milky Way.

“A veil of fiery hot gas keeps engulfing the Milky Way disk as a result of the star-forming activities in the stellar disk,” Bisht added .

This dual mechanism explains both the absorbing and emitting signals detected by astronomers, offering a comprehensive understanding of the dynamics within our galaxy.

Implications for galaxy evolution

The faint X-ray signals emitted by the hot gas offer valuable opportunities for further study. The research team plans to test their models at other frequencies, hoping to gather more clues about the composition and behavior of the gas.

Their work also raises questions about how these processes affect the larger-scale evolution of the Milky Way. Understanding the interplay between star formation, supernovae, and the surrounding gaseous envelope could provide insights into the life cycles of galaxies.

This research not only solves a long-standing mystery but also highlights the importance of gas dynamics in shaping the Milky Way. The interplay between supernovae and the surrounding gas regulates the galaxy’s star formation and energy distribution.

The enriched gas observed in the study also points to the ongoing recycling of materials within the galaxy, where elements created in stellar cores are redistributed through explosions, fueling new generations of stars.

Sources of the Milky Way’s fiery gas

The new model proposed by RRI and its collaborators sheds light on the sources of the fiery hot gas surrounding the Milky Way, offering answers to a decades-old mystery. 

By identifying supernovae as the primary heat engines, the study reveals the complex interplay between star formation, explosions, and gas dynamics that sustain the galaxy’s evolution.

With future research planned to explore these phenomena further, scientists are poised to gain even deeper insights into the processes that shape our galaxy and others like it.

The findings are published in two studies in The Astrophysical Journal.

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