Voids Made by Dying Stars Have Been Detected in Our Galaxy's Interstellar Gas

People's deaths frequently seem to leave a hole in their wake. That happens to be physically accurate for big stars.

When a huge star goes supernova at the end of its life, bubbles that spread into space leave their impression on the flimsy gas that floats between the stars in the Milky Way galaxy, according to a new examination of the gas. According to experts, these eerie remnants preserve the Milky Way's rotation and a history of star death.

There is some matter in the region between the stars. Gas flows across the voids in space, occasionally gathering into broader clouds made primarily of atomic hydrogen. When these clouds are dense enough, stars can form there. When stars die, they leave behind the elements they formed in their centers, which then become part of these clouds.

But it's not quite clear how these clouds develop, organize, and recycle themselves around the galaxy. In order to better understand the features observed in the neutral atomic hydrogen that penetrates our galaxy, a group of astronomers under the direction of Juan Diego Soler of the Italian National Institute for Astrophysics (INAF) in Italy began their research.

The group made advantage of information gathered by the HI4PI project, an all-sky survey that examined the radio spectrum to create a map of neutral atomic hydrogen distribution throughout the Milky Way.

It is the most thorough study of its sort to date, documenting not just the hydrogen's distribution but also its velocity throughout the galaxy. The scientists are able to determine the separation between objects in the gas by combining this with a model of the Milky Way's rotation.

With the use of this data and a frequently used technique for satellite photo analysis, the team was able to reveal subtle hydrogen structural details that were difficult to see with the naked eye.

These were made up of a vast network of filaments, or thin threads of gas, most of which were directed randomly away from the disk but generally perpendicular to the plane of the Milky Way galaxy. Beyond 33,000 light-years out from the Milky Way's disk, the filaments were generally perpendicular to the galactic plane.

These networks were interpreted by the researchers as the Milky Way's gas bearing the fingerprint of supernova feedback.

"These are likely the remnants of multiple supernovae explosions that sweep up the gas and form bubbles that pop when they reach the characteristic scale of the galactic plane, like the bubbles that reach the surface in a glass of sparkling wine," stated Ralf Klessen, an astronomer at Heidelberg University in Germany.

"The fact that we see mostly horizontal structures in the outer Milky Way, where there is a strong decrease in the number of massive stars and consequently fewer supernovae, suggests that we are registering the energy and momentum input from stars shaping the gas in our galaxy."

According to the scientists, this might provide a fresh look at the dynamical processes that produced the Milky Way's disk and a tool for galactic archaeology, the study of old processes' preserved traces to piece together the past of our galaxy.

It also provides a fresh perspective for understanding any side effects that may occur close to the filaments.

"The interstellar medium, which is the matter and radiation that exist in the space between the stars, is regulated by the formation of stars and supernovae, with the latter being the violent explosions that occur during the last evolutionary stages of stars that are more than ten times more massive than the Sun," claimed French astronomer Patrick Hennebelle from the Saclay Nuclear Research Center.

"Associations of supernovae are very efficient at sustaining turbulence and lifting the gas in a stratified disk. The finding of these filamentary structures in the atomic hydrogen is an important step in understanding the process responsible for the galaxy-scale star formation."

The research has been published in Astronomy & Astrophysics.