The Importance of Gas in Galaxy Breathing

Gas plays a crucial role in the life cycle of galaxies. While stars and galaxies have long been the focus of astronomical observation, it is only recently that researchers have been able to see and better understand the significant impact of gas on the formation and evolution of these cosmic structures.

The term “gas” refers to the interstellar medium, which can be found between galaxies, as well as the circumgalactic gas that closely surrounds a galaxy. These terms are used by astronomers to describe the different regions of gas, but there is no strict boundary between them.

Astronomers have begun to unravel the intricate flow of gas between galaxies, their circumgalactic medium, and the intergalactic medium. This flow plays a vital role in regulating star formation, as the on-going breathing of gas is essential for the birth of new stars. When this flow ceases, so does the formation of stars.

The process of galaxy breathing involves the interplay between stars, gravity, and the temperature and density of gas. When the Universe formed, gas collected within galaxies and gave birth to stars. As stars die, they expel gas back into the surrounding space, which is initially hot and diffuse. However, as the gas leaves the galaxy, it cools down and its density increases. This allows gravity to draw the gas back into the galaxy, where it can collapse and form new stars.

Astronomers have managed to observe the flow of gas into and out of galaxies since the 1960s using the light from distant quasars. They have discovered that the circumgalactic gas close to galaxies has a higher metallicity, indicating that it is the result of gas being expelled by stars. The gas in the circumgalactic medium also acts as a source of fuel for star formation in galaxies.

Large-scale surveys have revealed that the gas in the circumgalactic medium is up to 1,000 times denser than the gas found in the intergalactic medium. The temperature of this gas ranges from 10,000 to 1 million Kelvins, making it both hotter and cooler than the intergalactic gas. However, studying the in-flowing gas is challenging due to signals being overlapped by those from the galaxies themselves.

The cause of the outflowing gas, which is easier to observe, remains uncertain. It could be a result of supernovae, stellar winds, or even black hole feedback. Nonetheless, regardless of the specific cause, the flow of gas eventually ceases, leading to the quenching of star formation in galaxies. Once a galaxy becomes quiescent, it will no longer form stars and will appear red in color.

While there is still much to learn about galactic breathing, simulations are providing astronomers with helpful insights. The FIRE simulation, for example, models the formation and evolution of galaxies over billions of years, allowing researchers to visualize the flow of gas in and out of these cosmic structures.

In conclusion, gas is a vital component in the process of galaxy breathing. Understanding the role of gas in star formation and the life cycle of galaxies is crucial for comprehending the origins of stars, planets, and even life itself.

– Tumlinson, J. et al. “The Circumgalactic Medium of Milky Way Mass Galaxies” (2017)