Gas plays a crucial role in the life cycle of galaxies. While stars and galaxies have long been the focus of astronomical observation, researchers have recently been able to see and 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 continuous breathing of gas is essential for the birth of new stars. When this flow ceases, star formation also ceases.

The process of a galaxy’s breathing involves the interaction between stars, gravity, and the temperature and density of the gas. When the Universe formed, gas accumulated within galaxies and gave rise to stars. As stars die, they expel gas back into the surrounding space, which is initially hot and diffuse. However, as the gas exits the galaxy, it cools down and its density increases. This allows gravity to attract the gas back to the galaxy, where it can collapse and form new stars.

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

Large-scale surveys have revealed that 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 Kelvin, making it hotter and colder than intergalactic gas. However, studying incoming gas is challenging because its signals overlap with those from the galaxies themselves.

The exact cause of outgoing gas, which is easier to observe, remains uncertain. It could be the result of supernovae, stellar winds, or even feedback from black holes. However, regardless of the specific cause, the flow of gas eventually ceases, leading to the cessation of star formation in galaxies. Once a galaxy becomes quiescent, it will stop forming stars and appear red in color.

Although there is still much to learn about galactic breathing, simulations are providing astronomers with valuable insights. For example, the FIRE simulation 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 galactic breathing. Understanding the role of gas in star formation and the life cycle of galaxies is crucial for understanding the origins of stars, planets, and even life itself.

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