Gas plays a crucial role in the evolution of galaxies. While stars and galaxies are more visible to us, the majority of matter in a galaxy is actually gas. Astronomers are now able to better observe and understand how galaxies breathe in and out this gas, which in turn affects star formation.
Gas exists in two forms in relation to galaxies. When it resides in the space between galaxies, it is referred to as the intergalactic medium (IGM). On the other hand, when the gas surrounds a galaxy closely, it is known as circumgalactic gas (CGM). The flow of gas between a galaxy, its CGM, and the IGM plays a vital role in regulating star formation.
The process of galactic breathing involves several factors, including stars, gravity, gas temperature, and density. In the early stages of the Universe, gas gathered in galaxies and formed stars. When stars die, particularly through supernovae, they expel gas back out into the surrounding space. This gas, initially hot and diffuse, eventually cools and increases in density as it moves away from the galaxy. The gravity of the galaxy then pulls the cooled gas back in, where it can collapse into clouds and ignite star formation once again. This cycle of gas recycling, or breathing, is essential to the life cycle of galaxies.
Astronomers began studying the flow of gas in and out of galaxies in the 1960s using the light from distant quasars. Through these observations, they discovered evidence of gas outflows and inflows. More recent research has focused on the CGM, which is a smaller and fainter region compared to the IGM. It has been recognized as a crucial source of star-forming fuel for galaxies, as well as a key regulator of the galactic gas supply.
However, understanding the complete process of galactic breathing remains a challenge due to the difficulty in observing in-falling gas. Astronomers have turned to large-scale surveys and simulations to gain more insights. These investigations have revealed that the gas in the CGM is up to 1,000 times denser than the gas in the IGM, with a range of temperatures. While the exact causes of gas outflows are still uncertain, supernovae, stellar winds, black hole feedback, and even supermassive black holes have been considered potential contributors.
The eventual cessation of gas breathing is known as “quenching,” which leads to the transformation of a galaxy into a quiescent state that no longer forms stars. Understanding quenching is of great interest to astronomers, but strong evidence is still lacking. Some research has found dense gas gravitationally bound to red, quenched galaxies, even though it has the potential to form stars. The difficulty in understanding in-falling gas and quenching prevents a complete comprehension of galactic breathing at present.
Nevertheless, simulations like FIRE (Feedback In Realistic Environments) provide helpful visualizations of the formation and evolution of galaxies over billions of years. These simulations contribute to our understanding of warm gas leaving galaxies while cool, dense gas flows back in. By gaining a deeper understanding of galactic breathing, astronomers hope to unlock the secrets of how stars, planets, and life itself are formed and sustained in the universe.
– Tumlinson, J. et al. (2017), Annual Review of Astronomy and Astrophysics, Vol. 55: The Circumgalactic Medium, arXiv:1707.05326 [astro-ph.CO]