Galaxies breathe gas, and when they stop, no more stars form
For most of the history of astronomy, all we could see were stars. We could see them individually, in clusters, in nebulae, and in fuzzy blobs that we thought were clumps of stars but were actually galaxies. The thing is, ...
Now that astronomers can see gas better than ever, we can see how galaxies breathe it in and out. When they stop breathing it, stars stop forming.
In the universe's early days, everything was gas. There were no stars or galaxies. Fast forward 13.5 billion years or so, and we see galaxies and their stars almost wherever we look. But the huge majority of matter in a galaxy is still gas.
Gas is everywhere. When it's in the space between galaxies, we call it the intergalactic medium. When the gas closely surrounds a galaxy, we call it circumgalactic gas. There's no barrier between these, they're just names astronomers use so they can talk about them.
Astronomers are starting to understand the flow of gas between a galaxy, its circumgalactic medium, and the intergalactic medium. The flow regulates star formation, and when that flow stops, the galaxy stops breathing.
Artist concept of how a galaxy might accrete mass from rapid, narrow streams of cold gas. These filaments provide the galaxy with continuous flows of raw material to feed its star-forming at a rather leisurely pace. Credit: ESA–AOES Medialab
This illustration shows gas flowing in and out of a galaxy. Pink and orange outflows emerge from the galaxy’s disk. The blue gas is from the intergalactic medium (IGM) and flows back into the galaxy to be recycled into new galaxies. Credit: Tumlinson et al. 2017