There are some science stories that are pleasing because the universe is doing something that just makes sense. In a new paper in The Astrophysical Journal, researchers led by Anshu Gupta and Kim-Vy Tran document how tightly formed massive galaxies blow out their own star formation in ways that more puffy galaxies do not.

Puffy is actually the word scientists use to explain this work, although in the science paper they instead referred to these galaxies as more extended. We’re going to go with puffy.

Here’s what’s happening. Early in the universe, a large population of galaxies grew up together, reaching large sizes and maximum star formation about four billion years after the universe formed. While many galaxies, including our own, continued to form later, we weren’t among the cool kids who did everything as a pack early in the universe.

Some of those early galaxies packed their material tightly near their core. In these systems, the combination of massive star formation, periodic supernovae, and all the crazy things that go on when stars are young, those activities send material into the galaxies’ supermassive black holes. As material falls toward a black hole, it forms a bright accretion disk, and the light from that bright disk can actually exert enough force on surrounding material that it can push that material out of the system.

With that material gone, star formation comes to a halt as the galaxy quiets down.

In more extended or puffy systems, the star formation is spread out, and what is going on in one place is less likely to affect what is going on in another place. This paper documents how puffy galaxies essentially protect their star formation. Material isn’t pushed toward the black hole. It isn’t flung out of the system. Everything just putters along. This is essentially the galaxy example of how a small fire in a dense forest can burn down everything, while that same fire in a parking lot with a few trees here and there for shade might catch one or two trees on fire, but generally won’t burn down everything.

Bottom line, puffy galaxies are the place to be if you want long periods of star formation over billions and billions of years. Those tightly packed systems? They’re where you’ll see all the active galaxy fireworks before everything grinds to a halt, early in the Universe.

More Information

Astro3D press release

“MOSEL and IllustrisTNG: Massive Extended Galaxies at z = 2 Quench Later Than Normal-size Galaxies,” Anshu Gupta et al., 2021 February 2, The Astrophysical Journal