Cities have their heydays and their downfalls. But that’s not merely an earthly phenomenon.

In fact, billions of years ago, galaxies that dwelled in crowded areas called clusters also experienced a kind of opulence, with lots of cold gas, or fuel, for making stars.  Today, however, these galactic cities are ghost towns, populated by galaxies that can no longer form stars.  How did they get this way and when did the fall of galactic cities occur?

A new study from NASA’s Spitzer Space Telescope finds evidence that these urban galaxies, or those that grew up in clusters, dramatically ceased their star-making ways about 9 billion years ago (our universe is 13.8 billion years old). These galactic metropolises would have either consumed or lost their fuel. Galaxies in the low-density “countryside,” by contrast, are still actively forming stars – these are the beautiful spiral disk galaxies like our own Milky Way.

“We know the cluster galaxies we see around us today are basically dead, but how did they get that way?” wondered Mark Brodwin of the University of Missouri-Kansas City, lead author of this paper, published in the “Astrophysical Journal.”Read more on Phys.org.