How do galaxies form and grow? The theory behind it is simple and straightforward. You start with a collection of normal-and-dark matter, the normal matter collects in the center, pancakes, and forms stars over time. A little later, more gas falls in, and the galaxy continues to form new stars over time. It’s only when a gravitational interaction or a major merger occurs that a galaxy goes from active to “dead and red,” in the form of a giant elliptical.

Galaxy mergers are common, and as time goes on, all the gravitationally bound galaxies in groups and clusters will eventually merge into a single galaxy at the core of each bound structure. 
When major mergers occur, the result is almost always a giant elliptical. 
Image credit: NASA, ESA and the Hubble Heritage Team.

At least, that’s the theory. But a new analysis of a very unusual galaxy, MACS2129-1, shows that despite being just 3 billion years old, it’s already “dead and red,” but it’s somehow still a spiral! With no evidence for a major merger and no conventional explanation for this otherwise, this young, dead galaxy creates a huge puzzle for astronomers.

This artist’s concept shows what the young, dead, disk galaxy MACS2129-1, right, would look like when compared with the Milky Way galaxy, left. Although three times as massive as the Milky Way, it is only half the size. MACS2129-1 is also spinning more than twice as fast as the Milky Way. Note that regions of Milky Way are blue from bursts of star formation, while the young, dead galaxy is yellow, signifying an older star population and no new star birth. MACS2129-1 appears redder overall because of its cosmic redshift. Image credit: NASA, ESA and Z. Levay (STScI).

From the distant Universe, light has traveled for some 10.7 billion years from distant galaxy MACSJ2129-1, lensed, distorted and magnified by the foreground clusters imaged here.
Image credit: NASA, ESA, and S. Toft (University of Copenhagen) 
Acknowledgment: NASA, ESA, M. Postman (STScI), and the CLASH team.