Discovery of a dead galaxy creates a huge puzzle for astronomers

At just 3 billion years old, this galaxy should be blue and full of new, young stars. Instead, it’s already out of fuel. What gives?

“This new insight may force us to rethink the whole cosmological context of how galaxies burn out early on and evolve into local elliptical-shaped galaxies. Perhaps we have been blind to the fact that early “dead” galaxies could in fact be disks, simply because we haven’t been able to resolve them.” -Sune Toft
The life-cycle of a galaxy is straightforward and inevitable: normal and dark matter gravitationally attract, creating a large collection of mass.

Within a dark matter halo, the normal matter collects towards the center. When the densities reach large enough amounts, gas clouds collapse, forming new stars within. Image credit: J. Turner.
The normal matter collects in the center, forming stars and pancaking into a disk.

Over time, more gas falls into the core and smaller, young galaxies merge, building up the large galaxies we see today.
The most widely accepted theory of galaxy formation and evolution says that galaxies formed from “collisions” of smaller structures, which then evolved over the past 8–11 billion years into the galaxies we see today. Image credit: Gemini Observatory Illustration / Jon Lomberg.
This pattern of hierarchical mergers creates modern spirals when a single mass dominates, or giant ellipticals when multiple large ones merge.
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: A. Gai-Yam / Weizmann Inst. of Science / ESA / NASA.
When major mergers happen, stars form all at once, expelling the remaining gas that would be used for future generations of stars.
The Cigar Galaxy, M82, and its supergalactic winds (in red) that showcase the rapid new star formation occurring within it. This is the closest massive galaxy undergoing rapid star formation like this to us. Image credit: NASA, ESA, The Hubble Heritage Team, (STScI / AURA); Acknowledgement: M. Mountain (STScI), P. Puxley (NSF), J. Gallagher (U. Wisconsin).
But one newly-discovered galaxy challenges that entire picture.
This is a wide-field view of galaxy cluster MACS J2129–0741, located in the constellation Aquarius. The massive galaxy cluster magnifies, brightens, and distorts the images of remote background galaxies, including the far-distant, dead disk galaxy MACS2129–1, in red at the upper right. Image credit: NASA, ESA, M. Postman (STScI), and the CLASH team.
Younger spiral galaxies are smaller, bluer, gas-rich and less massive, in general.
Galaxies comparable to the Milky Way today are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, and richer in gas in general than galaxies today. Image credit: NASA and ESA.
Except, apparently, for MACS2129–1, which we see at a redshift of z=2.15, when the Universe was just 3 billion years old.
A blown-up view of the gravitationally lensed galaxy, MACS2129–1, which rotates extremely rapidly and is completely devoid of new, young stars. Image credit: NASA, ESA, and S. Toft (University of Copenhagen) Acknowledgment: NASA, ESA, M. Postman (STScI), and the CLASH team.
It is gas-poor and devoid of young, blue stars, and only half the physical size of the Milky Way despite being three times its mass.
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. Levy (STScI).
It appears stretched and distorted due to the gravitational lensing of a nearby, massive cluster.

By mapping the matter distribution of the foreground, lensing cluster, astronomers can reconstruct what the galaxy looks like without the effects of the gravitational lens. Image credit: NASA, ESA, and S. Toft (University of Copenhagen); Acknowledgment: NASA, ESA, M. Postman (STScI), and the CLASH team.
With no new stars, this young, massive spiral is a cosmic mystery, challenging our theories of galaxy evolution.
Mostly Mute Monday tells the cosmic story of an astronomical object or phenomenon in pictures, visuals and no more than 200 words.
Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.