File:Galaxy Cluster MACS J2129-0741.jpg

The galaxy, called MACS 2129-1, is considered “dead” because it is no longer making stars. The existence of dead galaxies so early—when the universe was just one-quarter its current age—has long been a puzzle, as the Universe at that time was full of gas and at the peak of the cosmic star formation history. The leading theory has been that they formed in galaxy collisions that efficiently drove all the gas into the center of the collision and turned it into stars.

“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,” said study leader Sune Toft of the Dark Cosmology Centre at the Niels Bohr Institute, University of Copenhagen. “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.”

When the universe was just 3 billion years old, half of the most massive galaxies were extremely compact and had already completed their star formation. Astronomers believe that they ultimately grew into the most massive elliptical galaxies seen in the nearby universe today. Scientists theorize they did this through mergers with small companion galaxies, which added to the stars on the galaxy’s outskirts.

Confirming this scenario requires more powerful telescopes than are currently available, whether on Earth or in space. However, through the phenomenon known as “gravitational lensing,” a massive, foreground cluster of galaxies functions as a natural telescope, magnifying and stretching images of far more distant background galaxies. By joining this natural lens with the resolving power of Hubble, scientists were able to see into the center of the dead galaxy.

The astronomers expected to see a chaotic ball of stars formed through mergers. Instead, they found a pancake-shaped stellar distribution, meaning that the stars must have been born in a disk. The remote galaxy is three times as massive as the Milky Way but only half the size. Rotational velocity measurements made with the European Southern Observatory’s Very Large Telescope (VLT) showed that the disk galaxy is spinning more than twice as fast as the Milky Way.

This is observational evidence that at least some of the earliest “dead” galaxies must have gone through major makeovers, changing not just their structure, but also the motions of their stars, to evolve into the giant elliptical galaxies we see today.

Using archival data from the Cluster Lensing And Supernova survey with Hubble (CLASH), Toft and his team were able to determine the stellar mass, star formation rate, and the ages of the stars.

“We can see that the star formation stopped first in the center of the galaxy and then moved outwards through the galaxy,” Toft said. “In other words, it died inside out. Until now it had been impossible to resolve them, so we didn’t know if star formation stopped all at once in the whole galaxy, or if it stopped first in the center, or if it stopped first in the outskirts.”

Why this galaxy stopped forming stars is still unknown. It may be the result of an active galactic nucleus, where energy is gushing from a supermassive black hole. This energy inhibits star formation. Or it may be the result of “halo quenching,” where, when cold gas tries to stream into the galaxy’s center, it’s rapidly heated. This prevents it from reaching the center and cooling down into star-forming clouds.

But how do these young, massive, compact disks evolve into the elliptical galaxies we see in the present-day universe? “Probably through mergers,” Toft said. “If these galaxies grow through merging with minor companions, and these minor companions come in large numbers and from all sorts of different angles onto the galaxy, this would eventually randomize the orbits of stars in the galaxies. You could also imagine major mergers. This would definitely also destroy the ordered motion of the stars.”

The findings are published in the June 22 issue of the journal Nature. Toft and his team hope to use NASA’s upcoming James Webb Space Telescope to look for a larger sample of such galaxies.