Massive Disk-Shaped Dead Galaxy Discovered

Astronomers have detected a first-of-its kind compact yet massive, fast-spinning, disk-shaped galaxy that stopped making stars only a few billion years after the Big Bang.

Finding such a galaxy early in the history of the universe challenges the current understanding of how massive galaxies form and evolve, the researchers said.

The finding, published in the the journal Nature, was possible with the capability of NASA's Hubble space telescope.

When Hubble photographed the galaxy, astronomers expected to see a chaotic ball of stars formed through galaxies merging together.

Instead, they saw evidence that the stars were born in a pancake-shaped disk.

This was the first direct observational evidence that at least some of the earliest so-called "dead" galaxies - where star formation stopped - somehow evolve from a Milky Way-shaped disk into the giant elliptical galaxies we see today.

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"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," said study leader Sune Toft from University of Copenhagen, Denmark.

"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," Toft said.

The remote galaxy was 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 was spinning more than twice as fast as the Milky Way.

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.

Why this galaxy stopped forming stars was still unknown. It might be the result of an active galactic nucleus, where energy was gushing from a supermassive black hole.

This energy inhibits star formation by heating the gas or expelling it from the galaxy. Or it might be the result of the cold gas streaming onto the galaxy being rapidly compressed and heated up, preventing it from cooling down into star-forming clouds in the galaxy's centre.

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.