Using a powerful astronomical technique called gravitational lensing, the NASA/ESA Hubble Space Telescope made a stunning discovery — the first example of a compact yet massive, fast-spinning, disk-shaped galaxy that stopped making stars only a few billion years after the Big Bang.
The galaxy in question, called MACS 2129-1, lies approximately 10 billion light-years away from Earth.
It is three times heavier than our own Milky Way Galaxy but only half the size.
“So MACS 2129-1 is extremely compact,” said Dr. Sune Toft, associate professor at Dark Cosmology Centre at the Niels Bohr Institute, the University of Copenhagen, Denmark.
“Speed limits also seem to differ somewhat between MACS 2129-1 and the Milky Way.”
“We were able to establish that the stars in MACS 2129-1 rotate in circles around the center of the galaxy at a speed of over 500 km per second, more than twice as fast as stars rotate in the Milky Way.”
When Hubble photographed the galaxy, Dr. Toft and co-authors 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 is 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.
This is a surprise because elliptical galaxies contain older stars, while spiral galaxies typically contain younger blue stars.
At least some of these early ‘dead’ disk galaxies must have gone through major makeovers. They not only changed their structure, but also the motions of their stars to make a shape of an elliptical galaxy.
“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,” Dr. Toft said.
“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.”
“Previous studies of distant dead galaxies have assumed that their structure is similar to the local elliptical galaxies they will evolve into.”
“Confirming this assumption in principle requires more powerful space telescopes than are currently available.”
Why MACS 2129-1 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 by heating the gas or expelling it from the galaxy.
Or it may 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 center.
But how do these young, massive, compact disks evolve into the elliptical galaxies we see in the present-day Universe?
“Probably through mergers,” Dr. 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 team’s findings were published in the June 22 issue of the journal Nature.
Sune Toft et al. 2017. A massive, dead disk galaxy in the early Universe. Nature 546: 510-513; doi: 10.1038/nature22388