Astronomers have discovered an exceptional object: a large compact spiral galaxy from the very early universe that seems to have spent all its gas and stopped forming new stars. This object is a progenitor for one of the most massive galaxies we see in the universe today, but it’s the wrong shape.
The biggest galaxies in the universe are elliptical, so astronomers assumed they came from smaller elliptical galaxies. However, if the new study published in Nature is anything to go by, this might be only part of the full picture.
Elliptical galaxies were thought to be good candidates because they are often “red and dead”, lacking new stars that make them “younger”. Spirals are often blue, with new stars constantly forming. The new object is instead an anomaly – a spiral galaxy three times as massive as the Milky Way, but only half its size.
"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," lead author Sune Toft of the Dark Cosmology Center at the Niels Bohr Institute, University of Copenhagen, said in a statement. "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."
Studying these giant galaxy seeds has been a challenge. Their light comes from when the universe was less than 3 billion years old. On top of that, these galaxies are quite dense and red, so it's difficult for astronomers to assess their shape.
Luckily, the universe lends a hand. The light of this particular compact galaxy experienced an effect called gravitational lensing. As it passed near a massive galaxy cluster, the light was magnified, giving researchers a better view of the galaxy.
Using the Hubble Space Telescope, the team saw that the object had a very thin disk (as one would expect from a spiral galaxy) and was rotating fast, which is another big clue. When the scientists looked at other properties such as star formation rate, mass, and age of the stars, they pointed at an already evolved spiral galaxy.
The researchers don’t know why this galaxy stopped forming stars and if this object is the exception or the norm. Maybe there are multiple ways for the massive galaxies in the universe to form. Future observatories like the James Webb Space Telescope will hopefully see these objects in more detail and help us work out the missing pieces from our theory of galaxy evolution.