ALESS 073.1, a star-forming galaxy located 12.5 billion light-years away, has all of the features expected of a much more mature galaxy and has led astronomers to question how it grew so fast.
Galaxies come in a variety of shapes, sizes and colors, and are made up of different components such as rotating disks, spiral arms, and bulges (tightly packed groups of stars usually situated within the center of galaxies).
Cosmological models predict that galaxies forming in the early Universe experience a chaotic phase of gas accretion and star formation, followed by gas ejection.
Bulges of these galaxies may assemble later via mergers with other galaxies or internal evolution.
“We discovered that a massive bulge, a regular rotating disk, and possibly spiral arms were already in place in ALESS 073.1 when the Universe was just 10% of its current age,” said Dr. Federico Lelli, an astronomer in the School of Physics and Astronomy at Cardiff University.
“In other words, this galaxy looks like a grown adult, but it should be just a little child.”
Using data from the Atacama Large Millimeter/submillimeter Array (ALMA), the astronomers measured the kinematic properties of the ALESS 073.1 galaxy.
They then modeled the mass distribution within the galaxy and found that it contains a massive stellar bulge and a uniformly rotating disk, features that models predict take billions of years to form.
“A galaxy like ALESS 073.1 just defies our understanding of galaxy formation,” Dr. Lelli said.
“This spectacular discovery challenges our current understanding of how galaxies form because we believed these features only arose in mature galaxies, not in young ones,” said Dr. Timothy Davis, also from the School of Physics and Astronomy at Cardiff University.
The study was published today in the journal Science.
Federico Lelli et al. 2021. A massive stellar bulge in a regularly rotating galaxy 1.2 billion years after the Big Bang. Science 371 (6530): 713-716; doi: 10.1126/science.abc1893