Astronomers have observed signs of rotation in a galaxy which existed a mere 500 million years after the Big Bang. This is by far the earliest galaxy with possible rotation. But the speed of the rotation is slower than modern galaxies, indicating that the observed galaxy is still in the process of revving-up to speed. This is an important example for understanding the early development of galaxies.
Many galaxies in the modern Universe, including our own Milky Way, rotate about a central area. When and how galaxies start to rotate is an important question in astronomy because it can affect the environment where later stars, planets, and possibly even life-forms will develop.
A team led by Tsuyoshi Tokuoka at Waseda University in Tokyo used ALMA (Atacama Large Millimeter/submillimeter Array) to conduct observations over a two-month period of a galaxy known as MACS1149-JD1, or JD1 for short. JD1 existed in the early Universe, only 500 million years after the Big Bang. By fitting a model to their observations, the team found that a small, slowly rotating galaxy provides the best fit to the data. The model indicates that JD1 is 3,000 light-years across, compared to 100,000 light-years for the Milky Way; and its rotation speed is only 50 kilometers per second, again compared to 220 kilometers per second for the Milky Way.
“The rotation speed of JD1 is much slower than those found in galaxies in later epochs and our [Milky Way] Galaxy and it is likely that JD1 is at an initial stage of developing a rotational motion,” says Akio K. Inoue, a co-author of the paper, also at Waseda University. With the recently launched James Webb Space Telescope, the team now plans to investigate the structure of JD1 in more detail to refine their scenario for its formation.
These results appeared as Tokuoka et al. “Possible Systematic Rotation in the Mature Stellar Population of a z = 9.1 Galaxy” in The Astrophysical Journal Letters on July 1, 2022.