The Earliest Galaxies Rotated Slowly, Revving up Over Billions of Years

A team of astronomers have used the ALMA telescope to find a slowly-rotating galaxy in the early universe. That galaxy is the youngest ever found with a measured rotation, and it’s much slower than present-day galaxies.

All galaxies rotate, usually at incredible speeds. For example, the Milky Way galaxy has a rotation speed of over 200 kilometers per second. But astronomers do not yet understand how galaxies build up to these speeds. The only way to tell is through measurements of galaxies throughout cosmic time, building up a map of galactic evolution.

Recently a team of astronomers based at Waseda University in Tokyo used ALMA (the Atacama Large Millimeter/submillimeter Array) in Chile to observe an extremely distant galaxy. This galaxy, MACS1149-JD1, is so far away that it’s normally far too dim to be observed. But the light from that galaxy passes through a giant galaxy cluster, and the gravitational lensing from that cluster magnifies MACS1149-JD1. Astronomers can use this magnification to see the galaxy.

Measuring galaxy rotation

MACS1149-JD1 existed when the universe was only 500 million years old, making it among the youngest known galaxies. The team used ALMA to study O III, or doubly-ionized oxygen, in the disk of the galaxy. They then developed a model of the size and rotation speed of the disk of the galaxy to compare against observations. They reported their results in a paper recently appearing in The Astrophysical Journal Letters.

The team found that MACS1149-JD1 is only 3,000 light-years across. That is far smaller than the Milky Way galaxy, which is over 100,000 light-years across. They also found that MACS1149-JD1 rotates at only 50 kilometers per second, which is less than a quarter of the Milky Way’s rotation speed.

“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.

These results suggest that galaxies start out small and rotate slowly. Then, over the course of billions of years, they accumulate more matter and increase their rotation rate. The team hopes to use the James Webb Space Telescope to conduct further studies of galaxy rotation rates over cosmic time.