The first galaxies rotated slowly, accelerating over billions of years

This image shows the spiral galaxy NGC 691, captured in fantastic detail by Hubble’s Wide Field Camera 3 (WFC3). This galaxy is the namesake member of the NGC 691 group of galaxies, a group of gravitationally bound galaxies that lie about 120 million light-years from Earth. Hubble observes objects like NGC 691 using a variety of filters. Each filter only allows certain wavelengths of light to reach Hubble’s WFC3. Images collected with different filters are then colored by specialized visual artists who can make informed decisions about which color best corresponds to which filter. By combining the color images of the individual filters, a full color image of the astronomical object can be recreated. In this way, we can get a very good insight into the nature and appearance of these objects.

A team of astronomers 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 is much slower than current galaxies.

All galaxies rotate, usually at incredible speeds. For example, the Milky Way galaxy has a rotation speed of more than 200 kilometers per second. But astronomers still don’t understand how galaxies stack up at these speeds. The only way to know is through measurements of galaxies over cosmic time, building a map of galactic evolution.

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

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

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

“JD1’s rotation rate is much slower than that found in galaxies at later epochs and our [Milky Way] galaxy, and JD1 is likely to be in an early stage of developing a rotational motion,” he says. Akio K. Inoue, a co-author on the paper, also at Waseda University.

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