Slow spin of an early galaxy observed for the first time

For the study, published in The Astrophysical Journal Letters, an international team of researchers analyzed data from a galaxy known as MACS1149-JD1 (JD1), obtained from observations by the Atacama Large Millimeter/ submillimeter Array (ALMA), an assembly of radio telescopes in Chile.

The galaxy is so distant that its light comes to us from a time when the Universe was only 550 million years old, or 4% of its current age.

The researchers, led by Tsuyoshi Tokuoka of Waseda University, found subtle variations in the wavelengths of light indicating that parts of the galaxy were moving away from us while others were moving towards us. From these variations, they concluded that the galaxy was disk-shaped and spinning at a speed of 50 kilometers per second. In contrast, the Milky Way, at the position of the Sun, rotates at a speed of 220 kilometers per second today.

From the size of the galaxy and the speed of its rotation, the researchers were able to deduce its mass, which allowed them to confirm that it was probably 300 million years old and therefore formed around 250 million. years after the Big Bang.

“This is by far the farthest in time that we have been able to detect the spin of a galaxy,” said co-author Professor Richard Ellis of University College London (UCL). “It allows us to trace the development of rotating galaxies over 96% of cosmic history – rotations that started slowly at first, but got faster as the Universe got older.

“These measurements confirm our earlier findings that this galaxy is well established and likely formed about 250 million years after the Big Bang. On a cosmic timescale, we see it spinning shortly after the stars have turned on the Universe for the first time.

“Our findings shed light on how galaxies evolved in the early Universe,” said the co-author Dr Nicolas Laportefrom Cambridge Kavli Institute of Cosmology. “We see that 300 million years after massive molecular clouds condensed and merged into stars, a galactic disk grew and the galaxy acquired shape and rotation.”

“Determining whether distant galaxies rotate is very difficult because they only appear as tiny dots in the sky,” said co-author Professor Akio K. Inoue, from Waseda University, Tokyo. high resolution obtained by combining the 54 radio telescopes of the ALMA observatory.

The farther a galaxy is from Earth, the faster it seems to be moving away from us. As receding objects emit light shifted to longer wavelengths (“redshift”), this means that we can calculate their distance, and in turn their age, from the extent of the redshift.

Previous studies have shown the redshift of JD1 to be 9.1, meaning what we see is from when the Universe was 550 million years old. In the latest study, the team identified variations in the redshift across the galaxy, indicating differences in the rate at which the galaxy was moving away from us, meaning that, relatively speaking, one side moved away while the other approached. .

From the new observations, the team concluded that JD1 was only 3,000 light-years in diameter (in comparison, the Milky Way is 100,000 light-years in diameter) and that its total mass was equivalent to 1 to 2 billion. times the mass of the Sun.

This mass is consistent with the galaxy being about 300 million years old, with most of the mass coming from mature stars that formed near the start of the galaxy’s life.

The finding supports previous evidence from the same researchers, who proposed the same age estimate for JD1 in a paper published last year, using a different technique based on the brightness of light at different frequencies. They determined the ages of six galaxies, including JD1, concluding that cosmic dawn – the moment when stars first illuminated the Universe – occurred 200 to 300 million years after the Big Bang. .

The research was supported by MEXT in Japan, the Japan Society for the Promotion of Science, the European Research Council under the EU’s Horizon 2020 research and innovation program and the Kavli Foundation.


Tsuyoshi Tokuoka et al. “Possible systematic rotation in mature stellar population of az=9.1 galaxy.” The Astrophysical Journal Letters (2022). DO I: