Astronomers observed the slow spin of early galaxy

One of the most distant known galaxies, observed in the very earliest years of the Universe.

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Using data from the Atacama Large Millimeter/submillimeter Array (ALMA), an international team of researchers observed one of the most distant known galaxies in the very earliest years of the Universe. The observations reveal that the early galaxy MACS1149-JD1 (JD1) has a slow spin. The galaxy appears to be rotating at less than a quarter of the speed of the Milky Way today.

The findings came out from a new study involving University of Cambridge researchers.

MACS1149-JD1 (also known as PCB2012 3020) is the most distant object. Light from the young galaxy captured by the orbiting observatories first shone when our 13.7-billion-year-old Universe was just 500 million years old, i.e., 4% of its present age.

Researchers detected subtle variations in the wavelengths of the light coming from a galaxy that signifies parts of the galaxy were moving away from us while other parts were moving toward us. Based on these variations, researchers concluded that the galaxy was disc-shaped and rotating at 50 kilometers a second. In comparison, the Milky Way today rotates at a speed of 220 kilometers per second.

Based on its size and rotational speed, researchers could infer its mass. From this, they calculated the galaxy’s age- almost 300 million years old and formed about 250 million years after the Big Bang.

Co-author Professor Richard Ellis from University College London (UCL) said, “This is by far the furthest back in time we have been able to detect a galaxy’s spin. It allows us to chart the development of rotating galaxies over 96% of cosmic history – rotations that started slowly initially, but became more rapid as the Universe aged.”

“These measurements support our earlier findings that this galaxy is well-established and likely formed about 250 million years after the Big Bang. On a cosmic time scale, we see it rotating not long after stars first lit up the Universe.”

Co-author Dr. Nicolas Laporte from Cambridge’s Kavli Institute for Cosmology said, “Our findings shed light on how galaxies evolved in the early Universe. We see that a galactic disk has developed 300 million years after massive molecular clouds condensed and fused into stars, and the galaxy has acquired a shape and rotation.”

Co-author Professor Akio K. Inoue from Waseda University, Tokyo, said“Determining whether distant galaxies are rotating is very challenging because they only appear as tiny dots in the sky. Our new findings came thanks to two months of observations and the high resolution achieved by combining the 54 radio telescopes of the ALMA observatory.”

The further away a galaxy is from Earth, the faster it appears to move away from us. Since objects traveling away emit light that has been “redshifted” toward longer wavelengths, we can determine their distance and, consequently, their age from the degree of redshift.

According to earlier research, JD1 has a redshift of 9.1, which indicates that the Universe was 550 million years old when it was observed. In the most recent investigation, the scientists discovered changes in the redshift across the galaxy, indicating differences in the rate at which the galaxy was moving away from us. Relative to us, one side of the galaxy was moving farther away while the other was moving closer.

From the new observations, the team concluded that JD1 was only 3,000 light-years across (by comparison, the Milky Way is 100,000 light-years across) and that its total mass was equivalent to 1-2 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 close to the start of the galaxy’s life.

Journal Reference:

  1. Tsuyoshi Tokuoka et al. ‘Possible Systematic Rotation in the Mature Stellar Population of a z = 9.1 Galaxy.’ The Astrophysical Journal Letters (2022). DOI: 10.48550/arXiv.2205.14378