Astronomers have detected the rotation of a galaxy dating back to just 550 million years after the Big Bang, when the universe was 4% of its current age. The rotation suggests that this baby galaxy was not an amorphous blob, but rather an organized disk, much like the Milky Way and similar galaxies that took more than 13 billion years to mature. It’s even more proof that galaxies are growing and evolving faster than theorists expected.
Stars merging into a disk so early don’t contradict existing theories, but they do stretch them, says Sarah Bosman of the Max Planck Institute for Astronomy, who was not involved in the study. “It can be this early.”
The stars in this leading galaxy were also mature, indicating that they formed 300 million years earlier, when the universe was only 250 million years old. That’s good news for the upcoming James Webb Space Telescope, which launched last year and expected to begin scientific operations next month† If these first galaxies spawned many stars early in an outburst, they’ll likely be “bright enough for Webb to see,” said Richard Ellis, an astronomer at University College London and co-author of the new study. Astronomers hope the $10 billion instrument will reveal a wealth of early galaxies, rather than just the largest, brightest outliers, so they can understand how these early agglomerations formed and evolved.
The galaxy, MACS1149-JD1 or JD1 for short, is not the most distant galaxy known, but it is now the most distant galaxy with anything known about its dynamics. The team, including Ellis, discovered JD1 in 2018 as an indistinct blob in images from the Atacama Large Millimeter/submillimeter Array (ALMA), a collection of 66 radio telescopes high in the Chilean Andes, and the European Southern Observatory’s Very Large Telescope. in Chile. From its color, Ellis and his colleagues knew that at least some of its stars were mature, because they tend to turn red as they age. But the team didn’t know how those stars were organized.
They decided to look longer and more closely with ALMA. Such telescope arrays can “zoom in” by spreading the dishes further apart and combining their signals through a process known as interferometry. The team observed JD1 when the dishes were spread over an area 2.5 kilometers wide, providing the sharpness of vision of a single dish of that size.
When astronomers look at such distant objects, their light is shifted toward the red end of the spectrum because photons are stretched in flight by the expansion of the universe. The degree of this “redshift” tells astronomers how far away the object is. With the new high-resolution image from JD1, the researchers were also able to see small differences in redshift across the face of the galaxy, the kind of variations you’d expect from a rotating object with one part moving toward the observer and the other away. As the team reports today in Astrophysical Journal Lettersthere was a difference in speed of 120 kilometers per second between one side of the galaxy and the other† “We were really excited when we saw the speed map,” says Ellis.
The researchers placed the velocity distribution in a galaxy model and it fitted well with a disk galaxy with a mass between 1 billion and 2 billion suns. This is small compared to the Milky Way, which contains 1 trillion solar masses, and therefore JD1 rotates more slowly than our galaxy, at about a quarter the speed of our galaxy. Finding a rotating galaxy so early in the universe’s history is “surprising,” Bosman says, but the data is “completely consistent with a disk.”
Astronomers won’t have to wait long to find out if JD1 is unique or if disk galaxies were common in the universe’s youth. The Webb telescope is expected to find many more galaxies in the first half billion years of the universe within a few weeks. “We’re going from single-digit numbers to hundreds,” Bosman says. “At least I hope so!”
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