ALMA discovers the most massive spinning galaxy in the early universe


In our universe, which is 13.8 billion years old, most galaxies like our Milky Way gradually form, reaching their large mass relatively late. A new discovery made with the Atacama Massive Millimeter / Submillimeter Array (ALMA), a massive rotating disk galaxy, seen at a stage when the universe was only ten percent of its current age, challenges traditional galaxy formation models. This study appeared on May 20, 2020 in the journal Nature.

Galaxy DLA0817g, nicknamed the Wolfe Disk in honor of the late astronomer Arthur M. Wolf, is the most distant spin-disk galaxy ever seen. The unprecedented power of ALMA allowed us to see the rotation of this galaxy at a speed of 272 kilometers per second, like our Milky Way.

“While previous studies hinted at the existence of these early rotating gas-rich disk galaxies, thanks to ALMA, we now have clear evidence that they formed already 1.5 billion years after the Big Bang,” said lead author Marcel Nilman of Max Planck Institute in Heidelberg, Germany.

How was the galaxy DLA0817g formed?

The discovery of the galaxy poses a challenge to many simulations of galaxy formation that predict that massive galaxies at this point in evolution grow due to the multitude of mergers of smaller galaxies and hot clusters of gas.

“Most of the galaxies that we find in the initial universe resemble train wrecks because they underwent a consistent and often“ violent ”merger,” Nilman explained. “These hot fusions make it difficult to form well-ordered disks, which we observe in our galaxy.”

In most galaxy formation scenarios, they begin to show a well-formed disk about 6 billion years after the Big Bang. The fact that astronomers discovered such a disk galaxy when the universe was only ten percent of its current age indicates that its growth process should have been different.

“We believe that it has grown mainly due to continuous growth due to cold gas,” said J. Xavier Prochaska of the University of California, Santa Cruz and co-author of the study. “However, one of the questions that remains is how to collect such a large mass of gas while maintaining a relatively stable rotating disk.”

Star formation

The team also used the Very Large Antenna Array (VLA) and the Hubble Space Telescope to learn more about star formation in the DLA0817g galaxy. In radio waves, ALMA looked at the movements of the galaxy and the mass of atomic gas and dust, while the VLA measured the amount of molecular mass – fuel for star formation. In the ultraviolet light, Hubble observed massive stars. “Star formation in DLA0817g is at least ten times faster than in our own galaxy,” said Prochaska. “It must be one of the most productive disk galaxies in the early Universe.”

“Normal” galaxy

Wolf’s disc was first discovered by ALMA in 2017. Nilman and his team discovered the galaxy when they explored light from a more distant quasar. The light from the quasar was absorbed when it passed through a huge reservoir of gaseous hydrogen surrounding the galaxy – this is how it manifested itself. Instead of looking for direct light from extremely bright, but rarer galaxies, astronomers used this “absorption” method to find weaker and more “normal” galaxies in the early Universe.

“The fact that we found Wolf’s disk using this method tells us that it belongs to the normal population of galaxies present in the early days,” Nilman said. “When our latest observations with ALMA unexpectedly showed that it rotates, we realized that the early spinning disk galaxies are not as rare as we thought, and that there should be many more.”

“This observation is the epitome of how our understanding of the universe improves thanks to the increased sensitivity that ALMA brings to radio astronomy,” said Joe Pesche, director of the National Science Foundation’s astronomy program, which finances the telescope. “ALMA allows us to make new, unexpected discoveries with almost every observation.”


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