New discoveries

Early disk galaxy found by ALMA is hard to explain

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Disk galaxies like our own Milky Way grew faster than most astronomers expected. That’s the surprising conclusion of a new ALMA discovery. After the Big Bang, some 13.8 billion years ago, atoms first clumped together into small, irregular galaxies. These ‘galactic building blocks’ later merged into
larger and larger systems. Eventually, after everything came to rest, the galaxies developed flat, rotating disks, comparable to the central disk of our Milky Way. The whole process may take billions of years – at least, that’s what astronomers thought.

But a remote, massive galaxy found and studied by ALMA shows otherwise. This galaxy is so far away that its light took 12.3 billion years to reach us. In other words: we see the galaxy as it was 12.3 billion years ago. Back then, the universe was only 1.5 billion years old – just over one-tenth of its present age.
Surprisingly, ALMA found that the gas in the galaxy is already rotating in an orderly way, like the gas in our disk-like Milky Way galaxy. Never before has a rotating disk galaxy been found at such an early stage in the life of the universe.
It’s a bit like finding a young but almost full-grown bird in a newly-built nest. No one understands how the galaxy could have formed so quickly. If it only grew through the merging of smaller ‘galactic building blocks’, it could never become so big and regular in just 1.5 billion years. Perhaps there also was a
steady inflow of cold gas from the surrounding area. But even then, you would expect the growing galaxy to be pretty chaotic for a very long time.
Astronomers hope to find much more remote galaxies in the future. Together, they will paint a picture of how disk galaxies like our own Milky Way came to be.

What?

The remote galaxy, known by its catalogue designation DLA 0817g, was first discovered by ALMA in 2017. It is sometimes called the Wolfe Disk, after astronomer Arthur Wolfe, who died in 2014. Later observations revealed that the gas in the massive galaxy is rotating in an orderly, disk-like fashion, at velocities of about 270 kilometers per second. That’s comparable to the rotational velocities in our own Milky Way galaxy. ALMA was able to study the motions of gas atoms in the galaxy. Meanwhile, the Very Large Array radio telescope observed gas molecules, while the Hubble Space Telescope studied light from the many newborn, hot stars in the galaxy.

Who?

The ALMA observations of DLA 0817g were carried out by a team of astronomers led by Marcel Neeleman of the Max Planck Institute for Astronomy in Heidelberg, Germany. Marcel worked together with Xavier Prochaska of the University of California in Santa Cruz, Nissim Kanekar (Tata Institute of Fundamental Research in Pune, India), and Marc Rafelski (Space Telescope Science Institute in Baltimore). The four astronomers published their results in the weekly science magazine Nature.

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