The first spiral galaxy
Well, it may not have been the first spiral galaxy researchers have now discovered in data from the Atacama Large Millimeter/submillimeter Array (ALMA), but it was the oldest and most distant (which is synonymous in astronomy) to date. We observed it at a time when the universe was only 1.4 billion years old. Today it is almost ten times as old. The discovery of a galaxy with a spiral structure at such an early date is an important clue to solving the classic questions of astronomy: «How and when did spiral galaxies form?»
«I was excited because I had never seen such clear evidence of a rotating disk, spiral structure, and centralized mass structure in a distant galaxy in the previous literature,» said Takafumi Tsukui, lead author of the research published in the journal Science. «The quality of the ALMA data was so good that I could see so much detail that I thought it was a nearby galaxy.»
The Milky Way galaxy we live in is a spiral galaxy. Spiral galaxies are fundamental objects in the universe and account for up to 70% of the total number of galaxies. However, other studies have shown that the proportion of spiral galaxies is rapidly decreasing as we look back in the history of the universe. So when did the spiral galaxies form?
It is also interesting to note how Tsukui made the discovery. Namely, it was an archival find. The astronomer found a galaxy in the ALMA Science Archive called BRI 1335-0417, which existed 12.4 billion years ago and contained a large amount of dust that obscures starlight. This makes it difficult to study this galaxy in detail with visible light. On the other hand, ALMA can detect radio emissions from carbon ions in the galaxy, which allows us to study what is going on in the galaxy. The researchers found a spiral structure extending 15,000 light-years from the center of the galaxy. That’s one-third the size of the Milky Way. The estimated total mass of stars and interstellar matter in BRI 1335-0417 is about the same as that of the Milky Way.
«Since BRI 1335-0417 is a very distant object, we may not be able to see the true edge of the galaxy in this observation,» Tsukui comments. «For a galaxy in the early universe, BRI 1335-0417 was a giant.»
The question now is how this distinctive spiral structure was formed just 1.4 billion years after the Big Bang. Researchers considered several possible causes and suggested it could be due to an interaction with a small galaxy. BRI 1335-0417 is actively forming stars, and the researchers found that the gas in the outer part of the galaxy is gravitationally unstable, which favors star formation. This situation likely occurs when a large amount of gas is supplied from outside, possibly through collisions with smaller galaxies.
The fate of BRI 1335-0417 is also shrouded in mystery. Galaxies containing large amounts of dust and actively producing stars in the early universe are thought to be the ancestors of the giant elliptical galaxies in the universe today. In this case, BRI 1335-0417 would change its shape from a disk galaxy to an elliptical galaxy. Or, contrary to the conventional view, the galaxy would remain a spiral galaxy for a long time.