The most ancient spiral galaxy yet
Swirly and beautiful, spiral galaxies are what we often think of when someone mentions the word galaxy. Our own Milky Way is a spiral galaxy. These galaxies are pretty common in the nearby universe. But the farther back in time and distance astronomers look, the fewer spiral galaxies they see among the multitudes of galaxies in our universe. Instead, as we go out into space – and back in time – galaxies appear more irregular in shape. And thus how and when spiral galaxies formed is one of astronomy’s classic questions. And so it was with some excitement on May 20, 2021 that astronomers reported the most ancient spiral galaxy yet found.
This galaxy is labeled BRI 1335-0417. It existed only 1.4 billion years after the Big Bang, which equals a distance from us of 12.4 billion light-years. So far away – so far back in time – and yet this galaxy has clearly visible spiral arms! Clearly, this galaxy has an important contribution to make in answering questions about spiral galaxies’ origins.
What is a spiral galaxy?
Galaxies come in many different shapes and are classified by their morphology, meaning how they look. There are elliptical, spiral and strangely irregular galaxies, all with different features. Spiral galaxies consist of a central bulge of older stars, a flat rotating disk, and arms spiraling around the disk. Spiral galaxies exist primarily in the nearby universe. As you go out far in distance, back in time, the fewer spirals you see.
Takafumi Tsukui at the university SOKENDAI in Japan is the lead author of the new paper. He said in a statement:
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 any previous literature.
Observing the most ancient spiral galaxy
The astronomers used a radio telescope, the Atacama Large Millimetre Array or ALMA telescope, to study galaxy BRI 1335-0417. This observatory – located in the Atacama Desert of northern Chile – is able to reach a high level of resolution (detail), despite the enormous distance the the galaxy. Tsukui said:
The quality of the ALMA data was so good that I was able to see so much detail that I thought it was a nearby galaxy.
Due to both the galaxy’s distance, and the early age of the universe at that distance, galaxy BRI 1335-0417 contained a lot of dust that obscures the light from it. The dust makes the galaxy structure hard to see using visible-light telescopes like Hubble. But, at radio wavelengths, astronomers can observe specific elements within the galaxy. And so they can look past the obscuring dust.
In this case, the astronomers looked at the emission from carbon ions for information.
Using the carbon ions as a tool for tracing the galaxy’s structure, the astronomers could see the spiral shape of BRI 1335-0417. They could see this structure extends about 15,000 light-years from the center of the galaxy. This is about 1/3 of the size of the Milky Way, as a comparison. But BRI 1335-0417 is about as massive as our Milky Way galaxy, including its number of stars and amount of interstellar matter. Just because you don’t see it extend farther doesn’t mean it isn’t larger. Tsuki explained:
As BRI 1335-0417 is a very distant object, we might not be able to see the true edge of the galaxy in this observation. For a galaxy that existed in the early universe, BRI 1335-0417 was giant.
How did a spiral galaxy form so early?
Simulations show that interacting galaxies can form an end-product galaxy with spiral arms. Galaxies interacted much more in the early universe, and so might explain the presence of BRI 1335-0417 so far back in time. There are more clues to that scenario as well: BRI 1335-0417 has a large supply of gas in its outskirts, for example. That’s an indication that there’s some kind of supply delivery coming in from the outside, possibly because this galaxy has been colliding with other, smaller galaxies.
The video below is a simulation that shows how many small galaxies interact to form a larger spiral galaxy.
Video ©2007 T. Takeda, S. Nukatani, T. R. Saitoh, 4D2U Project, NAOJ.
What happened next?
What happened next is the interesting question. According to conventional theory, star-forming galaxies (like BRI 1335-0417) with lots of dust in the early universe would evolve into giant ellipticals as they age. But maybe that might not happen? Maybe a galaxy like BRI 1335-0417 would remain a spiral for a much longer time? Spirals arms are of special interest to us because, as Tsukui said:
Our solar system lodges in one of the Milky Way spiral arms. Tracing the roots of spiral structure will provide us with clues as to the environment in which the solar system was born. I hope that this research will further advance our understanding of the formation history of galaxies.
Bottom line: Astronomers were surprised to discover spiral arms in a galaxy located in the very early universe. This makes the galaxy, BRI 1335-0417, the most ancient spiral galaxy found so far and provides clues to how and when spiral galaxies formed.