The first twin of the Milky Way is discovered: the oldest known barred spiral contradicts common models of galaxy evolution

November 14, 2023, reading time: 4 minutes.

Contradiction to theory: Astronomers have discovered a surprisingly early twin of the Milky Way: a barred spiral galaxy that is more than 11.7 billion years old. The galaxy called ceers-2112 is not only the oldest known barred spiral, but it also existed much earlier than current models of galactic evolution predict. Furthermore, this early galaxy is almost exactly the shape and size that our Milky Way must have been, as the team reports in Nature.

Our Milky Way is one of the barred spirals: a straight structure with a greater density of stars passes through its center, to which the arms of the spiral are attached. This bar forms an important material transport route and promotes star formation in the inner region of the galaxy. However, it is still unclear why only some spiral galaxies have this bar and when it appears in galaxy evolution. According to current models, these barred spirals have only existed for a few billion years, because only then did galaxies have the necessary order and calm.

Our Milky Way is a barred galaxy: its spiral arms begin at the end of a straight structure of dense stars and gases. © NASA/JPL-Caltech/ESO/R. Hurt

“It was thought that such bars could not form in early galaxies or at least did not last long because they were still too chaotic and turbulent,” explains co-author Alexander de la Vega of the University of California, Riverside. But in March 2023, new images from the James Webb Space Telescope raised the first doubts about this. In them, astronomers discovered several barred spirals more than ten billion light years away.

The furthest bar spiral yet

Now there is another even older discovery: Luca Costantin’s team at the Centro Astrobiológico de Madrid has discovered another early barred spiral galaxy in near-infrared images taken by the James Webb Telescope. The galaxy ceers-2112 has a redshift greater than z=3 and therefore existed about two billion years after the Big Bang. However, more detailed analyzes of its structure and light spectrum suggest that this galaxy was already 620 million years old at that time.

Contrary to what was initially assumed, this galaxy already has a clearly pronounced bar despite its early formation, as astronomers discovered. It has a length of 10,700 light years and is distinguished from the rest of the galaxy by its greater stellar density and greater brightness. “Finding a bar in ceers-2112 is surprising because galaxies were much more chaotic in the early universe,” says de la Vega.

Density contours of the galaxy ceers-2112 (a) determined from the images and spectral analyzes (b) show a bar structure in the center of the galaxy. © Constantin et al./Nature, CC-by 4.0

One of the first twins in our home galaxy

But the primitive galaxy ceers-2112 is not only very similar to our Milky Way in its shape. Its other structure and its stellar mass of around 3.9 billion solar masses also make it one of the first twins of our galaxy: “If we compare ceers-2112 with the history of the development of the Milky Way, then we can consider it as the most distant and first predecessor of the Milky Way, both in terms of its structure and its growth,” say Constantin and his colleagues.

“The discovery of ceers-2112 demonstrates that galaxies in the early universe could also be as ordered as the Milky Way,” says de la Vega. As astronomers determined, the stellar disk of this early Milky Way twin could have formed between one billion and half a billion years after the Big Bang. The formation of the ceers-2112 beam began about 200 million years later and concluded in about 400 million years.

“It is necessary to review the theories”

According to astronomers, this discovery sheds new light on the evolution of galaxies in the early cosmos. “The bar in ceers-2112 suggests that galaxies matured and organized more quickly than we thought,” says de la Vega. Until now it was assumed that this process was not completed until after several billion years, approximately half of our universe. Only then did the strong gas flows that allowed rapid star formation in the early cosmos but also caused galactic turbulence subside.

“But the discovery of this galaxy shows that this can happen in a fraction of that time, in just a billion years or less,” says the researcher. “Therefore, our theories about the formation and evolution of galaxies must be revised at least in some aspects.” Therefore, the first turbulent phase of star formation could have occurred much faster than previously thought, at least in some galaxies. (Nature, 2023; doi: 10.1038/s41586-023-06636-x)

Source: University of California-Riverside

November 14, 2023 – Nadja Podbregar

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