A Milky Way analogue in the early cosmos

Galaxy SPT0418-47

The light of the galaxy SPT0418-47 is distorted by the gravitational lens effect and appears as a nearly perfect ring of light. (Image: ALMA (ESO / NAOJ / NRAO), Rizzo et al.)

Our home galaxy has some typical features: it has a central thickening, it rotates and it has spiral arms. Now, for the first time, astronomers have discovered a galaxy in the early cosmos that already has two of these three characteristics – rotation and thickening. The galaxy is more than twelve billion light years away and dates from just 1.4 billion years after the Big Bang. It is the most distant image of the Milky Way – and causes a surprise. Because, according to current models, such early galaxies should be far more chaotic and turbulent.

When the cosmos was still young and the first galaxies were only gradually forming and growing, many astrophysical processes were far more dynamic and intense than they are today. Many of the first stars were extremely massive and therefore exploded again as a supernova after a few million years. In the young galaxies, an abundant supply of interstellar gases ensured that star formation took place at a significantly higher rate than in most of today’s star clusters. In addition, there were frequent collisions and mergers of galaxies, through which they grew. All of this, however, repeatedly led to strong turbulence in the galactic environment, which is why current models assume a poorly ordered and rather chaotic structure of early galaxies. It was only when the events calmed down with advancing age of the universe that the galaxies were able to form an ordered rotation, a central bulge and spiral arms – according to the theory.

From a ring of light to a galaxy image

But now astronomers working with Francesca Rizzo from the Max Planck Institute for Astrophysics in Garching have discovered an early galaxy that does not fit into the picture. The galaxy SPT0418-47 is around twelve billion light years away from us and therefore existed around 1.4 billion years after the Big Bang. Its light shows it as it looked back then. Because of this great distance, even the most powerful telescopes can barely resolve the features of such early galaxies. But a lucky coincidence came to the rescue of Rizzo and her team: A closer galaxy pushed itself between the earth and the distant galaxy SPT0418-47 and acted as a gravitational lens: The gravity of the foreground galaxy distorted and intensified the light from SPT0418-47 and served so, as it were, as a cosmic magnifying glass. With the help of the Atacama Large Millimeter / Submillimeter Array (ALMA), the researchers were able to determine some of the key features of the distant galaxy.

To do this, however, they first had to correct the image of SPT0418-47, which was strongly distorted by the gravitational lens. Because in the raw images, the distant galaxy does not appear as a cluster of stars, but as an almost perfectly circular ring of light around the foreground “lens”. The researchers used a specially developed computational modeling technique to reconstruct the true appearance of the distant galaxy. “When I first saw the reconstructed picture of SPT0418-47, I couldn’t believe it: a treasure chest opened,” reports Rizzo. As the images revealed, SPT0418-47 seems to have at least two features that are typical of our Milky Way: On the one hand, it already has a bulge, the large collection of stars that are tightly packed around the galactic center. This is the first time a bulge has been observed this early in the history of the universe, according to astronomers. On the other hand, however, their rotation corresponds to the distribution of the movement speeds across the disk of today’s spiral galaxies.

Contradicting current theory

Although this galaxy comes from the still young universe, SPT0418-47 is surprisingly similar to our Milky Way and other galaxies in today’s local cosmos in these key features. Overall, this early cluster of stars is far more ordered than expected based on current models for this period. It thus contradicts the theory that all galaxies in the early universe were turbulent and unstable. “What we found was quite puzzling: Although stars form at high speed and therefore highly energetic processes take place, SPT0418-47 is the most highly ordered galaxy disk that was ever observed in the early universe, explains Rizzo’s colleague Simona Vegetti. “This result is quite unexpected and has important implications for how we believe galaxies evolve.”

The discovery of SPT0418-47 and its surprising features could suggest that the current model may not be complete. Because apparently the early universe was not as chaotic as previously assumed. “This result represents a breakthrough in the field of galaxy formation and shows that the structures that we observe in nearby spiral galaxies and in our Milky Way existed twelve billion years ago,” says Rizzo. With this, however, SPT0418-47 also raises the question of how such a well-ordered galaxy could have formed so shortly after the Big Bang. The astronomers are hoping for an answer from future observations with new telescopes, some of which are still under construction. They could also help to find out how common such unusually ordered clusters of stars were in the young cosmos.

Source: Francesca Rizzo (Max Planck Institute for Astrophysics, Garching) et al., Nature, doi: 10.1038 / s41586-020-2572-6

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