REBELS-25: multi-phase morphology and kinematics at z = 7.31

In the earliest moments of the universe, the cosmos was a dark, formless place filled only with hydrogen and helium gas. Over hundreds of millions of years, gravity began to pull this gas together, igniting the first stars and forging the first galaxies. For a long time, astronomers believed that these early galaxies were chaotic, messy places. The prevailing theory suggested that in the first billion years of cosmic history, galaxies were too young to have settled into orderly shapes. They were thought to be turbulent, unstable collections of gas and stars, constantly colliding and churning, lacking the smooth, spinning discs that characterize mature galaxies like our own Milky Way today. This view held that it took billions of years for galaxies to cool down, organize their gas, and develop the elegant, rotating structures we see in the nearby universe.

However, the arrival of the James Webb Space Telescope and the Atacama Large Millimeter/submillimeter Array has begun to rewrite this timeline. These powerful instruments allow scientists to peer back to the epoch of reionization, a time when the universe was less than a billion years old, and see galaxies that appear surprisingly mature. The question now is not just whether these ancient galaxies exist, but how they managed to assemble such complex structures so quickly. Do they truly possess the calm, rotating discs of later galaxies, or is their apparent order an illusion created by the way we observe them? To answer this, researchers must look beyond the visible light of stars and examine the invisible dust and gas that often hide the true nature of these cosmic giants.

A team of astronomers has now turned their attention to a specific galaxy named REBELS-25, located at a redshift of 7.31. This distance means the light we see from it left the galaxy when the universe was only about 700 million years old. Previous studies had already hinted that REBELS-25 was a massive, star-forming system with a rotating structure, but those observations were limited. To get a complete picture, the researchers combined new, high-resolution images from the Atacama Large Millimeter/submillimeter Array with data from the James Webb Space Telescope. They looked at the galaxy across a wide spectrum of light, from the ultraviolet glow of young stars to the far-infrared heat radiating from dust and gas. By stitching these different views together, they created a detailed, three-dimensional map of the galaxy's shape, its internal conditions, and the motion of its gas.

The results revealed a striking contrast between what the galaxy looks like in different types of light. When viewed in ultraviolet and optical light, which traces the hot, young stars, REBELS-25 appears clumpy and irregular. The brightest spots are scattered across the galaxy, with some bright clumps located about two thousand light-years away from the center. It looks chaotic, much like the turbulent systems astronomers expected to find at this early time. However, when the researchers switched their view to far-infrared light, which traces the cold dust and gas, the picture changed completely. In this view, the galaxy is not a scattered mess but a smooth, rotating disc. The dust and gas are concentrated in a central region, forming a near-perfect exponential disc that is much more orderly than the scattered stars suggest.

This dramatic difference in appearance is caused by dust. The study found that the central regions of the galaxy are heavily shrouded in dust, which blocks the ultraviolet and optical light from the stars hidden inside. The researchers calculated that dust obscuration hides between 55 and 98 percent of the galaxy's total star formation. In the very center, where the dust is thickest, almost all the star formation is invisible to the optical telescopes. The bright clumps seen in the ultraviolet are likely just the tips of the iceberg, representing the less obscured outer edges of the galaxy where stars are visible. The true heart of the galaxy, where the most intense activity is happening, is hidden behind a thick veil of dust, visible only to the infrared eyes of the observatories.

Despite the chaotic appearance of the stars, the gas within REBELS-25 tells a story of remarkable stability. The researchers analyzed the motion of two different types of gas: the warm, ionized gas that glows in a specific infrared line, and the colder, neutral gas that emits a different infrared signal. They found that both types of gas are moving in the same large-scale, rotating pattern. The gas is not tumbling randomly; it is spinning in a coherent disc. The ratio of the ordered spinning motion to the random, chaotic motion is very high, indicating that the gas is "dynamically cold." This means the galaxy has already settled into a stable, rotating structure, defying the expectation that galaxies at this age should be turbulent and disordered.

The study also looked at the chemical makeup of the galaxy to see if the different clumps were separate systems crashing into each other, or parts of a single, unified galaxy. They measured the ratios of different chemical elements in the gas across the various clumps. The results showed that the chemical conditions are broadly similar everywhere in the galaxy. The metal content, which indicates how many heavy elements the galaxy has produced, is high and consistent across the different regions. This uniformity suggests that the bright clumps seen in the ultraviolet are not separate, merging galaxies with different histories. Instead, they are likely star-forming regions embedded within the same single, chemically enriched disc. The galaxy appears to be a unified system that has already processed a significant amount of its gas into heavier elements, reaching a level of chemical maturity comparable to galaxies found much later in the universe's history.

While the galaxy is largely a smooth, rotating disc, the data does show some signs of complexity. There are hints of non-circular motions and small-scale structures that suggest the galaxy is not perfectly smooth. There may be a bar-like structure in the center, and the gas shows some signs of inflows or outflows. These features indicate that while the galaxy has achieved a stable state, it is still actively assembling and evolving. It is not a static object but a dynamic system that is continuing to build itself up, even as it maintains its overall order.

The findings for REBELS-25 challenge the long-held view that the early universe was a place of pure chaos. This galaxy, existing less than a billion years after the Big Bang, is massive, chemically rich, and dynamically cold. It possesses a rotating disc structure that is surprisingly similar to galaxies found billions of years later. The study demonstrates that dust plays a crucial role in hiding the true nature of these early systems. Without looking through the dust with infrared telescopes, astronomers would have seen only a messy, clumpy collection of stars and missed the elegant, spinning disc hidden beneath.

This research highlights the importance of looking at galaxies across multiple wavelengths to understand their true nature. A single view can be misleading, hiding the underlying order behind a veil of dust or obscuring the chaotic details of star formation. By combining the power of different telescopes, scientists are beginning to see that the universe may have organized itself much faster than previously thought. REBELS-25 stands as a testament to this rapid evolution, showing that even in the earliest epochs of cosmic history, some galaxies had already found their rhythm, spinning calmly in the dark.