A two-stage settling of the Milky Way disk revealed by precise ChronoGal ages

Our galaxy, the Milky Way, is not a static island of stars but a living, breathing structure that has grown and changed over billions of years. At its heart lies a vast, flat disk where our Sun and most of the stars we see at night reside. Astronomers have long known that this disk is not uniform; it is composed of two distinct layers. There is a younger, thinner layer where stars like our Sun orbit in a relatively calm, flat plane, and an older, thicker layer where stars move in more erratic, up-and-down paths. For decades, scientists have debated how these two layers formed. Did the thin disk form first and then get heated up and puffed out over time by gravitational tugs from passing clouds and spiral arms? Or did the galaxy start as a thick, turbulent cloud that gradually cooled and settled down into the thin disk we see today? Answering this question requires knowing the precise ages of stars and how far they wander from the galactic center, but until now, the data has been too fuzzy to tell the difference between these two very different stories of cosmic history.

A team of researchers has now solved this puzzle by looking at the Milky Way with unprecedented clarity. Using data from the European Space Agency's Gaia mission, which has mapped the positions and motions of billions of stars, the team developed a new method to determine the ages of stars with extreme precision. Instead of guessing the age of a single star, they analyzed the entire population of stars in the solar neighborhood, grouping them into clusters based on their age and chemical composition. By measuring how far these groups of stars spread out vertically from the galactic plane, they could trace the history of the disk's thickness. The results reveal a dramatic shift in how the galaxy evolved, showing that the Milky Way did not simply heat up over time. Instead, it underwent a two-stage birth process: a chaotic, rapid settling phase followed by a long, quiet period of gradual thinning.

The researchers focused on a specific region of the galaxy near our Sun, dividing the stars into two groups based on how they move. One group consists of stars that orbit in a calm, cold disk, while the other group contains stars that move in a hotter, more chaotic fashion. For each group, they calculated the average age of the stars and measured the "scale height," which is a way of describing how thick the layer of stars is. If the stars are tightly packed near the center, the layer is thin; if they are spread far above and below the center, the layer is thick. By plotting the thickness of these layers against the age of the stars, the team discovered a clear pattern that changes abruptly at a specific point in time.

The data shows that for the youngest stars, the disk has been getting thinner very slowly over the last several billion years. This gradual thinning is consistent with the idea that the disk has been slowly cooling and settling down. However, the story changes completely for the oldest stars. The researchers found that more than 10.85 billion years ago, the rate at which the disk was thinning was drastically faster. In a span of just two billion years, the thickness of the disk dropped from over 1.26 kiloparsecs to about 0.5 kiloparsecs. This rapid collapse suggests that the early galaxy was not a calm place that slowly cooled down. Instead, it was a turbulent environment where the gas was thick and chaotic, and stars were born with wild, high-energy orbits.

This finding strongly supports a model where the galaxy formed "upside down." In this scenario, the early Milky Way was a thick, turbulent disk of gas, likely stirred up by frequent collisions with other galaxies and intense bursts of star formation. As this gas cooled over time, it settled down into a flatter plane, and the stars born from this cooling gas ended up in a thinner, more orderly disk. The rapid drop in thickness observed in the oldest stars indicates that this settling happened very quickly, within a few billion years of the galaxy's birth. Once this chaotic phase ended, the galaxy entered a quieter era where the disk continued to thin, but at a much slower, more gradual pace, eventually forming the thin disk we see today.

The study also clarifies the relationship between the thick and thin disks. The researchers found that the transition between the two phases happened around 10.85 billion years ago. Before this time, the stars formed a thick, hot component that settled rapidly. After this time, the galaxy transitioned to a state where new stars formed in a much colder, thinner disk. This means that the thick disk is not just an older version of the thin disk that has been puffed up by time; it is a distinct component with its own unique history of formation. The rapid settling of the early disk suggests that the conditions in the early universe were fundamentally different from what they are today, with a much more violent and energetic environment that shaped the structure of our galaxy in its first few billion years.

By combining precise age measurements with detailed maps of stellar positions, the team has provided the first clear observational evidence for this two-stage history. The results rule out the idea that the thick disk was formed solely by the slow heating of a thin disk over billions of years. Instead, the data points to a dynamic early history where the galaxy assembled itself from a turbulent, thick cloud that quickly settled into a stable, flat structure. This new understanding helps astronomers piece together the complex assembly history of the Milky Way, revealing that our galaxy's past was marked by a dramatic and rapid transformation from chaos to order. The findings align with computer simulations of galaxy formation, which also predict that galaxies go through an early, turbulent phase before settling into the calm, rotating disks we observe in the nearby universe.

The implications of this discovery extend beyond just the Milky Way. It suggests that the formation of disk galaxies in the universe may follow a similar pattern, with an early period of rapid settling followed by a long period of gradual evolution. The ability to measure the ages of stars with such precision opens up new possibilities for studying the history of our galaxy and others. It allows astronomers to test theories of galaxy formation with real data, moving beyond speculation to a concrete understanding of how the structures we see today came to be. The work highlights the power of modern astronomical surveys to reveal the hidden history of the cosmos, turning the faint light of distant stars into a detailed timeline of galactic evolution.

In the end, the story of the Milky Way is one of transformation. From a thick, turbulent cloud of gas and stars, the galaxy settled down into the flat, graceful disk that surrounds us today. The rapid thinning of the early disk, captured in the ages and positions of the oldest stars, tells us that this transformation happened quickly and decisively. The galaxy did not just slowly cool down; it underwent a fundamental change in its structure, driven by the cooling of gas and the settling of stars into a stable orbit. This two-stage process, with its distinct phases of rapid and gradual thinning, provides a clear picture of how our galaxy grew up, offering a new perspective on the dynamic history of the universe.