Stormy teenage years of our Milky Way

Our home galaxy appears relatively stable and peaceful today. However, it has a rich history behind it – and astronomers have shed more light on part of it for the first time. Their spectral analyzes of some 250,000 stars allowed them to date the events, revealing that the oldest, fattest part of the galactic disk of stars was formed as early as 13 billion years ago – just 800 million years after the Big Bang. However, the greatest impetus in star formation came about two billion years later when our galaxy collided with a smaller neighboring galaxy and merged. The galactic halo also formed during this time.

Our Milky Way is made up of billions of stars of all ages, interstellar gases, and loads of dark matter. All these ingredients were accumulated in the course of turbulent development. Initially, the ancestral gas-rich galaxies merged into an initially even smaller and less structured galaxy. During this time, the so-called thick disk of our Milky Way galaxy was formed, the main part of the star’s disk, which is about 100,000 light years wide and 6,000 light years thick. Later, by merging with other, smaller neighboring galaxies, a halo was added, which lies like a coating around the star’s disk, and the so-called a thin disk that is only about 2,000 light years thick. However, when these particular phases took place, and what caused them, it has only been clarified to a limited extent.

Star Podgiants as Dating Helpers

Maosheng Xiang and Hans-Walter Rix of the Max Planck Institute of Astronomy now provide a more detailed insight into the early years of our Milky Way, 13 to 8 billion years ago. For the first time, they have been able to more accurately date important milestones in the history of galaxies. This was made possible by assessing data from two large sky surveys, the European Gaia mission and the LAMOST survey, spectral analysis of approximately nine million stars using the Large Sky Area Multi-Object Fiber Spectroscopic Telescope in China. The combination of the two datasets has provided astronomers with information about the position, motion, temperature and chemical composition of stars in different regions of the Milky Way.

Using these data, scientists were able to identify the type of star that is crucial for dating the so-called sub-giants. These stars have already used up most of the hydrogen in their cores, so nuclear fusion slows down there and the core shrinks. At the same time, nuclear fusion begins in the shell surrounding the star’s core – and with it, the transition to the giant star. During this transitional phase, which only lasts a few million years, the age of these sub-giants can be inferred directly from their surface temperature and brightness. “This makes sub-giants a valuable dating tool for galactic archeology,” explain Xiang and Rix. The downside, however, is that sub-giants are very rare. Therefore, both astronomers had to analyze data from millions of stars to track down about 250,000 of these sub-giants in our galaxy and use them to reconstruct the teenage years of the Milky Way.

Building structures and accelerating star formation

The estimates showed that the oldest members of the sub-giants lie in the thick disk of the Milky Way and are up to 13 billion years old. Accordingly, this part of our galaxy did not form until about 800 million years after the Big Bang. Based on the large total number of stars formed, astronomers conclude that the thick disk contained large amounts of gas from the outset, and thus the raw material for the new stars. This would also explain their relatively large thickness. A little later, the galactic halo began to form. ‘The oldest stars in the thick disk are on average one to two billion years older than the main population of halo stars,’ the researchers say. The formation of this outer shell of gas and stars was completed about eleven billion years ago.

Around this time, the rest of the Milky Way also changed dramatically. Since about eleven billion years ago the data showed the conspicuous “maximum production” in star formation, at the same time, the orbits of many stars suddenly changed. Xiang and Rix attribute this to the fact that the Milky Way collided with the slightly smaller neighboring galaxy Gaia-Enceladus / Sausage at that time. “The obvious interpretation of this temporal coincidence is that the perturbations caused by the Gaia-Enceladus / Kiełbasa galaxy strongly stimulated star formation in the thick disk,” astronomers write. The shockwaves from the collision caused more clouds of gas to collapse and new stars to form. This spurt in star formation lasted about five to six billion years, but gradually faded away over that time.

Quiet late phase

Then, eight billion years ago, the tumultuous and productive “teenage years” of the Milky Way were over. Most of the interstellar hydrogen gas in the thick disk has been used up, leaving a few new stars there. However, as some fresh gas was still pouring in from intergalactic space, star formation in part of the galactic disk remained active longer – a thin disk formed. As this structure developed, a long, quiet adult phase of our parent galaxy began. Since the Milky Way hasn’t had any major collisions since then, its structure has remained largely unchanged to this day. The development of the Milky Way therefore corresponds to what the models also predict for galaxies: a productive early phase followed by a calm, less disturbed late period.

Source: Maosheng Xiang and Hans-Walter Rix (Max Plank Institute for Astronomy, Heidelberg), Nature, doi: 10.1038 / s41586-022-04496-5

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