The wild years of our Milky Way galaxy
Astronomers reconstruct the turbulent past life of our home galaxy
A very long ago, our Milky Way had a truly eventful life: between about 13 and 8 billion years ago, it lived hard and fast, merging with other galaxies and consuming a lot of hydrogen to form stars. With the help of a new data set, Maosheng Xiang and Hans-Walter Rix from the Max Planck Institute for Astronomy in Heidelberg have reconstructed the turbulent teenage years of our home galaxy. To do this, the researchers had to precisely determine the ages of 250,000 Milky Way stars.
Understanding the formation history and evolution of our home galaxy is a major goal for astronomy and astrophysics, and one where a flood of high-quality “big data” over the past years has led to impressive progress. The new study by Xiang and Rix constitutes a big step forward by putting much more precise dates onto the different phases of early Milky Way history. This was made possible by a unique analysis that managed to determine the ages of 250,000 stars.
In our current understanding, our home galaxy went through several phases. During the “baby phase” (not an official astronomy term), small, gas-rich progenitor galaxies merged to form a conglomerate that subsequently grew into our Milky Way. As those galaxies did not collide head-on, they imparted a spin on the resulting structure, presumably flattening its out into what we now see as the so-called thick disk of our Milky Way: gas and stars in a flat pancake, 100,000 light-years in diameter and 6000 light-years thick.
There were also a number of additional mergers with galaxies that were somewhat smaller than the Milky Way. They created the so-called stellar halo surrounding the Milky Way disk, and generally shook things up. The “adult years” that followed were much more quiet, and involved steady star formation activity in the so-called thin disk, which is younger and only about 2000 light-years thick.
The new result by Xiang and Rix now fleshes out in more detail than ever before, the history of the Milky Way’s productive teenage years, from about 13 to about 8 billion years ago. Key to that reconstruction was that the astronomers managed to precisely determine the ages of roughly 250,000 individual stars. In astronomy, that is anything but an easy task. But there is one type of star, so-called “sub-giants,” where one can directly tell the age by looking at a star's surface temperature and brightness. The drawback is that sub-giants are rare – only a few per cent of the stars in our Milky Way are in that brief stage of their life at any given time.
Fortunately, recent comprehensive surveys provide high-quality data for an impressive number of stars – enough to include numerous examples even of the rarer types of star: The Early Data Release 3 of ESA’s Gaia mission, published in December 2020, provides position data and distances for nearly 1.5 billion stars, and the 7th data release of the LAMOST survey, published in 2021, more than 9 million stellar spectra that contain information about the stars’ temperature and chemical composition. Combining the information from these surveys, Xiang and Rix were able to construct their extensive data set of stars with known ages.
The resulting picture is remarkably clear. Sorting the stars by age and chemical composition, the astronomers were able to read at least the outline of teenage galaxy history like an open book – and with the age information, they could tell when the different stages had happened.
During the early times, all the action was in what we now call the stellar halo and the thick disk, which formed from an initial inflow of gas. What Xiang and Rix found was that about 11 billion years ago, exceptionally many new stars formed in our galaxy in a short period of time. That peak very likely is a consequence of one merger in particular: the so-called Gaia Enceladus/Sausage, a satellite galaxy whose merger-disrupted remnants were discovered and named by two competing groups using Gaia data in 2018.
In their data, the Xiang and Rix could see that a prominent star formation peak in the thick disk 11 billion years ago coincided with the sudden appearance of numerous stars whose orbits had suddenly and drastically changed. The latter is an obvious consequence of the gravitational disruption by the merger, suggesting that the star formation peak in the Milky Way was not only contemporaneous with the Gaia Enceladus/Sausage merger, but plausibly a consequence of it: Shockwaves from the collision of the gas masses of the merging galaxy with the gas in our own galaxy may have triggered gas cloud collapse and star formation.
After the turbulent merger-dominated era had ended, the thick disk continued to form stars in an unusually productive way. The total amount of stars formed suggests that right from the beginning, the thick disk contained impressive amounts of gas. That would also explain its thickness: The gas did not need to settle vertically into a thin disk to create conditions that were right for the formation of lots of stars. With that much suitable gas around, making new stars was apparently easy.
As new stars form, massive stars in particular produce lots of elements heavier than hydrogen and helium – what astronomers, somewhat confusingly, call “metals”. The heavier elements tend to collect near the central regions of the galaxy. Hence, stars newly forming in those regions will contain more metals than stars formed in the outskirts.
But the sample collected by Xiang and Rix shows something different: From the earliest possible times visible in the data – 13 billion years ago, a mere 800 million years after the Big Bang! – until the change in pace 8 billion years ago, all stars forming at a specific time appear to have the same metal content. The metal content itself changes over time: the older a star, the less metal it contains.
The easiest explanation is that, during all that time, there was a thorough mixing of gas throughout the thick disk. This is a key result of the new study. That way, all the stars born at the same time would inherit the same chemical composition, with the proportion of heavy elements increasing with time, as the gas gradually got more and more polluted – astronomers call it enriched – with the products of nuclear fusion processes of earlier generations of stars.
About 8 billion years ago, the new data shows, the productive teenage years came to an end. Presumably, this was because the thick disk had exhausted much of its initial supply of hydrogen gas. Evidently though, there was still a steady inflow of moderate amounts of fresh hydrogen gas from intergalactic space. With star formation activity in the thick disk all but ended, that gas could slowly settle into a disk of its own. But since there was not that much gas coming in, this disk needed to contract much further, to a thickness of only about 2000 light-years, to achieve the right conditions for (moderate) star formation.
The result was what we now call our galaxy's extended thin disk. The long, almost boring adulthood of our home galaxy had begun. Another serious collision and merger with a smaller galaxy might have livened things up a little, but for our galaxy, that did not happen – a rather unusual fate, compared with other galaxies. What is anything but unusual, though, is the general trend: A productive earlier phase followed by a quiet life appears to be the norm for galaxy evolution, going by current computer simulations.
That is the newly refined reconstructed version of our galaxy’s history. And what might sound about par for the course for an account of human history – major events and their dates – is rather unusual for astronomy. It is very difficult to put reliable dates to events in our home galaxy’s cosmic history. That the new study managed to do so, and was thus able to construct a detailed timeline of our galaxy's teenage years, is major progress.
MP / HOR