Astronomers Identify The Ancient Heart of The Milky Way Galaxy: From Gaia DR3 data, a map of especially metal-poor giant stars shows the “poor old heart” of the Milky Way galaxy as a concentrated area (marked with a circle). The map shows the whole night sky, just like some world maps show the surface of the Earth. In the middle of the map is the direction to our home galaxy’s center. Photo by H.-W. Rix for MPIA
A group of MPIA astronomers has found the “poor old heart of the Milky Way,” made up of stars that have been around since the beginning of our home galaxy and live in its core.
For this “galactic archaeology” feat, the researchers looked at the most recent data from the ESA’s Gaia Mission. They used a neural network to figure out the metal content of two million bright giant stars in the inner part of our galaxy. The discovery of these stars and what we know about them gives cosmological simulations of the early history of our home galaxy a welcome boost.
Our galaxy, the Milky Way, has been around for almost all of the 13 billion years that the universe has been around. Over the past few decades, astronomers have been able to piece together different times in the galaxy’s history. This is similar to how archaeologists piece together the history of a city since some buildings have clear dates of when they were built.
For others, using more basic building materials or older building styles is a sign that they were there before, as is finding ruins underneath other, newer buildings. Last but not least, spatial patterns are essential. In many cities, there is an old town in the middle surrounded by clearly more unique areas.
Cosmic archaeology works the same way for galaxies and especially for our galaxies. The stars make up the main parts of the universe. Astronomers can figure out the exact age of a small number of stars. For example, this is true for so-called “sub-giants,” a short stage in a star’s life when its brightness and temperature can be used to figure out how old it is.
Estimating Age From Chemistry
More generally, there is a “building style” that can be used to estimate the age of almost all stars. This is a star’s “metallicity,” the number of chemical elements heavier than helium in its atmosphere. These elements, which astronomers call “metals,” are made inside stars through nuclear fusion and are released near or at the end of a star’s life. Some are released when a low-mass star’s atmosphere breaks up, while the heavier elements are released more violently when a high-mass star explodes as a supernova. In this way, each generation of stars “seeds” the gas between the stars, then used to make the next generation of stars. In general, each generation will have a higher metallicity than before.
As for larger structures, space distribution is essential, just like in a city. But because a galaxy is less stable than a city—buildings don’t move around, but stars do—movement patterns can also tell us important things. Stars in the Milky Way might only be in the center or moving in a steady routine in the thin or thick disc. Or, they could be part of the jumbled orbits of the many stars in our galaxy’s large halo. Some of these stars have strange orbits that take them back and forth between the inner and outer regions.
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How Large Galaxies Grow Over Time?
The history of galaxies is shaped by mergers and collisions and by the huge amounts of fresh hydrogen gas that flow into galaxies over billions of years. Hydrogen is the raw material that galaxies use to make new stars. The history of a universe begins with smaller proto-galaxies, which are places where there is too much matter right after the Big Bang. This is where gas clouds collapse to form stars.
So, when proto-galaxies crash into each other and join together, they make more giant galaxies. If you add another protogalaxy to these slightly bigger ones, one that moves in from the side (“large orbital angular momentum”), you might get a disc of stars. When two large galaxies merge (called a “major merger”), their gas reservoirs heat up. This makes an elliptical galaxy with few new stars and a complicated pattern of orbits for the old stars that are already there.
To put together this kind of history, you need to combine observations with simulations that are more and more accurate. The big picture of how galaxies form and change has been known for a long time, but the details have only been known for a short time, thanks mainly to surveys that have led to better and more complete data.
The Milky Way, where we live, is an important part of this. By definition, this is the galaxy whose stars we can study best and in the most detail. Galactic archaeology is the study of the history of our galaxy. It helps us piece together parts of our history and learn more about how galaxies change over time (“local cosmology“).
What came before the Milky Way’s exciting teenage years?
This particular episode of galactic archaeology started with a reconstruction published in the Spring of 2022. MPIA researchers Maosheng Xiang and Hans-Walter Rix used data from the ESA’s Gaia satellite and the LAMOST spectral survey to figure out the ages of stars in a sample of 250,000 so-called sub-giants. This was the first time the generations of this many stars had been found. Astronomers could figure out what happened to the Milky Way during its exciting teen years 11 billion years ago and its more settled (or boring) adult years that followed.
(When people were in their teens, the Milky Way and another galaxy called Gaia Enceladus/Sausage, whose remains were found in 2018, merged for the last time in a big way. It started at a time when a lot of stars were being made, which made the disc of stars we see today. Adulthood was marked by a moderate hydrogen gas flow, which settled into our galaxy’s extended, thin disc. Over billions of years, new stars formed slowly but steadily.
Back then, astronomers noticed that the oldest stars in their sample of young stars already had a lot of metal, about 10% as much as our sun. Before those stars were made, there must have been even earlier generations of stars that put metals into the space between the stars.
What simulations tell us about the Milky Way’s ancient core?
Simulations of the universe’s history showed that these earlier generations should have existed. Also, these simulations were able to figure out where people from those earlier generations might still be living. In these simulations, the Milky Way was first made up of three or four proto-galaxies that formed close to each other and then merged. The stars of these galaxies then settled into a relatively small core, no bigger than a few thousand light-years across.
Later, the different disc structures and the halo would be made when smaller galaxies were added. But the simulations showed that part of the original core could be expected to survive these last changes without too much damage. Even now, billions of years later, we should still be able to find stars from the Milky Way’s ancient heart, the compact core, in or near the center of our galaxy.
In search of ancient core stars
At this point, Rix became interested in finding ways to find stars from the very center of our galaxy. But he knew that if he wanted to see more than a few dozen of these stars, he would have to change how he looked at the sky. The LAMOST telescope, which was used in the last study, can’t look at the centre of the Milky Way because it is on Earth and can’t see during the summer monsoon months. And sub-giants, which used to be the best choice for a probe, are too dim to be seen from more than 7,000 light-years away. This makes it impossible to get to the centre of our galaxy.
Remember that, in addition to the few stars where we can figure out their exact ages, there is a much more general way to tell how old a star is: the “different building styles” that show how old a star is. The Data Release 3 (DR3) of the ESA’s Gaia mission came out in June 2022, which was a good thing. Since 2014, Gaia has been taking very accurate measurements of the position, motion, and distances of more than a billion stars. This has changed many parts of galactic astronomy, among others. DR3 was the first time that Gaia’s actual spectra for 220 million astronomical objects were included in a data release.
Red Giants From Gaia
Astronomers use spectra to learn about the chemical makeup of a star’s atmosphere, such as its metallicity. Gaia’s spectra are high quality, and there are more than anyone else’s. However, the spectral resolution, which is how finely an object’s light is split by wavelength into the primary rainbow colors, is designed to be low. More analysis would be needed to get reliable metallicity values from the Gaia data, which is what Hans-Walter Rix, a Gaia researcher at MPIA, and René Andrae, a Gaia researcher at MPIA, did in a project with their Harvard University summer student Vedant Chandra.
Since the three astronomers knew their analysis had to go to the centre of the Milky Way, they looked at red giant stars in the Gaia sample. Red giants are usually about a hundred times brighter than sub-giants and are easy to see even in the farthest parts of our galaxy’s core. The spectral features that show how metallic these stars are also pretty obvious, making them a good choice for the kind of analysis the astronomers were planning.
Extracting metallicities with machine learning
Astronomers used machine learning methods to do the actual analysis. By now, many people will have seen how this new technique has been used. For example, software like DALL-E can make good images from simple text descriptions, and ChatGPT can more or less answer questions and write when asked. The most important thing about machine learning is that the solutions are not programmed in advance. Instead, the algorithm is built around a “neural network,” which is similar to how neurons are arranged in our brains. Then, that neural network is trained. The web is given sets of tasks and their answers, and the connections between input and output are changed so that, at least for the training set, when certain information is given, the correct result is given.
In this case, selected Gaia spectra were used to train the neural network. These were Gaia spectra for which the correct answer, the metallicity, was already known from another survey (APOGEE, high-resolution spectral observations as part of the Sloan Digital Sky Survey [SDSS]). The network’s internal structure changed so that, at least for the training set, it could make the correct metallicities.
Reliable metallicities for 2 million bright giants
A general problem with using machine learning in science is that the neural network is, by definition, a “black box,” meaning that its internal structure has been formed by the training process and is not directly under the scientists’ control. So, Andrae, Chandra, and Rix only initially used half of the APOGEE data to train their neural network. The algorithm was put to the test with the rest of the APOGEE data in a second step. The results were amazing: the neural network could figure out the exact metal content of stars it had never seen before.
Now that the researchers had trained their neural network and made sure it could get accurate results for spectra it hadn’t seen before, they used the algorithm on their complete Gaia spectra from their red giant data set. Once the results were in, the researchers had a sample of accurate metallicities from 2 million bright giants in the inner galaxy. This was the most significant sample of its kind ever made.
Mapping the ancient heart of the Milky Way
With that sample, it was easy to find the old centre of the Milky Way galaxy. Rix calls this group of stars the “poor old heart” because they don’t have much metal in them and are in the middle of the galaxy. On a sky map, these stars look like they are all near the galaxy’s center. The parallax method, which Gaia uses to measure distances, makes it easy for a 3D reconstruction to show that those stars are all in a small area around the centre, about 30,000 light-years across.
The stars in question fit in well with what Xiang and Rix knew about the Milky Way when it was young. They have just the right amount of metal to have given birth to the metal-poorest stars that formed the thick disc of the Milky Way. Since that earlier study provided a timeline for how thick discs included, the Milky Way’s ancient heart is older than 12.5 billion years.
Corroboration from chemistry
For the small number of objects for which APOGEE spectra are available, it is possible to go one step further: These spectra give us more information about the poor-old-heart stars in this subset, like how much oxygen, silicon, and neon they have. In a process called “alpha enhancement,” alpha particles (nuclei of helium-4) are added to existing nuclei one at a time to make these elements. Their presence in such large amounts shows that the early stars got their metals from a place where supernova explosions of massive stars made heavier elements in a relatively short amount of time.
This fits much better with the idea that these stars formed right after the first few proto-galaxies came together to form the Milky Way’s initial core, rather than being present in the dwarf galaxies that came together to form the Milky Way’s initial core or that joined the Milky Way later. It’s more evidence that what cosmological simulations say about the early history of our home galaxy is true.
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A path to finding the Milky Way’s progenitor galaxies?
Even though the information from Gaia’s global view is groundbreaking because it shows that our Milky Way’s “poor old heart” still exists, this discovery makes astronomers want to learn more right away: Can more detailed spectra be made for many or even all of these stars, so that their chemical makeup can be studied in more depth? Will they all show alpha enhancement, which would make sense since they all formed in the centre of the Milky Way? Follow-up spectra were taken as part of the recently started SDSS-V survey or the upcoming 4MOST survey, in which MPIA is a partner. They promise to give the group the necessary information to answer these important questions.
If everything goes well, the researchers might even be able to figure out which stars in the core region belong to which of the galaxies that came before the Milky Way: For an older star, like those in the poor old heart, the extra information about its chemical makeup and temperature lets us make a good guess about how bright it is. You can figure out how far away a star is by its brightness in the sky. The farther away a star is, the dimmer it will look to us. Gaia’s parallax measurements are much less accurate when figuring out how far away these stars are. This method is much more accurate.
The position of a star in the sky and how far away it is tells us where it is in the Milky Way in three dimensions. Using the Doppler shift of their spectral lines to measure how fast the stars are moving toward or away from us and their apparent motions in the sky, we can figure out where the stars are moving in our galaxy. If this kind of analysis shows that the stars in the old Milky Way’s heart belong to two or three groups, each of which moves in another way, these groups are likely to be the two or three progenitor galaxies that merged to make the Milky Way.
Hans-Walter Rix et al. “The Poor Old Heart of the Milky Way” has been published in the Astrophysical Journal. It talks about the results that have been found.
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