The Milky Way is older than astronomers thought, or part of it. A newly published study shows that part of the disk is two billion years older than we thought.
The region, called the thick disk, only began to form 0.8 billion years after the Big Bang.
A few astronomers have summarized the history of the Milky Way in more detail than ever before. Their results are based on detailed data from ESA’s Gaia mission and China’s Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). The key to this discovery lies in sub-giant stars.
The newspaper is “A Time-Resolved Picture of Our Milky Way’s Early Formation History,” and it’s online in the journal Earth. The authors are Maosheng Xiang and Hans-Walter Rix, both of the Max Planck Institute of Astronomy (MPIA.)
One of the most difficult things to determine from a star is its age. A star’s composition, or metallicity, is the key to determining its age. The more accurately astronomers can measure metallicity, the more accurately they can determine its age.
The early universe contained almost exclusively hydrogen and helium. Elements heavier than hydrogen and helium are produced in stars and spread to the universe when those stars die and explode. Astronomers call each element heavier than the two primordial elements “metals”.
Milky Way at the edge. (Stefan Payne-Wardenaar / MPIA)
Stars with lower metallicity are older because they formed when mostly only hydrogen and helium were available. Thus, when astronomers identify a population of stars that contain mainly hydrogen and helium, they know that those stars are older. When they find a population of stars with higher ratio metals, they know those stars must be younger.
Precision age measurements are the holy grail in some aspects of astronomy, which is true in this case. Xiang and Rix used more than just metallicity to determine stars’ ages. They focused on a specific type of star: subgiants.
The subgiant phase in a star’s life is relatively short, so astronomers can determine a star’s age most accurately when it’s a subgiant. Sub-giants switch to red giants and no longer produce energy in their nucleus. Instead, fusion in a shell moved around the nucleus.
In this study, the few scientists used LAMOST data to determine the metal content of about 250,000 stars in different parts of the Milky Way. They also used Gaia data to give the exact position and brightness data for about 1.5 billion stars.
The ESA’s Gaia mission is responsible for increased accuracy in this study and many others. Before Gaia, astronomers regularly worked with uncertainties in the star age range between 20 and 40 percent. This meant that ages could be down by one billion years, which is a lot.
But Gaia changed it all. The current data release of the mission is Gaia EDR 3 or Early Data Release 3, and that’s a big improvement. EDR3 gives exact 3D positions of more than 330 000 stars. It also gives high-precision measurements of the stars’ movements through space.
The researchers used all this data from Gaia and LAMOST and compared it with known models of star parameters to determine the subgiants’ ages with greater accuracy. “With Gaia’s brightness data, we are able to determine the age of a sub-giant star to a few percent,” Maosheng said.
The sub-giants were spread over the different parts of the Milky Way, enabling the researchers to combine the ages of the other components and build a timeline of the Milky Way’s history.
The study shows two different phases in the history of our galaxy. The first phase began 0.8 billion years ago when the thick disk began to form stars. The inner regions of the galactic halo also began to develop.
Two billion years later, a merger drove the star formation in the hard disk to completion. A dwarf galaxy called Gaia-Sausage-Enceladus merged with the Milky Way.
The Gaia-Sausage-Enceladus (GSE) dwarf galaxy is not shaped like a sausage. It gets its name by drawing its stars on a velocity map, where their orbits are highly elongated. When GSE merged with the Milky Way, it helped create the thick disk, and the gas that came with it fueled star formation in that part of the galaxy.
The merger also filled the Milky Way’s halo with stars. Astronomers think the spherical cluster NGC 2808 may be the Gaia Wors’ remnant core. NGC 2808 is one of the most massive spherical swarms in the Milky Way.
The star formation caused by the GSE in the thick disk lasted about 4 billion years. About 6 billion years after the Big Bang, the gas was all used up. During that period, the metallicity of the thick disk increased by more than a factor of ten.
The study also found a very close correlation between the metallicity and the ages of the stars in the entire disk. This means that the gas that came with the GSE must have been turbulent, causing it to mix more thoroughly into the disk.
Astronomers only recently discovered the 2018 GSE merger. Similar discoveries have shaped our understanding of the history of the Milky Way, and the galaxy’s evolutionary timeline is becoming increasingly clear. This new study gives us a more detailed version.
“Since the discovery of the ancient fusion with Gaia-Sausage-Enceladus, in 2018, astronomers have suspected that the Milky Way was already there before the halo formed, but we have not had a clear picture of what that Milky Way looked like, says Maosheng.
“Our results provide excellent details about that part of the Milky Way, such as its date of birth, rate of star formation and metal enrichment history. The compilation of these discoveries using Gaia data is a revolution in our picture of when and how our galaxy was formed.”
In recent years, astronomers have discovered more details about the Milky Way. But it is challenging to map its structure because we are in the middle of it. The ESA’s Gaia mission is our best catalog yet of the stars in the Milky Way. And every data release gets better and better.
“With each new analysis and data release, Gaia enables us to compile the history of our galaxy in even more unprecedented detail. With the release of Gaia DR3 in June, astronomers will be able to enrich the story with even more details,” says Timo Prusti , Gaia project scientist for ESA.
The Gaia mission is essential, but observations of other galaxies such as the Milky Way also give astronomers insight into the structure and history of the Milky Way. But observing galaxies only two billion years after the Big Bang is difficult. It requires powerful infrared telescopes. Fortunately, one long-awaited infrared space telescope will soon begin observations.
The James Webb Space Telescope (JWST) has the power to look back in time to the universe’s early years. It will be able to see the universe’s earliest Milky Way-like galaxies.
Astronomers want to know more about the GSE merger and how it led to star formation and formed our galaxy’s thick disk just two billion years after the Big Bang. JWST observations of ancient, high-redshift galaxies similar to the Milky Way could help answer some questions and fill in a more detailed galactic history.
And in June, the ESA will release Gaia’s full third data release, called DR3. The DR3 catalog will contain ages, metallicity and spectra for more than 7 million stars. DR3 and the JWST will be a powerful combination.
What will all that data tell us? As the Universe evolves, galaxies must either eat or be eaten. Gravity pulls galaxies together, but the Universe also expands thanks to dark energy, and the dark energy pushes galaxies apart. Thus galaxies tend to cling together in groups. The Milky Way is part of the Local Group.
The groups remain internally coherent due to the combined gravity of the galaxies, but the groups drift away from each other due to expansion. Eventually, the largest galaxies in a group consume the smaller ones.
The Milky Way digested the GSE and spherical clusters. And it consumes the Great Magellanic Cloud, which consumes its even smaller neighbor, the Little Magellanic Cloud.
Eventually, the Milky Way will consume both, and then in about 4.5 billion years it will merge with the even larger Andromeda Galaxy, another member of the Local Group.
This is a strange situation, because the future of the Milky Way may be easier to distinguish than its past. This is the mystery of an expanding Universe: the evidence we seek continues to deviate from us, lost in time and distance.
But the JWST and the Gaia DR3 have the potential to turn the table on the growing universe. Together they can shed more light on the history of the Milky Way and on the details of galaxy mergers in general. Hopefully we will end up with a much more thorough historical timeline.
This article was originally published by Universe Today. Read the original article.
