(ORDO NEWS) — The Milky Way is older than astronomers thought, or part of it. A recently published study shows part of the disk is two billion years older than we thought.
This region, called the thick disk, began to form just 0.8 billion years after the Big Bang.
A pair of astronomers has pieced together the history of the Milky Way in more detail than ever. 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 subgiant stars.
The work is titled “A Time-Resonant Picture of the History of the Early Formation of Our Milky Way” and is published in the journal Nature. The authors of the work are Maosheng Xiang and Hans-Walter Rix, both from the Max Planck Institute for Astronomy (MPIA).
One of the hardest things to determine about 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 formed in stars and spread throughout the universe when those stars die and explode. Astronomers call all elements heavier than the two primitive elements “metals”.
Lower metallicity stars are older because they formed when only hydrogen and helium were mostly available.
So when astronomers discover a population of stars containing mostly hydrogen and helium, they know that those stars are older. When they find a population of stars with a higher proportion of metals, they know that those stars must be younger.
Accurate age measurements are the holy grail in some aspects of astronomy, and they are true here as well. Xiang and Ricks used more than just metallicity to determine stellar age. They focused on a specific type of star: subgiants.
The subgiant phase in a star’s life is relatively short, so astronomers can most accurately determine the age of a star when it is a subgiant. Subgiants move on to red giants and no longer produce energy in their cores. Instead, fusion has moved into a shell around the nucleus.
In this study, a pair of scientists used LAMOST data to determine the metallicity of about 250,000 stars in different parts of the Milky Way. They also used the Gaia data, which gives accurate position and brightness data for about 1.5 billion stars.
ESA’s Gaia mission is responsible for improving the accuracy of this study and many others. Prior to Gaia, astronomers typically worked with stellar age uncertainties between 20 and 40 percent. This meant that the age could differ by one billion years, which is a lot.
But Gaia changed all that. The current mission data release is Gaia EDR 3 or Early Data Release 3 and this is a significant improvement. EDR3 gives accurate 3D positions for over 330,000 stars. It also gives highly accurate measurements of the movement of stars in space.
The researchers used all of this data from Gaia and LAMOST and compared it to known models of stellar parameters to determine the age of the subgiants with greater accuracy. “With the Gaia brightness data, we can determine the age of the subgiant star to within a few percent,” Maosheng said.
The subgiants are scattered throughout the Milky Way, allowing researchers to piece together the ages of the other components and build a timeline of the Milky Way’s history.
The study shows two distinct phases in our galaxy’s history. The first phase began 0.8 billion years ago, when stars began to form in the thick disk. The inner regions of the galactic halo also began to develop.
Two billion years later, the merger resulted in the completion of star formation in the thick disk. A dwarf galaxy called Gaia Sausage Enceladus has merged with the Milky Way.
The dwarf galaxy Gaia-Sausage-Enceladus (GSE) is not shaped like a sausage. It got its name due to the fact that its stars are depicted on the velocity graph, where their orbits are very elongated. When the GSE merged with the Milky Way, it helped create a thick disk, and the gas that came with it fueled star formation in that part of the galaxy.
As a result of the merger, the halo of the Milky Way also filled with stars. Astronomers believe that the globular cluster NGC 2808 may be the remnant core of Guy’s sausage. NGC 2808 is one of the most massive globular clusters in the Milky Way.
GSE-induced star formation in the thick disk lasted about 4 billion years. About 6 billion years after the Big Bang, the gas was completely used up. During this period, the metallicity of the thick disk increased more than tenfold.
The study also found a very strong correlation between the metallicity and age of stars throughout the disk. This means that the gas coming from the GSE must have been turbulent, resulting in more thorough mixing in the disk.
Astronomers discovered the GSE merger as recently as 2018. Discoveries such as this have shaped our understanding of the Milky Way’s history, and the timetable for the galaxy’s evolution is getting clearer. The new study gives us a more detailed account.
“Since the discovery of the ancient Gaia-Sauzaj-Enceladus merger in 2018, astronomers have suspected that the Milky Way already existed before the formation of the halo, but we didn’t have a clear picture of what this Milky Way looked like,” says Maosheng.
Our results provide exquisite details about this part of the Milky Way, such as its birth date, star formation rate, and metal enrichment history. “Combining these discoveries with the Gaia data will revolutionize our picture of when and how our galaxy formed.”
In recent years, astronomers have uncovered more details about the Milky Way. But it is very difficult to map its structure, because we are located in the very center. The ESA Gaia mission is our best catalog of stars in the Milky Way. And every data release gets better and better.
“With each new analysis and data release, Gaia allows us to piece together the history of our galaxy in even more unprecedented detail. With the release of Gaia DR3 in June, astronomers will be able to enrich this history with even more detail,” says Timo Prusti, Gaia Project Scientist at ESA. .
The Gaia mission is very important, but observations of other galaxies like the Milky Way also give astronomers insight into the structure and history of the Milky Way.
But observing galaxies just two billion years after the Big Bang is very difficult. This requires powerful infrared telescopes. Fortunately, one long-awaited infrared space telescope is about to begin its observations.
The James Webb Space Telescope (JWST) is capable of looking back into the early years of the universe. He will be able to see the earliest galaxies in the universe, similar to the Milky Way.
Astronomers want to learn 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-shift galaxies similar to the Milky Way will help answer some questions and fill in a more detailed galactic history.
And in June, ESA will release the full third release of Gaia data, which is called DR3. The DR3 catalog will contain the ages, metallicities and spectra of over 7 million stars. DR3 and JWST will be a powerful combination.
What will all this data tell us? As the universe evolves, galaxies must either eat or be eaten. Gravity pulls galaxies together, but the universe is also expanding due to dark energy, and dark energy pushes galaxies apart. Therefore, galaxies tend to unite in groups. The Milky Way is part of the Local Group.
The groups remain internally intact due to the joint gravity of the galaxies, but due to the expansion of the groups, they move away from each other. After all, the largest galaxies in the group gobble up the smaller ones.
The Milky Way has engulfed the GSE and globular clusters. It engulfs the Large Magellanic Cloud, which engulfs its even smaller neighbor, the Small Magellanic Cloud.
Eventually, the Milky Way will swallow them both up and then, about 4.5 billion years later, 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 determine than its past. Such is the mystery of the expanding universe: the evidence we seek is constantly moving away from us, lost in time and distance.
But JWST and Gaia DR3 are able to turn the view of the expanding universe. Together, they may shed more light on the history of the Milky Way and on the details of galaxy mergers in general. I hope that in the end we will get a much more detailed historical chronology.
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