The Milky Way is older than astronomers thought, or part of it. A recently published study shows that part of the disk is two billion years older than we thought.
The region, called the thick disk, began to form only 0.8 billion years after the Big Bang.
A pair of astronomers have pieced together the history of the Milky Way in greater detail than ever before. Their results are based on detailed data from the ESA’s Gaia mission and that of China Large Celestial Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). The key to this discovery lies in subgiant stars.
The paper is “A time-resolved image of the early formation history of our Milky Way“, and it’s online in the newspaper Nature. The authors are Maosheng Xiang and Hans-Walter Rix, both of the Max-Planck Institute for Astronomy (MPIA).
One of the hardest things to determine about a star is its age. The composition or metallicity of a star is essential in 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 throughout the Universe when those stars die and explode. Astronomers call each element heavier than the two primordial elements “metals”.
Stars with lower metallicity are older because they formed when most of the time only hydrogen and helium were available. So when astronomers identify a population of stars containing mostly hydrogen and helium, they know those stars are older. When they find a population of stars with higher proportions of metals, they know those stars must be younger.
Accurate age measurements are the holy grail in some aspects of astronomy, which is true in this case. Xiang and Rix used more than metallicity to determine stellar ages. They focused on a specific type of star: subgiants.
The subgiant phase of a star’s life is relatively brief, so astronomers can determine a star’s age more accurately when it is a subgiant. The subgiants are becoming red giants and no longer produce energy in their core. Instead, the fusion moved into a shell around the core.
In this study, the pair of scientists used LAMOST data to determine the metallicity of around 250,000 stars in different parts of the Milky Way. They also used data from Gaia which gives precise data on the position and luminosity of about 1.5 billion stars.
ESA’s Gaia mission is responsible for the increased accuracy of this study and many others. Prior to Gaia, astronomers routinely worked with stellar age uncertainties of between 20 and 40 percent. This meant that the ages could be off by a billion years, which is a lot.
But Gaia changed all that. The current version of mission data is Gaia EDR 3 or Early Data Release 3, and that’s a big improvement. EDR3 gives precise 3D positions of over 330,000 stars. It also provides high precision measurements of the motions of stars in space.
The researchers used all this data from Gaia and LAMOST and compared it to known models of stellar parameters to determine the age of the subgiants with greater precision. “With luminosity data from Gaia, we are able to determine the age of a subgiant star to within a few percent,” Maosheng said.
The subgiants are distributed throughout the different parts of the Milky Way, allowing researchers to piece together the ages of other components and construct a timeline of the Milky Way’s history.
The study shows two distinct 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 have also begun to develop.
Two billion years later, a merger pushed star formation in the thick disc to completion. A dwarf galaxy named Gaia-Sausage-Enceladus merged with the Milky Way.
The dwarf galaxy Gaia-Sausage-Enceladus (GSE) is not shaped like a sausage. It takes its name from the plot of its stars on a velocity map, where their orbits are very elongated. When GSE merged with the Milky Way, it helped create the thick disk, and the accompanying gas fueled star formation in that part of the galaxy.
The merger has also filled the Milky Way’s halo with stars. Astronomers think the globular cluster NGC 2808 could be the remaining core of Gaia Sausage. NGC 2808 is one of the most massive globular clusters in the Milky Way.
Star formation triggered in the thick disk by the GSE lasted about 4 billion years. About 6 billion years after the Big Bang, the gas was exhausted. During this period, the metallicity of the thick disc increased by more than a factor of ten.
The study also found a very close correlation between the metallicity and age of stars across the disk. This means that the gas supplied with the GSE must have been turbulent, causing it to mix more completely within the disc.
Astronomers only recently discovered the GSE merger in 2018. Discoveries like this have shaped our understanding of the Milky Way’s history, and the galaxy’s development timeline is becoming clearer. This new study gives us a more detailed account.
“Since the discovery of the ancient Gaia-Sausage-Enceladus merger in 2018, astronomers have suspected that the Milky Way was already there before the halo formed, but we didn’t have a clear picture of what it looked like. this Milky Way,” said Maosheng.
“Our results provide exquisite detail about this part of the Milky Way, such as its anniversary, star formation rate, and metal enrichment history. Pooling these findings using data from Gaia is revolutionizing our image of when and how our galaxy formed.”
In recent years, astronomers have discovered more details about the Milky Way. But it is difficult to map its structure because we are right in the middle. ESA’s Gaia mission is our best catalog to date of stars in the Milky Way. And each data release gets better and better.
“With each new data analysis and release, Gaia allows us to piece together the history of our galaxy in even greater detail. With the release of Gaia DR3 in June, astronomers will be able to enrich the story in even greater detail.” , says Timo Prusti, Gaia project scientist for ESA.
The Gaia mission is essential, 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 difficult. This requires powerful infrared telescopes. Luckily, a long-awaited infrared space telescope is set to begin observations soon.
The James Webb Space Telescope (JWST) has the power to travel back in time to the earliest years of the Universe. He will be able to see the first Milky Way-like galaxies in the Universe.
Astronomers want to learn more about the GSE merger and how it led to star formation and shaped our galaxy’s thick disk just two billion years after the Big Bang. JWST Observations of Alumni, high redshift galaxies similar to the Milky Way could help answer some questions and fill in a more detailed galactic history.
And in June, ESA will release the third full version of the Gaia data, called DR3. The DR3 catalog will contain the ages, metallicities and spectra of more than 7 million stars. DR3 and the 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 brings galaxies closer together, but the Universe is also expanding due to dark energy, and dark energy pushes galaxies apart. Thus, galaxies tend to clump together in groups. The Milky Way is part of the local group.
The groups remain internally cohesive due to the combined gravity of the galaxies, but the groups move apart due to expansion. Eventually, the larger galaxies in a group consume the smaller ones.
The Milky Way has consumed the GSE and the globular clusters. And it consumes the Large Magellanic Cloud, which consumes its even smaller neighbor, the Small Magellanic Cloud.
Finally, the Milky Way will consume boththen in about 4.5 billion years it will merge with the even larger Andromeda Galaxy, another member of the Local Group.
It’s a strange situation because the future of the Milky Way might be easier to discern than its past. It’s the riddle of an expanding universe: the evidence we seek keeps slipping away from us, lost in time and distance.
But JWST and Gaia DR3 have the potential to turn the tables on the expanding Universe. Together, they can shed more light on the history of the Milky Way and the details of galaxy mergers in general. Hopefully, we’ll end up with a much more complete historical timeline.