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The movement of stars on the outskirts of our galaxy suggests important changes in the history of the Milky Way. These and other equally fascinating results come from a set of papers that demonstrate the quality of ESA’s Gaia Early Third Data Release (EDR3), which is released on December 3, 2020.
Astronomers from the Gaia Data Processing and Analysis Consortium (DPAC) saw evidence of the Milky Way’s past by looking at the stars in the direction of the galaxy’s “anticenter”. It is exactly in the opposite direction on the sky from the center of the galaxy.
The results on the anticenter come from one of four “demonstration papers” published with the Gaia data. The others use Gaia’s data to provide a huge extension to the nearby star census, derive the shape of the solar system’s orbit around the center of the galaxy, and probe structures in two galaxies near the Milky Way. Articles are designed to highlight improvements and the quality of newly published data.
What’s new in EDR3?
Gaia EDR3 contains detailed information on more than 1.8 billion sources, detected by the Gaia spacecraft. This is an increase of over 100 million sources from the previous data release (Gaia DR2), which went public in April 2018. Gaia EDR3 also contains color information for around 1.5 billion sources, that’s an increase of around 200 million sources compared to Gaia DR2. In addition to including more sources, the general precision and the accuracy of the measurements has also improved.
“The new Gaia data promises to be a treasure trove for astronomers,” says Jos de Bruijne, deputy scientist of ESA’s Gaia project.
Towards the galactic anticenter
The new data from Gaia has allowed astronomers to trace the various populations of older and younger stars to the very edge of our galaxy – the galactic anticenter. Computer models have predicted that the Milky Way’s disk will grow larger over time as new stars are born. The new data allows us to see the relics of the 10 billion-year-old ancient disk and thus determine its smaller extent compared to the current size of the Milky Way disk.
New data from these outer regions also strengthens evidence of another major event in the galaxy’s more recent past.
The data shows that in the outer regions of the disk there is a component of slowly moving stars above the plane of our galaxy that move downward toward the plane, and a component of fast moving stars below the plane that move upwards. This extraordinary model had not been foreseen before. This could be the result of the near collision between the Milky Way and the dwarf galaxy Sagittarius that took place in our galaxy’s more recent past.
The dwarf galaxy of Sagittarius contains a few tens of millions of stars and is currently being cannibalized by the Milky Way. Its last close passage to our galaxy was not a direct hit, but it would have been enough for its gravity to disturb some stars in our galaxy like a stone falling into water.
Using Gaia DR2, members of the DPAC had already found a subtle ripple in the movement of millions of stars that suggested the effects of meeting Sagittarius some time between 300 and 900 million years ago. Now, using Gaia EDR3, they have discovered more evidence that points to its powerful effects on our galaxy’s star disk.
“The movement patterns of record stars are different from what we used to believe,” says Teresa Antoja, University of Barcelona, Spain, who worked on this analysis with colleagues at DPAC. Although the role of the Sagittarius dwarf galaxy is still debated in some circles, Teresa says, “She could be a good candidate for all of these disturbances, as some simulations from other authors show.
Measuring the orbit of the solar system
The history of the galaxy is not the only result of the Gaia EDR3 demonstration papers. DPAC members across Europe have done further work to demonstrate the extreme fidelity of the data and the unique potential for limitless scientific discovery.
In an article, Gaia allowed scientists to measure the acceleration of the solar system relative to the resting frame of the Universe. Using the observed motions of extremely distant galaxies, the speed of the solar system has been measured to change 0.23 nm / s every second. Due to this tiny acceleration, the trajectory of the solar system is deviated by the diameter of a atom every second, and in a year, that’s about 115 km. The acceleration measured by Gaia shows good agreement with theoretical expectations and provides the first measurement of the curvature of the orbit of the solar system around the galaxy in the history of optical astronomy.
A new stellar census
Gaia EDR3 also made it possible to obtain a new census of the stars in the solar neighborhood. The Gaia catalog of nearby stars contains 331,312 objects, which is estimated to be 92% of stars within 100 parsecs (326 light years) of the Sun. The previous census of the solar quarter, called the Gliese Catalog of Nearby Stars, was taken in 1957. It initially had only 915 objects, but was updated in 1991 to 3,803 celestial objects. It was also limited to a distance of 82 light years: Gaia’s census is four times as far away and contains 100 times as many stars. It also provides location, motion, and light measurements that are orders of magnitude more accurate than old data.
Beyond the milky way
A fourth demo article analyzed Magellanic clouds: two galaxies orbiting the Milky Way. After measuring the movement of the stars of the Large Magellanic Cloud with greater precision than before, Gaia EDR3 clearly shows that the galaxy has a spiral structure. The data also resolves a stream of stars that is extracted from the Small Magellanic Cloud and hints at previously invisible structures on the outskirts of the two galaxies.
At 12:00 p.m. CET on December 3, the data produced by the many scientists and engineers of the Gaia DPAC Consortium becomes public for anyone to watch and learn. This is the first of a two part version; full version 3 of the data is scheduled for 2022.
“Gaia EDR3 is the result of a huge effort by everyone involved in the Gaia mission. This is an extraordinarily rich data set, and I look forward to the many discoveries astronomers around the world will make with this resource, ”said Timo Prusti, ESA’s Gaia Project Scientist. “And we haven’t finished yet; more interesting data will follow as Gaia continues to take measurements from orbit. “
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