Astronomy Galactic habitability Milky Way Galaxy

At Live Science: Mysterious ‘ancient heart’ of the Milky Way discovered using Gaia probe

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Ben Turner writes:

Astrophysicists investigating the origins of the Milky Way may have discovered our galaxy’s ‘old heart’ — the original, ancient nucleus around which all of its stars and planets grew.

The Milky Way’s central region, where Sagittarius and the group of ancient stars can be found, above Telluride, Canada. (Image credit: John Sirlin/Alamy Stock Photo)

The collection of 18,000 of our galaxy’s oldest stars are located in the constellation Sagittarius are from the Milky Way’s protogalaxy — a primordial mass of gas and dust forming the first stars of a young galaxy — that is more than 12.5 billion years old. Accounting for an estimated 0.2% of our galaxy’s total mass, the group is the kernel around which all of the Milky Way eventually grew, the researchers found. The findings were published on Sept. 8 on the preprint server arXiv, and are yet to be peer-reviewed.

To discover the primordial group of stars, the astronomers drew on data from the European Space Agency’s (ESA) Gaia observatory — a 3594-pound (1,630 kilograms) spacecraft launched in 2013 with the goal of creating the most detailed and accurate map of the Milky Way

“It has long been believed (on the basis of theory and simulations) that the very oldest stars are at the very center of a galaxy. We have now shown them to be there in great numbers,” study lead author Hans-Walter Rix, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany, told Live Science. “It’s like doing archeology in an old city. We have shown that the oldest and most primitive ruins are at the ‘modern’ city center.”

Finding our galaxy’s ancient heart began with searching the most crowded region, its central bulge, for the tiny proportion of stars around the same age as the roughly 13 billion-year-old Milky Way. 

To pluck this tiny group like a needle from a haystack, the researchers pulled together data collected from Gaia on 2 million stars that sit within 30 degrees of the galactic center, searching for lower-mass, longer-lived stars with low metal content. Stars matching this profile were birthed in a much younger universe that was not yet filled with heavy metals scattered far and wide by supernova explosions.

But this is only one half of the story, as metal-poor stars within the Milky Way may also have come from smaller dwarf galaxies that smashed into and merged with our galaxy throughout its life. By examining these stars’ paths through space while retaining only those that didn’t veer out into the metal-poor regions of the galaxy, the researchers were able to separate out the stars that form the ancient heart from the stars that originated in a dwarf galaxy. 

The researchers’ examination of the Milky Way’s now-exposed ancient heart revealed two things. Firstly, as stars of the old protogalaxy rotate much less around the galactic center compared with younger stars, it confirms past observations that the Milky Way’s core began its life stationary, eventually picking up rotational speed as the galaxy’s center of mass grew.

And secondly, in spite of multiple mergers with smaller galaxies, the close bunching of stars in the Milky Way’s center points to its core not having been invaded by collisions from other galaxies.

“The Milky Way never has been shook up dramatically,” Rix said. “Our galaxy has lived a sheltered life.”

With further study, the researchers hope the ancient heart can teach them even more about our galaxy’s earliest years, such as the types of supernovas that must have exploded during the time of its creation to produce the proportions of early chemical elements we see today.

Live Science

Studies of our galaxy have revealed that numerous features of the Milky Way appear to be unique compared to most galaxies in giving our solar system a unique environment suitable for sustaining life on Earth in its various forms over the course of Earth’s history. In connection with merger events with smaller galaxies, astrophysicist Hugh Ross provides this comment:

This “undisturbed” quality appears to be fine-tuned. An outstanding MWG feature is that throughout its past 11 billion years it has not suffered any merger events of sufficient magnitude to alter its spiral structure in any life-threatening manner. Nevertheless, it has accreted sufficient streams of gas and a sufficient number and rate of low-mass dwarf galaxies to sustain its spiral structure. Without such regular accretions a spiral galaxy’s spiral structure collapses after only 3–4 galactic rotations.

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