Cosmology: Milky Way's violent history (Introduction)

by David Turell , Thursday, September 28, 2023, 23:26 (211 days ago) @ David Turell

Adding new parts:

https://www.quantamagazine.org/in-the-milky-ways-stars-a-history-of-violence-20230928/

"A supermassive black hole churns at its center, surrounded by the “bulge,” a knot of stars containing some of the galaxy’s oldest stellar denizens. Next comes the “thin disk” — the structure we can see — where most of the Milky Way’s stars, including the sun, are partitioned into gargantuan spiraling arms. The thin disk is encased in a wider “thick disk,” which contains older stars that are more spread out. Finally, a mostly spherical halo surrounds these structures; it is mostly made of dark matter, but also contains stars and diffuse hot gas.

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"Everything changed in 2016, when the first data from the European Space Agency’s Gaia satellite came back to Earth. Gaia precisely measures the paths of millions of stars throughout the galaxy, allowing astronomers to learn where those stars are located, how they move through space, and how fast they are going. With Gaia, astronomers could paint a sharper picture of the Milky Way — one that revealed many surprises.

"The bulge is not spherical but peanut-shaped, and it’s part of a larger bar spanning the middle of our galaxy. The galaxy itself is warped like the brim of a beat-up cowboy hat. The thick disk is also flared, growing thicker toward its edges, and it may have formed before the halo. Astronomers aren’t even sure how many spiral arms the galaxy really has.

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"In the intervening century, astronomers have calculated that the Milky Way’s bulge is about 12,000 light-years across, that the disk spans 120,000 light-years, and that the halo of dark matter and ancient star clusters extends hundreds of thousands of light-years in every direction.

"A recent observation found that some halo stars are scattered as far as 1 million light-years away — halfway to Andromeda — which suggests that the halo, and therefore the galaxy, is not quite an island universe unto itself.

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"In work published on September 14, Han and his team also showed that the dark matter halo might be tilted by about 25 degrees, causing the entire galaxy to look warped.

"And while that might seem weird enough, the tilt itself may be evidence of the Milky Way’s violent past.

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"The first hints of violence came when astronomers peering through the storied 200-inch telescope at Palomar Observatory (which Hubble was the first to use) found evidence in 1992 that the Milky Way was ripping apart some of the globular clusters in its halo. The Sloan Digital Sky Survey confirmed that observation, and radio telescopes later found that the galaxy was also inhaling streams of nearby gas.

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"As astronomers pored over the detailed motions and positions of about a billion stars, signs of a major disturbance in the galaxy emerged — they saw galactic wreckage in the halo. There, some stars orbit at extreme angles and have different compositions than others, suggesting that they originated somewhere else.

"Astronomers took these oddball stars as evidence of a titanic collision between the Milky Way and another galaxy. The merger, which probably happened between 8 billion and 11 billion years ago, would have catastrophically disrupted the young Milky Way, ripped the other galaxy to shreds, and sparked a firestorm of new star formation.

"The colliding galaxy’s remains are now called Gaia-Sausage-Enceladus, a result of two teams independently discovering the remnants of the merger.

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"When Xiang and Rix used those clues to infer the migration histories of a quarter of a million subgiant stars, they found that the thick disk formed earlier than expected in galaxy formation theories — 13 billion years ago, barely an eye-blink after the Big Bang.

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"The evidence for Gaia-Sausage-Enceladus continues to pile up. But what astronomers still don’t understand is why things have been calm ever since. The Milky Way’s chemical history and structural history seem atypical, Lu said.

"Andromeda, for instance, has a much more violent history than the Milky Way. It would be odd for our galaxy to be left alone so long, considering other galaxies’ histories and the prevailing cosmological model that says galaxies grow by smashing into each other, Wyse said. “The merging history is unusual, and the assembly history. Whether we are actually unusual in the universe … I would say is still an open question,” she said.

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"Here, around one particular star on the Local Arm, eight planets formed around the sun — four rocky and four gaseous. But other arms may be different. Those environments might produce different populations of stars and planets in the same way that specialized flora and fauna evolve on continents with distinct biospheres.

“'Maybe life can only arise in a really quiet galaxy. Maybe life can only arise around a really quiet star,” said Jessie Christiansen, an astronomer at the California Institute of Technology who studies galactic conditions and their effects on planet-building." (my bold)

Comment: we are here on Earth going 'round a quiet star. In a very special galaxy. Serendipity, contingency or design? Only one cause must exist.