Milky Way collided with another galaxy

Christopher Davidson
November 3, 2018

According to scientists, the giant halo of the milky way could appear as a result of clashes with smaller galaxies 10 billion years ago.

"The discovery that. the inner halo of the Milky Way turns out to be a different galaxy that's basically contributed all of the stars to our own galaxy, I think that was a big surprise", said lead author Amina Helmi, an astronomer with the Kapteyn Astronomical Institute at the University of Groningen in the Netherlands.

The peculiar motion of those stars pointed researchers to the ancient collision that consumed them in the first place.

The thickness of the thick disk increased by as much as a factor of ten after the collision, ... Instead, massive galaxies bulk up by catching and consuming smaller galaxies.

The study, if correct, firstly confirms what theorists have long thought: that galaxies like the Milky Way grow to enormous proportions by devouring many smaller ones. For U.S. readers snacking their way through a Halloween haul, consider the injection of caramel that turns a simple 3 Musketeers bar into the cosmic candy we know and love called Milky Way. His simulations of the merging of a large disc-shaped galaxy with the young Milky Way produced a distribution of stars from both objects, which is totally in line with the Gaia data. The collision's huge injection of stars and gas may also explain why the Milky Way has such a thick disk, Helmi said, although Johnston said there's more work to be done before scientists can fully flesh out how that might have worked. "It's like uncovering a fossil or an archaeological piece of evidence for how the galaxy got started", says James Bullock, an astronomer at the University of California, Irvine, who is unaffiliated with the new research. To find out, they chose to compare the chemical properties of the stars with the remaining ones in the Milky Way.Chemical cluesThe team used spectroscopic observations from the APOGEE-2 survey-which measures the amounts of different elements in individual stars.

The team knew the stars had formed elsewhere, but they didn't know how they ended up floating around the Milky Way.

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Then, Helmi and her colleagues noticed something else. "It was really the tip of the iceberg", Helmi says. "We're seeing the inner workings of a galaxy".

The team's observations matched simulations of a collision with a smaller galaxy, and based on the ages of those strangely constructed stars travelling in the wrong direction, the team calculated that the smash-up would have occurred about 10 billion years ago, and that the dwarf galaxy would have been comparable in size to the Large Magellanic Cloud, a present-day satellite galaxy. The galaxy is called Gaia-Enceladus, after the Giant Enceladus who in Greek mythology was born of Gaia (the Earth goddess) and Uranus (the Sky god).

In the past, Amina and her research group had used computer simulations to study what happens to stars when two large galaxies merge.

Such events are not as inconceivably prehistoric as they may seem.

RG: What do you make of the predicted collision between the Andromeda and the Milky Way in four billion years? There are two small gaseous galaxies that orbit our own known as the Large and Small Magellanic Clouds, which also orbit each other. The Milky Way, researchers determined, is now sipping gas from the Small Magellanic Cloud, using the material to produce new stars and planets.

It occurred about 10 billion years ago, and was nearly exclusively responsible for the stars in the inner halo - the dome-like structures that extend above and below the galactic plane - as well as the increased thickness of the galactic disc.

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