Galactic crash gave Milky Way its cream

James Marshall
November 3, 2018

Astronomers using the European Space Agency's Gaia space telescope have discovered that our galaxy was involved in a cosmic merger 10 billion years ago.

That is when our galaxy's gravity pulled a smaller companion, roughly one quarter its mass, into a unsafe dance: One where the dwarf galaxy plunged into and out of the Milky Way's disk, oscillating back and forth until it was finally swallowed whole. Although our galaxy survived, it has never been the same. The stars now form most of our Galaxy's inner halo - a diffuse component of old stars that were born at early times and now surround the main bulk of the Milky Way known as the central bulge and disc.

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.

The research is described in a paper published today (Oct. 31) in the journal Nature. Most recently, an global team of scientists led by Amina Helmi of the University of Groningen identified one of the major galactic collisions (or "mergers") that shaped the Milky Way as we know it. "The [inner] halo of our galaxy really is made up of stars that were not born in our galaxy but were born in another galaxy that merged with our own ... all of the halo that we thought was ours actually belongs to that other galaxy", Helmi said.

Large galaxies get that way by absorbing lesser ones. Helmi's study, on the other hand, brings multiple lines of evidence together to paint the most compelling portrait yet, says Kathryn Johnston, an astronomer at Columbia University who was not involved in the work. And it is one that relies on a single-yet massive-merger.

'And they are a fairly homogenous group, which indicates they share a common origin, ' Helmi said. "The disk is ordered; you have 100 million stars moving orderly around the galactic center".

"If you were there ... you would see bright, blue young stars. That hints that these stars could not have been formed in the Milky Way".

Another piece of evidence was the composition of the stars themselves. In every galaxy the abundance of elements heavier than hydrogen and helium gradually increases over time due to cycles of stellar death and rebirth. It is the equivalent of being sat on the roof of Buckingham Palace in London, staring out more than 5,500 kilometres (3,400 miles) across the Atlantic Ocean.Funny moving stars This illustration shows a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years.

Helmi & Co. also give a nod to Greek mythology, in which Enceladus, the giant son of Gaia, was buried below Sicily, causing seismic activity. The fact that so many clusters could be linked to Gaia-Enceladus is another indication that this must have once been a big galaxy in its own right, with its own entourage of globular clusters.

"The next steps are to characterise the galaxy Enceladus better, and also the proto-milky Way now that we know how to separate the two", Helmi told ScienceAlert.

If you want to see what a galaxy merger looks like from a God's-eye perspective, check out this simulation created by the research team (below).

"By reading the motions of stars scattered across the sky, we are now able to rewind the history of the Milky Way and discover a major milestone in its formation, and this is possible thanks to Gaia", concludes Timo Prusti, Gaia project scientist at ESA. The sheer confirmation alone has sent many astronomers into a state of euphoria. They think that a dwarf galaxy with about a fourth the mass of the Milky Way was pulled inside our home galaxy and then torn apart by our greater gravitational force, its constituent stars spread out and integrated into their new home. Even our nearest neighbor, Andromeda, is too far off for robust studies of mergers there.

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