Milky Way devoured a galactic challenger, astronomers reveal

Our home galaxy swallowed up a dwarf galaxy about 10 billion years ago—and it has its next victims in sight.

By Nadia Drake
Published 1 Nov 2018, 15:06 GMT
Powerful radiation from newborn stars blasts a cavity in a star-forming region inside the Small Magellanic ...
Powerful radiation from newborn stars blasts a cavity in a star-forming region inside the Small Magellanic Cloud in this image from the Hubble Space Telescope.
Photograph by ESA, NASA, Nat Geo Image Collection

Ten billion years ago, our home galaxy smashed into a smaller cluster of stars—a dwarf galaxy whose remains have recently been spotted among the glittering masses swarming the Milky Way. With a good telescope, that dwarf galaxy’s bizarre stars are still visible in our night sky; they’ve taken up residence in the solar neighbourhood and in the halo swaddling the Milky Way’s spiral arms.

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

“The stars move in the opposite [direction] than the majority of those in the Milky Way,” says astronomer Amina Helmi of the University of Groningen in the Netherlands, who reported the observations in the journal Nature.

That galaxies collide is hardly surprising. With space telescopes, we can see large stellar conglomerates in the act of merging. The smash-ups appear violent from here on Earth, but space is vast and stars are far apart, so individual celestial bodies rarely obliterate one another. Instead, massive galaxies bulk up by catching and consuming smaller galaxies.

As scientists scrutinise these events, two major questions linger: firstly, how often do these collisions happen? And secondly, are existing galaxies constructed from a few large collisions or a series of smaller ones?

“We would like to reconstruct the history, the genealogy tree as it were, of the Milky Way,” says Helmi, whose work falls into a field known as galactic archaeology.

Strange stars

Astronomers know the Milky Way has devoured several galaxies already. Some two dozen ribbons of stars streaming around the galaxy’s disc betray its predatory appetite, as do other oddly behaving, oddly composed stars. As further evidence, an enigmatic cluster of stars called Omega Centauri, which now sparkles in the southern constellation Centaurus, is thought to be the core of a mostly digested galaxy.

But recently, the European Space Agency’s Gaia spacecraft, which is taking an exquisitely detailed look at the positions and movement of more than a billion nearby stars, shook a new clue loose from the heavens. When Helmi and her team looked at the most recent Gaia catalogue, released earlier this year, they noticed a population of stars moving backward relative to almost everything else trekking around the Milky Way’s core. If those stars had been born in this galaxy, they would be marching around the core in the same direction as the rest of us.

Then, Helmi and her colleagues noticed something else.

“When we looked at their chemical composition, we saw that the stars defined a separate sequence in ‘chemical space,’” Helmi says. “Such a sequence can only be found if the stars formed elsewhere in a smaller galaxy.”

Those chemically distinct stars are distributed mostly in the Milky Way’s halo, or the region surrounding the thick stellar disk that weaves its way through a dark sky.

The Milky Way (white) gobbles up the dwarf galaxy Gaia-Enceladus (red) in scenes from a computer simulation of the ancient event.
Photograph by H.H. Koppelman, A. Villalobos and A. Helmi

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.

Helmi and her colleagues named the dwarf galaxy Gaia-Enceladus, after Gaia, the Greek mother of all life, and Enceladus, the son she had with Uranus, the sky god.

A growing galactic appetite

“This paper is suggesting that the stellar halo is dominated by the cannibalisation of at least one fairly large dwarf galaxy,” says astrophysicist Gurtina Besla of the University of Arizona. “So it looks like the assembly of the stellar halo is controlled by a few large-ish destruction events.”

Such events are not as inconceivably prehistoric as they may seem. Our night sky is still being shaped by galactic collisions, and it will continue to be a shifting canvas for billions of years. Take, for example, the Large and Small Magellanic Clouds, two dwarf galaxies that are now satellites of the Milky Way. Hanging unobtrusively in the southern sky, these small galaxies recently collided with one another—a direct hit betrayed by a small river of stars moving toward the Large Magellanic Cloud, Besla notes.

Simulations suggest that in another two billion or so years, the Milky Way will swallow the Large Magellanic Cloud, forever erasing it from the southern sky. Its partner will suffer the same fate.

“The structure of the Milky Way’s disc and stellar halo is going to change again,” Besla says.

If that sounds extreme, don’t forget that in roughly five billion years, the Milky Way will collide with our nearest neighbour, the giant Andromeda galaxy. Currently hurtling toward one another at 250,000 miles an hour, these galaxies will merge and change our night sky forever—that is, if there are any sentient beings left clinging to this rock we call home.

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