Mounting evidence indicates that our galaxy smashed up another smaller galaxy roughly 10 billion years ago.

We live in a big disk galaxy, a whirligig pancake that’s enshrouded in a big halo of old stars. And increasingly, astronomers suspect that very early in our cosmic pancake’s history, a collision messed up the serene stellar disk and donated the detritus that makes up much of the halo.

Milky Way ancient merger
This artist's concept shows the simulated positions and motions (yellow arrows) of thousands of stars that could be debris from an ancient merger with the Milky Way.
Composition: ESA, simulation: Koppelman, Villalobos and Helmi, galaxy image: NASA / ESA / Hubble CC BY-SA 3.0 IGO

The disk has two overlapping sections: the thin disk and the thick disk. The thin disk is roughly 3,000 light-years thick and embedded in the thick disk, which is twice as thick as the thin one (hence the names). The whole thing is kind of like an Oreo, except that the cookie goes through the crème. (Remember, there’s a lot of space between stars, so interweaving is easy.) Young stars relatively rich in elements heavier than helium dance in the thin disk; old, chemically anemic stars make up the thick one.

Both theorists and observers have struggled to explain why our stellar disk looks this way. Making a thin disk is easy: As gas first collapses to form a galaxy, it naturally flattens out into a nice, skinny saucer. And gas continues to rain down over time, continuously fueling a thin disk of star formation. But something has to puff the old stars up to make the thick disk, explains Puragra GuhaThakurta (University of California, Santa Cruz). That something could be anything from the galaxy’s central bar fluffing the disk up like an eggbeater does whipping cream, to a fender-bender with another galaxy.

To solve this mystery, astronomers are using larger and larger surveys, including that being done by the European Gaia satellite, which is mapping the positions and motions of some billion stars. These investigations are turning up a possible answer not in the disk itself, but in the halo.

Recent work using Gaia and other data has found that halo stars within a few thousand light-years of the Sun rotate around the galactic center in the opposite, or retrograde, direction as the disk does. These stars also have different chemical compositions than those in the disk. Combined, the strange characteristics suggest that these stars aren’t indigenous to the Milky Way — rather, they’re probably crumbs from when our galaxy ate a galactic snack very early in its history.

Amina Helmi (University of Groningen, The Netherlands) and colleagues have now taken a closer look at the retrograde stars’ motions and compositions. Reporting in the November 1st Nature, they confirmed that these halo stars are something quite unusual. First, they move together as a big unit. Second, they have a distinct pattern of heavy elements, and a range in their heavy element levels that suggests these stars didn't all form in a single burst but in an extended period of star formation, creating more heavy elements and increasingly infecting themselves with time. Third, the stars have a range of ages, which matches the chemistry predictions.

Taken together, these quirks set the stars apart from those born in the Milky Way. They very likely come from somewhere else.

stellar detritus
This all-sky map plots the distribution of the retrograde, chemically "odd" stars. Colors indicate stars' parallaxes, which indicate their distance: the most nearby stars are purple, the most distant are yellow. Several globular clusters (white circles) and variable stars (turquoise) appear to to follow similar trajectories as the retrograde stars, indicating that they might come from the same satellite galaxy.
ESA / Gaia / DPAC, A. Helmi et al 2018

Based on the chemical characteristics, the team could infer how long all the stars took to form and how massive their parent galaxy was: roughly 600 million solar masses, or approximately the same as the Small Magellanic Cloud dwarf galaxy — quite a respectable contender, but no match for the Milky Way’s approximately 100 billion stars.

So many are the retrograde stars (about 30,000), that they form a huge swarm around the disk for thousands of light-years around the Sun (maybe farther, we don’t know yet). Helmi estimates that roughly 80% of our galaxy’s halo could be from this single ancient collision. That would jibe with another recent study that couldn’t find signs of a homemade inner halo, as well as a popular hypothesis that halos around galaxies like the Milky Way are made with the leftovers of smaller galaxies that the big ones tore up and ate. The Milky Way’s sibling galaxy, Andromeda (M31), for example, has a jumbled halo full of stellar streams, while the Pinwheel Galaxy (M101) appears to have no halo at all.

Helmi and her colleagues also used simulation results to confirm that a merger with such a sizable small galaxy roughly 10 billion years ago could explain the retrograde stars, in keeping with other recent work and making a “striking” parallel to the observations, writes Kim Venn (University of Victoria, Canada) in a Nature opinion piece.

And the Milky Way’s disk? The merger would naturally puff up the disk that existed at that time, creating the thick disk we have today. Other recent studies make the same case.

Whether this putative merger really did puff up the thick disk remains to be seen — astronomers are always digging for more data. Meanwhile, Helmi’s team is pursuing more complex simulations that will include gas physics to see what kind of star formation the merger might have spurred.


Below, you’ll find a video simulation of the merger. Credit: H.H. Koppelman, A. Villalobos and A. Helmi



A. Helmi et al. “The Merger that Led to the Formation of the Milky Way’s Inner Stellar Halo and Thick Disk.” Nature. November 1, 2018.

V. Belokurov et al. “Co-formation of the Disc and the Stellar Halo.” Monthly Notices of the Royal Astronomical Society. July 21, 2018.

M. Haywood et al. “In Disguise or Out of Reach: First Clues about In Situ and Accreted Stars in the Stellar Halo of the Milky Way from Gaia DR2.” Astrophysical Journal. August 20, 2018.

K. Venn. “Evidence of Ancient Milky Way Merger.” Nature. November 1, 2018.


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