Last updated May 2, 2018 at 12:35 pm
An ancient fly-by of our Milky Way by a neighbour may have warped our galaxy into a huge galactic wave, scattering stars 14,000 light years above and below the disc, according to new observations.
In this artist’s impression of our Milky Way the outer edge of the galaxy is flared by the tidal force from the Sagittarius Dwarf Galaxy, over 5 billion years ago. The tide causes a wave-like ripple that scatters stars from the disk to thousands of light years above and below. Credit: T. Mueller/C. Laporte/NASA/JPL-Caltech
It looks like our galaxy had a more violent past than previously thought.
The location of two collections of stars, A13 and TriAnd, have been a long-running mystery for astronomers studying the structure of the Milky Way.
One previous model suggested that they could have belonged to a much smaller satellite system, torn apart by the stronger gravity of our more massive galaxy.
Instead, researchers have used the most powerful ground based telescopes to offer a different explanation. The stars are in fact our own.
Something very dramatic has caused them to wander a long way from home.
Chemical fingerprints
Stars that form from the same gas cloud tend to share almost the same chemical make-up – a particular combination of trace amounts of elements such as neon, barium and europium among others.
No matter where the stars then wander in their travels around the galaxy, these ratios act as chemical fingerprints, indicating shared parentage of the same patch of the galaxy.
The spectrum of sunlight passed through a prism, stretching from longer wavelength in red at top to shorter wavelengths in blue at the bottom. The dark lines indicate transitions where photons of light are absorbed by certain elements. These act as a barcode of the composition of the star, a unique chemical fingerprint shared only by stars born from the same gas cloud.
Using the Keck and Very Large Telescope, astronomers could split the light from individual stars to reveal the absorption patterns of these elements, acting as a unique barcode.
Surprisingly, the cluster of stars named A13, 14,000 light years above the Milky Way’s disc, share the same chemical signature with each other as well as with TriAnd, sitting 14,000 light years belowthe disc.
They also contain a similar amount of heavier elements, known as the stellar metallicity, to stars in the thin disc of the Milky Way, which we see as a band of light stretching across the night sky.
It is incredibly unlikely that such a close match is chance, instead the stars above and below the disc were instead once part of the thin disc.
Sagittarius dwarf galaxy
To understand how they have travelled so very far from their birthplace, researchers created a simulation of a young Milky Way before and after being passed by a neighbouring dwarf galaxy.
This dwarf was modelled on the known Sagittarius dwarf galaxy, but modelled as it likely then was – containing 100 billion Sun’s worth of mass. The impact was catastrophic.
A simulation of our Milky Way’s disk of stars, in the grey points of light, before the passage of the Sagittarius Dwarf Galaxy (in the top) and then after (in the bottom panel). The tidal forces warp the disk, kicking stars into waves stretching nearly 30,000 light years from crest to trough. The groups of stars that make up A13 and TriAnd are well explained by just such a wave scattering them to heights of 14,000 light years above the disk. Credit: Bergemann et al.
The tidal forces from the passing galaxy caused a bulge of disc stars to form out on the near-side and a bulge of stars to explode outwards on the far-side, just as the Moon’s raises tides in the water on opposite sides of the Earth.
Galactic cannibalism
These stars were then left out of balance when the dwarf galaxy moved on and fell downwards back to the disk from either direction before sloshing through and beginning a wave-like oscillation.
These waves, caused by the galactic tidal force, take millions of years to move from crest to trough but in speeded-up computational simulations look uncannily like water, composed not of atoms but of individual stars.
The close flyby also resulted in the large scale stripping of most of the Sagittarius dwarf galaxy, its size shrinking to barely three billion times the mass of our Sun.
This act of galactic cannibalism is how galaxies like our Milky Way grow larger, but as this event shows not always without a little damage.
The study was published in Nature.
