An unknown dwarf galaxy, dubbed ‘Sausage’ galaxy, smashed into our own Milky Way Galaxy around 8 to 10 billion years ago, a team of astronomers suggests. The dwarf galaxy did not survive the impact: it quickly fell apart, and the wreckage is now all around us.
An impression of the encounter between the Milky Way and the Sausage dwarf galaxy. Image credit: V. Belokurov / Juan Carlos Muñoz / ESO.
“The collision ripped the dwarf to shreds, leaving its stars moving in very radial orbits that are long and narrow like needles,” said team member Dr. Vasily Belokurov, from the University of Cambridge and the Flatiron Institute.
“The stars’ paths take them very close to the centre of our Galaxy. This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed.”
The astronomers used data (parallaxes and velocities of Milky Way stars) from ESA’s Gaia satellite.
“We plotted the velocities of stars, and the sausage shape just jumped out at us,” said Professor Wyn Evans, from the Institute of Astronomy, Cambridge.
“As the smaller galaxy broke up, its stars were thrown onto very radial orbits. These Sausage stars are what’s left of the last major merger of the Milky Way.”
The Milky Way continues to collide with other galaxies, such as the Sagittarius dwarf galaxy.
However, the Sausage galaxy was much more massive. Its total mass in gas, stars and dark matter was more than 10 billion times the mass of our Sun.
When the Sausage crashed into the young Milky Way, its piercing trajectory caused a lot of mayhem. The Milky Way’s disk was probably puffed up or even fractured following the impact and would have needed to regrow.
And Sausage debris was scattered all around the inner parts of the Milky Way, creating the bulge at the Galaxy’s center and the surrounding ‘stellar halo.’
“Numerical simulations of the galactic mashup can reproduce these features,” said Dr. Denis Erkal, from the University of Surrey.
In simulations run by the team, stars from the Sausage galaxy enter stretched-out orbits. The orbits are further elongated by the growing Milky Way disk, which swells and becomes thicker following the collision.
“Evidence of this galactic remodeling is seen in the paths of stars inherited from the dwarf galaxy. The Sausage stars are all turning around at about the same distance from the center of the Galaxy,” said Dr. Alis Deason, from Durham University.
“These U-turns cause the density in the Milky Way’s stellar halo to decrease dramatically where the stars flip directions.”
The researchers also identified at least eight globular clusters that were brought into the Milky Way by the Sausage galaxy.
Small galaxies generally do not have globular clusters of their own, so the Sausage galaxy must have been big enough to host a collection of clusters.
“While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all,” said Dr. Sergey Koposov, of Carnegie Mellon University.
The scientists reported their results and analysis in five papers in the Monthly Notices of the Royal Astronomical Society and the Astrophysical Journal Letters.
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V. Belokurov et al. 2018. Co-formation of the disc and the stellar halo. MNRAS 478 (1): 611-619; doi: 10.1093/mnras/sty982
G.C. Myeong et al. 2018. The Shards of ω Centauri. MNRAS, in press; arXiv: 1804.07050
G.C. Myeong et al. 2018. The Milky Way Halo in Action Space. ApJL 856, L26; doi: 10.3847/2041-8213/aab613
Alis J. Deason et al. 2018. Apocenter Pile-Up: Origin of the Stellar Halo Density Break. ApJL, in press; arXiv: 1805.10288
G.C. Myeong et al. 2018. The Sausage Globular Clusters. ApJL, in press; arXiv: 1805.00453
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