The Milky Way has a bulge and now we know why.

The distinctive shape of our galaxy has puzzled astronomers for a long, long time. New evidence fills in missing pieces identifying an ancient collision between our galaxy and a small celestial body, dubbed the ‘Sausage’ galaxy. About 8 to 10 billion years ago, astronomers believed that an unknown dwarf galaxy crashed into the Milky Way. This obliterated the dwarf and gave our galaxy the distinctive bulge at its center and its outer halo. Vasily Belokurov of the University of Cambridge and the Center for Computational Astrophysics at the Flatiron Institute in New York City mentioned that remnants of the cosmic crash surround us. She said,

“The collision ripped the dwarf to shreds, leaving its stars moving in very radial orbits that are long and narrow like needles. The stars’ paths take them very close to the center of our galaxy. This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed.”

A graduate student from the Cambridge University, GyuChul Myeong, and his colleagues collected data gathered by the European Space Agency’s Gaia satellite. The mission of this satellite had been the mapping of our galaxy’s stars and recording their journeys as they travel through the Milky Way. Data gathered by Gaia enabled astronomers to know the positions and trajectories of celestial bodies with extreme precision. This identified both the cosmic collision and the anomalies it birthed. Scientists noticed that there was a set of stars that did not conform to the flight paths of their neighbors. Wyn Evans of Cambridge explained “the Gaia Sausage” in the following words:

“We plotted the velocities of the stars, and the sausage shape just jumped out at us. As the smaller galaxy is 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.”

Events such as this are not new to our galaxy or the cosmos in general. However, only a few of them have managed to leave such a long-lasting impact. Our galaxy continues to collide with other galaxies, the Sagittarius dwarf galaxy to name one. It was the mass of the Sausage galaxy that caused an event of such scale. Its total mass was more than 10 billion times the mass of our sun in gas, stars, and dark matter. The trajectory of the Sausage when crashed into a young Milky Way was piercing. It caused numerous disturbances within our galaxy. Following the impact, the disk of our galaxy was probably puffed up or even fractured and necessitated the need for it to re-grow. The debris of the Sausage was scattered all around the inner parts of the Milky Way which led to the creation of the ‘bulge’ at the center of the Milky Way and the ‘Stellar Halo’ that surrounds it.

Denis Erkal of the University of Surrey stipulates that numerical simulations of the cosmic event can reproduce these features. In simulations run by Erkal and colleagues, 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. Alis Deason, a Representative of the Durham University, predicted that 5 years ago by saying,

“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. These U-turns cause the density in the Milky Way’s stellar halo to decrease dramatically where the stars flip directions.”

At least eight large, spherical clumps of stars called globular clusters were identified by new research. These clusters were brought into the Milky Way by the Sausage galaxy. The finding of a collection of globular clusters hosted by the Sausage galaxy points to its considerable size as small galaxies seldom have globular clusters of their own. Sergey Koposov of Carnegie Mellon University who has studied the kinematics of the Sausage stars and globular clusters in detail said,

“While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all.”