“While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all,” said Sergey Koposov of Carnegie Mellon University, about the collision 10 billion years ago with the massive Gaia Sausage satellite galaxy, whose total mass in gas, stars and dark matter was more than 10 billion times the mass of our Sun, which lies in the center of this enormous cloud of stars.
When the Sausage crashed into the young Milky Way, its piercing trajectory caused mayhem — its 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.’
“The collision ripped the dwarf to shreds, leaving its stars moving in very radial orbits” that are long and narrow like needles, said Vasily Belokurov of the University of Cambridge and the Center for Computational Astrophysics at the Flatiron Institute in New York City. 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.”
Last July, The Galaxy reported that an international team of astronomers proposed that around 8 billion to 10 billion years ago, an unknown dwarf galaxy smashed into our own Milky Way. The dwarf did not survive the impact: It quickly fell apart, and the wreckage is now all around us. The team discovered an ancient and dramatic head-on collision between the Milky Way and a smaller object, dubbed the “Sausage” galaxy. The cosmic crash was a defining event in the early history of the Milky Way and reshaped the structure of our galaxy, fashioning both its inner bulge and its outer halo.
Papers in the Monthly Notices of the Royal Astronomical Society, The Astrophysical Journal Letters and arXiv.org outlined the salient features of this extraordinary event. Astronomers used data from the European Space Agency’s Gaia satellite. This spacecraft has been mapping the stellar content of our galaxy, recording the journeys of stars as they travel through the Milky Way. Thanks to Gaia, astronomers now know the positions and trajectories of our celestial neighbors with unprecedented accuracy.
The paths of the stars from the galactic merger earned them the moniker “the Gaia Sausage,” explained Wyn Evans of Cambridge. “We plotted the velocities of the stars, and the sausage shape just jumped out at us. 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.”
When looking at the distribution of star velocities in the Milky Way, the stars of the Sausage galaxy form a characteristic sausage-like shape. This unique shape is caused by the strong radial motions of the stars. As the sun lies in the center of this enormous cloud of stars, the distribution does not include the slowed-down stars currently making a U-turn back toward the galaxy’s center. Credit: V. Belokurov (Cambridge, UK) and Gaia/ESA
Numerical simulations of the galactic mashup can reproduce these features, said Denis Erkal of the University of Surrey. 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.
Evidence of this galactic remodeling is seen in the paths of stars inherited from the dwarf galaxy, said Alis Deason of Durham University. “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. This discovery was especially pleasing for Deason, who predicted this orbital pileup almost five years ago. The new work explains how the stars fell into such narrow orbits in the first place.
The new research also identified at least eight large, spherical clumps of stars called 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.
New Research Pinpoints Age of the Gaia Sausage
Astrophysicists at the University of Birmingham in collaboration with colleagues at European institutions in Aarhus, Bologna and Trieste, have been studying evidence of the chemical composition of stars in this area of the Milky Way to try to pinpoint more accurately the age of the smaller galaxy.
Using only the information about the chemical traces of Gaia Sausage stars coming from the international APOGEE astronomical survey, the Birmingham researchers have pinpointed more precisely the age of the galaxy. By developing detailed models of the production, or nucleosynthesis of chemical elements by all kinds of stars and supernovae in the cosmos, they estimate the Sausage was formed around 12.5bn years ago—2.5bn years older than suggested by previous estimates.
“Elements interact with light in different ways and so by studying the properties of light from the stars, we can infer the chemical make-up of those stars,” explains Fiorenzo Vincenzo, in the School of Physics and Astronomy at the University of Birmingham.
“All chemical elements heavier than helium are produced by stars via thermonuclear burning deep in the heart of the star. Different chemical elements are typically synthesized by different kinds of stars in the cosmos. The oxygen atoms that are so important for life processes, for example, were deposited in the interstellar medium by many successive generations of massive stars until they were incorporated by our planet about 4.5 billion years ago. We can measure the relative proportion of different chemical traces in the atmosphere of stars and use this measurement as a clock to determine their age.”
Calculating the ages of stars accurately is a complex process and the technique used by the Birmingham team provides one piece of the puzzle. The next step will be to cross reference the chemical data with evidence from other techniques, such as studying the relative speeds at which stars move—a project also underway at the University of Birmingham.
The merger between the two galaxies seems to have produced another effect, too. The team spotted a gap in the age distribution of stars in the Milky Way, that occurred at the same time as the merger, suggesting that the collision caused an interruption in star formation within the Milky Way.
“We speculate that the turbulence and heating caused by the merger of the Gaia Sausage with the Milky Way could have prevented the formation of stars at this time,” says Dr. Vincenzo. “However to confirm this we would need even more precise measurements of the ages of the stars in the Milky Way and in the smaller galaxy.”
The study is published in Monthly Notices of the Royal Astronomical Society and is part of the Asterochronometry project, funded by the European Research Council and led by the University of Birmingham. The main aim of the project is to pinpoint precise and accurate stellar ages—a keystone for understanding the assembly history of our galaxy.
In this study, the team focused on the chemical traces left by three elements—iron, silicon and magnesium. The next step will be to incorporate measurements from other elements to build an increasingly accurate picture.
Image at top of the page: ESA (ARTIST’S IMPRESSION AND COMPOSITION); H.H. KOPPELMAN, A. VILLALOBOS AND A. HELMI (SIMULATION); HUBBLE/ESA/NASA (GALAXY IMAGE)