The Milky Way and the Sausage

Where did our Galaxy come from? Over the last half a century, astronomers have realised that our Milky Way was seeded in the initial instants of the universe and has grown over the action of gravity ever since. Recent observations of a single star, one that is visible to the naked eye, has dated one of the most prominent events of accretion, known as the Gaia-Enceladus-Sausage, to the early life of the Milky Way.

Large galaxies like ours grow over cosmic time, accreting smaller galaxies that get torn apart due to Milky Way’s tidal forces. The Sagittarius Dwarf galaxy, discovered in the early 1990s, is a relatively recent accretion, has had many of its stars stripped away into immense tidal tails that circle the Milky Way. And it appears that the Magellanic Clouds will eventually share a similar fate.

As they become completely disrupted, older accretion events are harder to spot, with no distinguishing galaxy remaining in the sky. However, these ancient signatures can be found if you can chart out stellar velocities, as individual accretions can appear as clumps in “phase-space”, the combination of the distribution of stars through space and their speeds. These accretions can also mess up the stars in the Milky Way disk, leaving their own scars in phase-space.

The Milky Way Galaxy forms part of a Local Group of galaxies within about a 10-million light-year diameter. The cluster features about 54 galaxies all of which are centred around two major clusters - the first with the Milky Way and its surrounding satellite galaxies, and the other around the Andromeda Galaxy - some 2.5 million light-years away.

Both the Milky Way and Andromeda are large spiral galaxies, where most of the other galaxies in the Local Group are smaller dwarf galaxies (there is another spiral, albeit much smaller, known as the Triangulum Galaxy). 

The Milky Way cluster of galaxies and Andromeda cluster of galaxies are racing towards each other at approximately 440,000 km/h, due to their mutual gravitation, with the two heavyweights set to collide in about 4.5 billion years from now. When this occurs, both galaxies will forever be disturbed, warped and changed. Whilst this collision of billions of stars might sound like there will be violent crashing of stellar objects, the space between the stars in both galaxies is so big, that no two stars will “collide”. It’s a different story for the gas and supermassive black hole in both galaxies, though - where more interactivity will occur during the collision of these objects.

Instead, the interaction between both galaxies will cause the velocity of stars to radically change - in some cases, some stars will be ejected entirely from both galaxies. In other cases, the star will take on new speed and direction, from its original orbit.

Launched in 2013 by the European Space Agency (ESA), the goals of the GAIA space-observatory is to measure the distances, positions, and motion of the Milky Way’s stars. Over the course of its mission - expected to last until 2022, GAIA will observe each of its targets (a collection of stars, comets, planets, asteroids, etc.) approximately 70 times - collecting data about objects in unprecedented precision. 

So far, the mission has released two data packages, DR1 which covered the first 14-months of the mission through to September 2015, and DR2, which spanned July 2014  through to May 2016, unleashing a range of new discoveries, such as:

• Precise description of the highly elliptical orbital motion of the Sculptor Dwarf Galaxy with respect to the Milky Way
• Discovery and catalogue of 20 hyper-velocity stars in the Milky Way
• The discovery of a new galaxy, known as Antila 2 - which is roughly the size of our neighbouring Large Magellanic Cloud Galaxy, only many times fainter

Using the Gaia satellite, astrophysicists have also measured accurate velocities of many hundreds of thousands of stars and searched for the signature of ancient accretions onto the Milky Way - galactic collisions from times gone past, finding a prominent clumping quite distinct from the usual orbits of stars in the Galaxy.

Uncovered by several groups, this feature is known as the Gaia-Enceladus-Sausage, the “Sausage” referring to its apparent shape in phase-space is a result of one of these ancient mergers. Scientists believe this event added approximately 50 billion solar masses of stars, gas and dark matter to the Milky Way Galaxy, in addition to at least eight Globular Clusters.

But when in the long history of the Milky Way was the Sausage accreted?

This is where the single star, nu Indi, makes its entrance. As this star is nearby, only 95 light-years from Earth, astronomers have been able to use another space-based observatory, known as Transiting Exoplanet Survey Satellite (TESS) to obtain extremely accurate measurements of its brightness over time. Like all stars, nu Indi wobbles due immense waves of sound bouncing around inside, driven by the flow of heat from the stellar core. 

Just like quakes on Earth reveal the internal composition of the planet, quakes on stars reveal their internal make-up. As the structure of a star changes as it ages, as hydrogen is burnt into helium and then heavier elements, astronomers can use this “asteroseismology” accurately to age a star. And for nu Indi, they found that it was 11 billion years old, give or take a billion years. 

Intriguingly, both the chemistry and motion of nu Indi suggest that it is related to Gaia-Enceladus-Sausage. Astronomers concluded that it is not a star that was brought to the Milky Way by the accretion, but it is probably a star whose orbit was distorted by the galactic collision or possibly formed in the stellar turmoil. With this crucial information, the accretion of Gaia-Enceladus-Sausage would have begun about 12 billion years ago, occurring in the earliest life of the Milky Way.

This had a major impact on the Milky Way’s structure, puffing up a proportion of the stars in the stellar disk into a thick disk that still exists today, as well as dumping many stars into the Galactic Stellar Halo. 

Galactic collisions like this also have the ability to trigger new star formation, as gravitational disturbances cause giant gas reserved to contract into stellar nurseries, along with stars that contain a variety of different chemical compositions being introduced into the Milky Way.

Looking back at the history of galactic evolution through collisions and mergers, it’s fairly common to see the interaction of these events that play out over timescales in the millions of years, and distances that are incomprehensible to our everyday thinking. The tell-tale signature of these collisions has been observed and documented in all stages of galactic archeology - through methods like asteroseismology with  stars like nu Indi, or observations of merging galaxies from the Universe’s distant past, and yet there is so much more to learn about these events.

And after more than ten billion years, our Galaxy still bears these scars. 

Youtube video credit: ESA