Stellar Spectra reveal Milky Way Merger with Extragalactic Sausage

Australian astrophysicists and their international collaborators have obtained new evidence of galactic collisions which might help us understand the formation and early development of our home galaxy, the Milky Way. They analysed the light from over 600,000 stars to study their chemical compositions and determine which stars formed locally and which came from encounters with our galactic neighbours.

“The Milky Way ate up lots of smaller galaxies but, until recently, we did not have enough evidence of that to say for sure,” said leader author Sven Buder, an observational astrophysicist from the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and the Australian National University (ANU). “That’s because simple images of stars in our Milky Way look the same – whether they were born inside the galaxy or outside and then blended into the galaxy.”  

Our next major galactic interaction, in about 4.5 billion years, will be the merger of our Milky Way galaxy with the Andromeda galaxy, the closest large spiral galaxy to ours roughly 2.5 million light-years away. But there’s no need to start panicking or packing your suitcase. Galactic collisions are quite common in our Universe and are actually far less violent events than people might think.

You may be wondering how it’s possible for two massive objects like galaxies, each containing hundreds of billions of stars, to collide non-violently. While it’s not immediately obvious, the answer is a relatively simple one: Space is huge! It’s also very sparsely populated since most stars, which are around a million kilometres in diameter, are on average 100 billion kilometres away from one another. For scale, if stars were the size of golf balls they would be about 3 kilometres apart. This means that when galaxies collide there is a vanishingly small chance of stellar collisions, let alone planets from the different galaxies coming anywhere near one another.

The Milky Way is no stranger to this kind of intergalactic rendezvous and astronomers have even found evidence of dwarf satellite galaxies - smaller galaxies that orbit around larger galaxies like the Milky Way - that have made multiple trips through our galaxy throughout its roughly 13 billion year lifetime. These interactions create gravitational disturbances which can increase the density of dust and gas in some areas, leading to the creation of nebulae and the birth of new stars.  

Besides boosting star formation, such a collision can radically impact the balance of gravitational forces that determine the large-scale structure of a galaxy and often result in the collection or accretion of massive star systems from the passing dwarf galaxy.

One prominent example of this sort of interaction, which Buder’s group identified as an early building block of the Milky Way, is the Gaia-Sausage-Enceladus (GSE). The GSE, which are the remnants of a dwarf galaxy that merged with the Milky Way around 10 billion years ago, gets its name from the European Space Agency’s Gaia satellite survey and the elongated region that the stars occupy in velocity plot; a diagram that shows how fast the stars are orbiting the galactic centre and how fast they are moving towards or away from it, as measured by Gaia.

The study used the HERMES (High Efficiency and Resolution Multi-Element Spectrograph) instrument, which is situated on the Anglo-Australian Telescope (AAT) at Siding Spring Observatory, to split the light from each star into a rainbow or spectrum of different wavelengths, and understand which chemical elements and compounds were more or less abundant in a particular star.  

“If an image is worth a thousand words, these spectra are worth more than a thousand pictures,” said Buder. “By ‘scanning’ these stellar barcodes, we measured how abundant 30 elements, such as sodium, iron, magnesium, and manganese, were, and how they appeared in different concentrations depending on where the star was born.”

The group’s identification of the unique chemical markers in GSE stars, which they published in the Monthly Notices of the Royal Astronomical Society, confirmed the information was used in conjunction with Gaia data to confirm their extragalactic origins.  

“The Milky Way spread out across the night sky is a familiar sight, and when we look at it, we are actually gazing into the centre of our galaxy with its billions of stars,” says Buder. “But we are looking at two populations of stars, one much older than the other. The old stars have moved so they look like they bulge out of the main plane of the Milky Way, while the younger stars form a much thinner band in the plane.”