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Using Evidence of Chemical Composition of Stars in Milky Way to Accurately Pinpoint Gaia Sausage’s Age

New information related to a galaxy called the Gaia Sausage are being unraveled by using chemical traces in the atmospheres of stars. Billions of years ago, this galaxy was involved in a huge collision with the Milky Way galaxy.

(Image credit: University of Birmingham)

Astrophysicists from the University of Birmingham have collaborated with coworkers at European institutions in Aarhus, Bologna, and Trieste to analyze the evidence of the chemical composition of stars in this region of the Milky Way to make attempts to identify the age of the smaller galaxy more accurately.

An international team identified the Gaia Sausage last year using information from the European Space Agency’s Gaia satellite. It is considered that its merger with the Milky Way, predicted to have taken place nearly 10 billion years ago, contributed to the shape of the Milky Way as it is observed at present.

The Birmingham researchers used only the information related to the chemical traces of Gaia Sausage stars obtained from the international APOGEE astronomical survey to more precisely pinpoint the age of the galaxy. They developed elaborate models for the production or nucleosynthesis of chemical elements by stars and supernovae of all types in the cosmos and predicted that the Sausage was formed nearly 12.5 billion years ago—2.5 billion years older than proposed by earlier prediction.

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. 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 synthesised 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.

Dr Fiorenzo Vincenzo, School of Physics and Astronomy, University of Birmingham

It is a complicated process to accurately calculate the ages of stars, and the method adopted by the Birmingham researchers offers one piece of the puzzle. The subsequent step will be to cross reference the chemical data with evidence from other methods, for example, analyzing the relative speeds at which stars move—a project that is ongoing at the University of Birmingham.

The merger of the two galaxies appears to have created another effect as well. The researchers located a gap in the age distribution of stars in the Milky Way galaxy, which occurred at the same instance as the merger, indicating that the collision led to an interruption in star formation inside the Milky Way galaxy.

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. 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.

Dr Fiorenzo Vincenzo, School of Physics and Astronomy, University of Birmingham

The research has been reported in Monthly Notices of the Royal Astronomical Society and is part of the Asterochronometry project, financially supported by the European Research Council and headed by the University of Birmingham. The principal objective of the study is to identify accurate and precise stellar ages—a mainstay for gaining insights into the assembly history of the Milky Way galaxy.

As part of this research, the focus of the astrophysicists was on the chemical traces left behind by three elements—silicon, iron, and magnesium. The future step will be to include measurements from other elements to paint a highly accurate picture.

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