How our Milky Way grew after Big Bang

Astronomers have uncovered, for the first time, how our Milky Way galaxy formed and evolved over nearly 14 billion years of the Universe’s life. They were able to reproduce the pattern that allowed this city of hundreds of billions of stars to develop from the moment it began to form after the Big Bang.

The barred spiral galaxy NGC 1300, similar to the Milky Way, pictured by the Hubble Space Telescope. Credit: ESA/NASA/Hubble Heritage

The international team, led by astrophysicists at the UK’s University of Central Lancashire (UCLan) claim that it is an achievement that has eluded scientists until now.

Previous attempts to model this type of galaxy – a typical barred spiral – have failed to reproduce the result that actually exists today. They ususally produced too many stars, all in a tight ball.

Professor Brad Gibson and Dr Chris Brook, of UCLan’s Jeremiah Horrocks Institute, worked for four years on the research project with the catchy name of MaGICC – Making Galaxies in a Cosmological Context.

Their findings, produced with colleagues in Germany and North America, are published August 2012 issue of the Monthly Notices of the Royal Astronomical Society. The team say they could potentially change scientific thinking in one of the most fascinating fields in astronomy.

Professor Gibson said: “For more than 30 years, repeated attempts to model the formation of a galaxy like the Milky Way, with sophisticated programming aids and high-performance computers, have failed. While spiral galaxies could be generated with such tools, they all uniformly refused to cooperate and look like the Milky Way.”

UCLan’s video modelling development of the Milky Way. Credit: UCLan

The main problems revolved around the formation of far too many stars, and those stars which did form, seemingly always wanting to form in the wrong place. Specifically, they tended to form a massive ball of stars surrounding a fairly ‘stubby’ star disk. In reality, the Milky Way has a glorious extended disk, with only a small fraction of its light surrounding this disk of stars and gas.

The UCLan’s breakthrough came after they included in their calculations a figure for the amount of energy released by exploding stars that was 10 to 100 times greater than any previously used. This at last gave them a galaxy that began to resemble the Milky Way. What previously appeared like a blob of stars, with a little disk inside it, slowly disappeared, to be replaced by the opposite. It was exactly what they wanted to find.

Dr Brook said: “From our analysis we realised that all attempts were being far too conservative in their energy usage from very massive stars – those 10 to 100 times more massive than the Sun. When massive stars ‘die’ they explode spectacularly and return most of the chemical elements out of which us, our planet, and our solar system formed, as well as an enormous amount of energy into the surrounding gas. We decided to be more aggressive in our simulations of the energy released by both living and dying massive stars.”

Professor Gibson explained: “We experimented over a two-year period with a range of energy efficiencies and found that when we locked in on a high efficiency value, not only did the galaxies ‘look’ real, their mass, size, colour, and chemistry were also a natural fit. We have spent several months now trying to ‘break’ the simulations and find relationships for which they clearly fail, but so far we have not been able to do so.”

The results of the UCLan team’s research has enormous significance for the European Space Agency’s Gaia satellite mission which has been designed to mine the fossil record of the Milky Way on a star-by-star basis.

Professor Gibson added: “The physics driving the formation and evolution of galaxies is of huge importance in the scientific research journey to understanding our origins. By demonstrating that we finally have been able to simulate such systems, we can enter the Gaia era with a greater degree of confidence in our ability to maximise the science and data exploitation of the mission.”

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