Biggest Ever Simulations Help Uncover The History Of The Galaxy

Credit: Robert J. J. Grand, Facundo A. Gomez, Federico Marinacci, Ruediger Pakmor, Volker Springel, David J. R. Campbell, Carlos S. Frenk, Adrian Jenkins and Simon D. M. White. Click for a full size imageAstronomers study our own and other galaxies with telescopes and simulations, in an effort to piece together their structure and history.

Spiral galaxies like the Milky Way are thought to contain several hundred thousand million stars, as well as copious amounts of gas and dust.

The spiral shape is commonplace, with a massive black hole at the centre, surrounded by a bulge of old stars, and arms winding outwards where relatively young stars like the Sun are found.

A composite image of the stellar distribution shown face-on (top) and edge-on (bottom) at present day. Older stars are redder and younger stars are bluer. Note the concentration of old stars that make up the bulge in the centre. Younger stars make up a thin disc with clear spiral structure, and extend far beyond the central bulge. 
Credit: Robert J. J. Grand, Facundo A. Gomez, Federico Marinacci, Ruediger Pakmor, Volker Springel, David J. R. Campbell, Carlos S. Frenk, Adrian Jenkins and Simon D. M. White.

However understanding how systems like our galaxy came into being continues to remain a key question in the history of the cosmos.

The enormous range of scales (stars, the building blocks of galaxies, are each about one trillion times smaller in mass than the galaxy they make up), as well as the complex physics involved, presents a formidable challenge for any computer model.

Using the Hornet and SuperMUC supercomputers in Germany and a state-of-the-art code, the team ran 30 simulations at high resolution, and 6 at very high resolution, for several months.

The dark matter density 500 million years after the Big Bang, centred on what would become the Milky Way. Red, blue and yellow colours indicate low, intermediate and high density regions.

Credit: Robert J. J. Grand, Facundo A. Gomez, Federico Marinacci, Ruediger Pakmor, Volker Springel, David J. R. Campbell, Carlos S. Frenk, Adrian Jenkins and Simon D. M. White.

The code includes one of the most comprehensive physics models to date. It includes phenomena such as gravity, star formation, hydrodynamics of gas, supernova explosions, and for the first time the magnetic fields that permeate the interstellar medium (the gas and dust between the stars).

Black holes also grew in the simulation, feeding on the gas around them, and releasing energy into the wider galaxy.

Dr Grand and his team were delighted by the results of the simulation. “The outcome of the Auriga Project is that astronomers will now be able to use our work to access a wealth of information, such as the properties of the satellite galaxies and the very old stars found in the halo that surrounds the galaxy.”

The gas column density 800 Million years after the Big Bang. Lighter colours indicate higher density. The image is centred on the Milky Way. Many individual galaxies line the ‘cosmic web’ as they move toward each other.

Credit: Robert J. J. Grand, Facundo A. Gomez, Federico Marinacci, Ruediger Pakmor, Volker Springel, David J. R. Campbell, Carlos S. Frenk, Adrian Jenkins and Simon D. M. White.

The team also see the effect of those smaller galaxies, in some cases spiralling into the larger galaxy early in its history, in a process that could have created large spiral discs.

Dr Grand adds: “For a spiral galaxy to grow in size, it needs a substantial supply of fresh star-forming gas around its edges – smaller gas-rich galaxies that spiral gently into ours can provide exactly that.”

The gas column density 2.4 Billion years after the Big Bang. Lighter colours indicate higher density. The image is centred on the Milky Way. Galaxies have merged in the last 2 Billion years to form a few, larger galaxies. These are the progenitors of the Milky Way. 

Credit: Robert J. J. Grand, Facundo A. Gomez, Federico Marinacci, Ruediger Pakmor, Volker Springel, David J. R. Campbell, Carlos S. Frenk, Adrian Jenkins and Simon D. M. White.

The scientists will now combine the results of the Auriga Project work with data in surveys from observatories like the Gaia mission, to better understand how mergers and collisions shaped galaxies like our own.

The magnetic field strength in the present day. Streamlines indicate the direction of the magnetic field lines. 

Credit: Robert J. J. Grand, Facundo A. Gomez, Federico Marinacci, Ruediger Pakmor, Volker Springel, David J. R. Campbell, Carlos S. Frenk, Adrian Jenkins and Simon D. M. White.

Contacts and sources:
Dr Robert Massey
Royal Astronomical Society

Dr Morgan Hollis
Royal Astronomical Society
 
Prof Robert Grand
Heidelberger Institut fuer Theoretische Studien, Germany

Citation:  “The Auriga Project: the properties and formation mechanisms of disc galaxies across cosmic time”, Monthly Notices of the Royal Astronomical Society, vol. 467, pp. 179-207. A copy of the paper is available at no cost from https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stx071