New galaxy simulations shining light on stars

Although a galaxy like the Milky Way is made up of many hundreds of billions of stars, the life and death of a single star can impact its shape. But how does it happen? Oscar Agertz in Lund is using new kinds of models to try to link the large-scale galaxy-forming process to the small-scale star-forming one.

Oscar Agertz

PhD Theoretical Physics

Wallenberg Academy Fellow 2016

Institution:
Lund University

Research field:
Galaxy formation

Stellar feedback is the name given to the phenomenon by which individual stars impact their surroundings – and may even influence the shape of their host galaxy. A good example is when a massive star dies, ending its life as a supernova.

A supernova is an explosion that expels huge quantities of energy and matter into surrounding space. This may cause galactic winds that push out so much matter from the galaxy that its size and shape are affected.

Astronomers researching into galaxy formation have found that if they do not take the energy of supernovae into account in their models, the end result will be wrong. One of those researchers is Oscar Agertz, working at the Department of Astronomy and Theoretical Physics at Lund University.

“How important is stellar feedback in small and large galaxies? Perhaps it was most important when the universe was young, or it might be more important today. We don’t know. Nor is it certain that supernovae contribute most – it might be something else. We astronomers need more detailed models of this to explain the observations we make,” comments Agertz.

What is known is that without the impact of supernovae, galaxies would be smaller and have greater mass than they do. Stars would also be formed quite differently from the way astronomers believe they are formed, based on their observations.

Heavy stars live for only a few million or tens of millions of years. When they become supernovae, and expel matter and energy from the galaxy, they impact the next generation; new stars cannot form as quickly.

Simulation using zoom combining small and large

Agertz, who is a Wallenberg Academy Fellow, is developing new types of models and simulations linking the small scale, in which an individual star is formed, with the large, galactic scale.

In practice, these are two separate fields of astronomy. Researchers into star formation often incorporate simplified, approximate evolution of the entire galaxy in their models. Other researchers, specialized in galaxy formation, adopt an equally approximate approach to the life and death of individual stars.

In recent years researchers in the two fields have collaborated more closely, although it is still a major challenge to reconcile the two scales. An unbelievable amount of computing power would be needed if a digital galaxy formation model were to incorporate all the individual processes occurring in billions of stars. Not even the supercomputers that Agertz has at his disposal could manage it. But he has an idea about how to approach the problem.

“I want to create a simulation of an entire galaxy, using a kind of zoom technology that makes it possible to study star formation in certain regions. It’s like pointing a magnifying glass only at some parts of the galaxy, where stars and the galaxy can be seen to evolve exactly as they should,” Agertz explains.

In the simulation the supercomputer itself can work out what is happening on the basis of various assumptions. But it is not powerful enough to work out the composition and evolution of individual stars. Agertz has to borrow that information from experts in stellar evolution, and feed the data into the model.

He plans to use information both from galactic areas where many stars have formed, and from those with few stars. This will enable him to compare the two situations to learn more about how stars impact their surroundings.

“Being chosen as a Wallenberg Academy Fellow is a form of recognition, and has a huge impact on the prospects of a career path in Sweden. I feel the Foundation is looking after me well, and is deeply committed. It’s a luxury to have the backing of such a large private research funding source. It offers potential available in scarcely any other country.”

Astronomy – combining mathematics with beauty

Looking back, Agertz recalls how much fun he had with Space Lego when he was six or seven years old. But it was not until he was in his teens that he realized that some people actually had the job of exploring space.

“I read Stephen Hawking’s book A Brief History of Time. And a researcher from Linköping University visited our school in Eksjö to talk about solar winds. I was so excited when I realized that research could actually be a job.”

Agertz studied physics at the University of Gothenburg, but graduated at a time when there were not many Swedish doctoral positions in astronomy. So he moved to Zurich, received a PhD in physics, and began delving into supercomputer simulations. This was followed by research positions in Chicago and Surrey in the U.K., before he and his wife moved back to Sweden. The Wallenberg Academy Fellow grant took them first to Stockholm University, then to Lund.

“My work involves a combination of highly rigorous mathematics, and visually beautiful processes. I make films of my simulations on the computer. When I watch the film I make discoveries that I can use as the basis for further computations. And I also get to travel the world in my job, and meet loads of interesting people. Having ‘the world as your oyster’ may sound like a cliché, but that’s actually how it is. It’s enormously stimulating and enjoyable.”

Text Lisa Kirsebom
Translation Maxwell Arding
Photo Magnus Bergström