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Supernovae missing piece in galaxy formation theories
Posted: 14 January 2010

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New supercomputer simulations of galaxy formation show that supernova explosions blasting away gas from nearby galaxies explains the deficit of stars in dwarf galaxies that cannot be explained in the standard cold dark matter theory.

These images depict various stages of galaxy formation under the cold dark matter theory using new computer simulations that account for the effects of supernova explosions. Image: Katy Brooks.

The cold dark matter theory, which holds that around 75 percent of the Universe's matter is made of mysterious, invisible dark matter, has long been used to explain how the initially smooth Universe born 13.7 billion years ago in the big bang has evolved into a galaxy-rich cosmic web, with small structures forming first and evolving into progressively larger structure, but this theory is unable to explain why there are less stars and dark matter in the cores of galaxies than is actually observed. The problem is particularly notable for the most common type of galaxy found in our celestial neighbourhood: dwarf galaxies.

New supercomputer simulations led by the University of Washington have provided a solution to this cosmic conundrum. “Most previous work included only a simple description of how and where stars formed within galaxies, or neglected star formation altogether,” says Fabio Governato, lead author of the paper describing the findings that features in today's edition of the journal Nature. “Instead we performed new computer simulations, run over several national supercomputing facilities, and included a better description of where and how star formation happens in galaxies.”

Computer simulation of billions of years of galaxy formation, illustrating the role of cold dark matter. Image: Fabio Governato.

The simulations demonstrate the importance of considering supernova explosions of the most massive early stars in galaxy formation theories. The blast waves from the explosions generate winds that sweep vast amounts of gas away from centre of what are destined to become dwarf galaxies, preventing star formation there. The knock on effect is that the removed mass means there is less gravitational pull on the dark matter there, and the dark matter drifts away.

“The cold dark matter theory works amazingly well at telling where, when and how many galaxies should form,” adds Governato. “What we did was find a better description of processes that we know happen in the real Universe, resulting in more accurate simulations.”

The results of the new simulations closely match the observed low-density cores seen in dwarf galaxies.