galaxy formation2

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

WASHINGTON (US)—Using millions of hours on supercomputers, researchers have run simulations of galaxy formation and produced dwarf galaxies very much like those observed today by satellites and large telescopes around the world.

The results address a decades-old problem related to the cold dark matter theory, used by cosmologists to explain how the smooth universe born in the big bang more than 13 billion years ago evolved into the filamentary, galaxy-rich cosmic web that we see today.

The theory suggested most galaxies should have far more stars and dark matter at their cores than they actually do. The problem is most pronounced for dwarf galaxies, the most common galaxies in our own celestial neighborhood. Each contains less than 1 percent of the stars found in large galaxies such as the Milky Way.

“Most previous work included only a simple description of how and where stars formed within galaxies, or neglected star formation altogether,” says Fabio Governato, a University of Washington astronomer and lead author of the Nature paper. “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.”

A computer simulation of billions of years of galaxy formation, illustrating the role of cold dark matter. (Credit: Fabio Governato/Univeristy of Washington).

The simulations showed that as the most massive new stars exploded as supernovas, the blasts generated enormous winds that swept huge amounts of gas away from the center of what would become dwarf galaxies, preventing millions of new stars from forming.

With so much mass suddenly removed from the center of the galaxy, the pull of gravity on the dark matter there is diminished and the dark matter drifts away, Governato says. It is similar to what would happen if our sun suddenly disappeared and the loss of its gravitational pull allowed the Earth to drift off into space.

The cosmic explosions proved to be the missing piece of the puzzle, and adding them to the simulations generated formation of galaxies with substantially lower densities at their cores, closely matching the observed properties of dwarf galaxies.

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

The theory of cold dark matter, first advanced in the mid 1980s, holds that the vast majority of the matter in the universe—as much as 75 percent—is made up of “dark” material that does not interact with electrons and protons and so cannot be observed from electromagnetic radiation. The term “cold” means that immediately following the big bang these dark matter particles have speeds far lower than the speed of light.

In the cold dark matter theory, smaller structures form first, then they merge with each other to form more massive halos, and finally galaxies form within the halos.

Researchers from the Jeremiah Horrocks Institute in the United Kingdom; Institut für Astronomie and the Institute for Theoretical Physics in Switzerland; California Institute of Technology; University of Nevada; McMaster University in Canada; University of California, Santa Cruz; and Haverford College contributed to the work.

The research was funded by NASA and the National Science Foundation.

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