Supernova Winds Shape Galaxies

The Crab Nebula is a pulsar wind nebula associated with the 1054 supernova.From Discovery News:

The explosions of massive stars may be responsible for the formation of galaxies.

After years of struggling to understand how to properly assemble a galaxy, astronomers have discovered that the answer is blowin’ in the wind. The supernova wind, that is.

New computer simulations show that winds generated by supernovas, which are the explosions of massive stars, can push stars out from the center of a dwarf galaxy. This simulation of supernova winds redistributes both ordinary matter and invisible dark matter in a way that almost perfectly matches observations of the way matter is distributed in actual dwarf galaxies. Fabio Governato of the University of Washington in Seattle and his colleagues describe their simulations in the Jan. 14 Nature.

Previous attempts to model galaxy formation based on the highly successful theory of cold dark matter — which states that invisible material must account for 85 percent of the mass of the universe — have done “an awesome job” of explaining such global properties as where, when and how many galaxies should form, notes Governato. But the models have failed to reproduce some of the key features of individual galaxies.

In particular, those simulations have produced galaxies whose centers are stuffed with too much dark matter and that are surrounded by a spherical distribution of stars that actual dwarf galaxies don’t possess. Dwarf galaxies, which are low-mass bodies with relatively uniform distributions of stars, are the most common type of galaxy in the neighborhood of the Milky Way.

Most of these earlier models included either a simplistic description of star formation or neglected star formation altogether. “Our simulations achieve the necessary resolution to follow the formation of individual star forming regions — dense clouds of gas containing the equivalent of 100,000 suns,” says Governato. Star formation is concentrated in the center of a galaxy, and because massive stars live for only a short time, they go supernova in the same region where they were born. As a result, supernova winds are also concentrated in galaxy centers.

Governato’s team showed that the supernova winds are intense enough to push both stars and star-forming clouds out of a dwarf galaxy’s core. Dark matter responds to gravity but is impervious to the winds. As the stars exit the core, the dark matter there feels a smaller gravitational tug and expands outward.

In one fell swoop, the model’s successful simulation of supernova winds not only reduces the density of the dark matter at the core but also does away with the spherical distribution of stars around the core, matching the properties of actual dwarf galaxies, Governato says…

[continues at Discovery News]