Mystery Behind Galaxy Shapes Solved
ESO's Wide Field Imager has captured the intricate swirls of the spiral galaxy Messier 83, a smaller look-alike of our own Milky Way. Shining with the light of billions of stars and the ruby red glow of hydrogen gas, it is a beautiful example of a barred spiral galaxy, whose shape has led to it being nicknamed the Southern Pinwheel.
Credit: ESO

Galaxies come in many shapes and sizes, but until recently astronomers have been at a loss to explain why.

Now scientists have used dark matter theory to predict the menagerie of galaxies found in the universe. Their new model reproduces 13 billion years? worth of cosmic evolution, resulting in a surprisingly accurate tally of the different kinds of galaxies we see.

"We were completely astonished that our model predicted both the abundance and diversity of galaxy types so precisely," said researcher Nick Devereux of Embry-Riddle University in Arizona.

American astronomer Edwin Hubble first developed a classification system in the 1930s, known as the Hubble Sequence, which divides galaxies into two main types: spirals, and ellipticals.

Elliptical galaxies look like eggs of light ? a central, solid nest of stars. Spirals, on the other hand, are the stereotypical swirling galaxies that many people think of, and include our own Milky Way and our closest neighbor, Andromeda. Spiral galaxies come in two kinds ? with and without a bar of thick material in the center, from which the spiral arms wind out. (The Milky Way is a barred spiral.)

Researchers created a new supercomputer model, based on observational data and the "Lambda Cold Dark Matter" theory of the universe. This theory suggests that about 72 percent of the cosmos is made of up a mysterious force called dark energy, while another 23 percent is composed of an invisible type of matter called dark matter. That leaves only 4 percent of the universe made of normal, visible matter, including all the stars and planets that we see.

The new supercomputer model was able to predict roughly the right relative numbers of spirals and ellipticals that exist today.

"It really boosts my confidence in the model," said astronomer Andrew Benson of Caltech, a co-researcher in the study.

The researchers said the inclusion of dark matter is likely crucial to their results, because theory predicts that galaxies sit inside larger spheres, called halos, of the invisible stuff. The behavior of a galaxy's dark matter halo could affect its evolution and help determine whether it becomes a spiral or an elliptical, they added.

"These new findings set a clear direction for future research," Devereux said. "Our goal now is to compare the model predictions with observations of more distant galaxies seen in images obtained with the Hubble [Space Telescope] and those of the soon-to-be-launched James Webb Space Telescope."

The findings were published in the journal Monthly Notices of the Royal Astronomical Society.

  • Image Gallery: Amazing Galaxies
  • Video: Spiral Galaxy Evolution
  • Image Gallery: Andromeda Galaxy