Using computer simulations, Thomas Zeltwanger and Neil F. Comins at the University of Maine, Orono, ME, and Richard V.E. Lovelace at Cornell University, Ithaca, NY, have reproduced the shapes and various details of off-center spiral galaxies created when galaxies rapidly pass each other. The motions of the stars and gas in their model support the belief that lopsided spiral galaxies are sloshing about in halos of dark matter. By studying the behavior of such galaxies, researchers hope to help determine the nature of these as-yet unseen galactic halos. Lopsided spiral galaxies have spiral arms and interstellar gas and dust that are not distributed uniformly around the bright center, or nuclear bulge, of the galaxy.
Galactic halos are distributions of dark matter in which galaxies are embedded.
Observations reveal that up to 90 percent of a typical galaxy's mass is in its halo. Astronomers infer the existence of halos by their observed gravitational effects on visible matter, but the nature of the halo matter is still under investigation. Observations reveal that a third of all spiral galaxies may have lopsided spiral structures. Dark matter that is relatively hot moves rapidly and therefore responds relatively little to the gravitational tug of a passing galaxy. Since stars and cool gas in a galaxy respond more strongly than the halo, these latter elements are pulled slightly away from the halo as the other galaxy passes.