New Way of Describing How Galaxies Formed and Changed Over Time

Recently, a group of astronomers developed a new method of describing galaxy features, which could provide scientists with new information on how exactly galaxies formed and changed over time in different parts of the known Universe.

Dr. Xiaolei Zhang, of the Naval Research Laboratory, Washington, D.C., together with Dr. Ronald J. Buta, professor of astronomy at the University of Alabama, published a paper in which they describe how density wave features can be characterized in galaxies.

The density wave theory, developed in the 1960s, explains the spiral and bar structures of observed disk galaxies as the long-lived quasistatic density waves, which are sections of the galactic disk that have a mass density about 20% greater than the overall density of the galaxy. Some galaxies, for example NGC 1530, could contain strong bars and spirals
that have density enhancements up to 70%, Dr. Zhang explained for Softpedia.

The density wave arms themselves are not made always of the same material, but rather like the situation of the traffic jam on a busy highway, they are made of orbiting stars and gas clouds that stream in and out of these arms. The gravitational attraction between stars is what actually keeps the spiral pattern coherence.

These density waves appear in different regions of the galaxy's disk in the form of complex segmented patterns, each rotating rigidly around the center of the galaxy at a fixed angular speed, called pattern speed. Where the angular orbital speeds of stars and gas clouds equal the pattern speeds of the density waves are termed the corotation radii.

Drs. Zhang and Buta used near-infrared light to trace the mass density and proved that the new method could determine the corotation radii, through calculating the gravitational potential field produced by the patterns and observed the manner of the azimuth phase shift between the calculated potential and the original density traced by the infrared light.

After analyzing many images, Zhang and Buta came to the conclusion that observed spiral, bar and ring patterns are density wave modes (natural oscillations of a stellar disk) that are quasi-stationary. They also showed that these density waves are capable of inducing significant morphological changes in a galaxy over a long period of time.

The mass accretion process induced by the density waves may be apparently slow, but over long time span (on the order of the age of the universe) can significantly increase the size of a galaxy's bulge. "In certain cluster galaxies where the gravitational tidal interaction excites large amplitude density wave patterns, the processs appears to be responsible for transforming the galaxy morphology from a disk-dominated one into a bulge-dominated one within the past 5 billion
years," Dr. Zhang told Softpedia.

This provides an important link to our understanding how galaxies in the universe were formed and how they evolve over their lifetime.