Astronomers Develop New Method To Describe Galaxy Features

Dr. Ronald J. Buta, professor of astronomy at UA, and Dr. Xiaolei Zhang, of the Naval Research Laboratory, Washington, D.C., co-authored the paper detailing the new method of characterizing density wave features in galaxies. Density waves are mass enhancements in galaxies that appear in the forms of spiral arms, linear bar features, and ring-shaped patterns. Orbiting stars and gas clouds stream in and out of these features much like vehicles in heavy traffic.

Density waves occur within different regions of a galaxy's disk and often appear as intricately nested segments of patterns. Each segment rotates rigidly around the galaxy center with a fixed angular velocity, or “pattern speed,” and each has a “corotation” radius where the angular orbital speeds of stars and gas clouds equals the pattern speed.

Using near-infrared light as a mass-density tracer, the new method allows the corotations of the wave patterns to be determined via calculating the gravitational potential field produced by the patterns. Once located, the corotations can be compared with the structure of a galaxy and correlated with observed features. From analysis of many images, Zhang and Buta concluded that observed spiral, bar, and ring patterns are density wave modes (natural oscillations of a stellar disk) capable of influencing a galaxy over a long period of time.

Zhang and Buta also confirmed that a previously proposed internal physical process termed “secular dynamical evolution,” which is driven by these density waves, can significantly transform the shapes of galaxies over their lifetime. A phase shift between the stellar mass in the density wave patterns and the gravitational field of those patterns is at the heart of the process. Although the process is slow, it can produce significant changes over the 14 billion year age of the universe, including the buildup of a central bulge. This provides an important link to understanding how galaxies in the universe were formed and how they evolve.