U.Alabama Astronomer Co-Develops New Method to Describe Galaxy Features (Forwarded)

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Interview source:
Dr. Ronald Buta, 205/348-3792

July 9, 2007

UA Astronomer Co-Develops New Method to Describe Galaxy Features

TUSCALOOSA, Ala. -- A University of Alabama astronomer has co-developed a
new way to characterize galaxy features that is giving scientists additional
insight into how galaxies formed and changed over time, according to a paper
published in the June 2007 issue of The Astronomical Journal.

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.