* Astronomy

By analysing a sample of 200 galaxies from the early universe, astrophysicists from CNRS have reconstructed the history of the formation of the large spiral galaxies. They grew by collisions and successive fusions of smaller galaxies, during which many stars were born. But, it was thought that this mechanism had essentially ceased about 8 billion years ago. However, the research published in la revue Astronomy&Astrophysics, show that the galaxies continued to grow, giving rise to half of the current stars.

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Title: The formation of spiral arms and rings in barred galaxies
Authors: M. Romero-Gomez

We propose a theory to explain the formation of both spirals and rings in barred galaxies using a common dynamical framework. It is based on the orbital motion driven by the unstable equilibrium points of the rotating bar potential. Thus, spirals, rings and pseudo-rings are related to the invariant manifolds associated to the periodic orbits around these equilibrium points. We examine the parameter space of three barred galaxy models and discuss the formation of the different morphological structures according to the properties of the bar model. We also study the influence of the shape of the rotation curve in the outer parts, by making families of models with rising, flat or falling rotation curves in the outer parts. The differences between spiral and ringed structures arise from differences in the dynamical parameters of the host galaxies.

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In the early 1900s, Edwin Hubble made the startling discovery that our Milky Way galaxy is not alone. It is just one of many galaxies, or "island universes," as Hubble dubbed them, swimming in the sea of space.
Now, a century later, NASA's Galaxy Evolution Explorer is helping piece together the evolution of these cosmic species. Since its launch in 2003, the mission has surveyed tens of thousands of galaxies in ultraviolet light across nine billion years of time. The results provide new, comprehensive evidence for the "nurture" theory of galaxy evolution, which holds that the galaxies first described by Hubble the elegant spirals and blob-like ellipticals are evolutionarily linked.

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A University of Alabama astronomer has co-developed a new way to characterise 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 University of Alabama, and Dr. Xiaolei Zhang, of the Naval Research Laboratory, Washington, D.C., co-authored the paper detailing the new method of characterising 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 galaxys disk and often appear as intricately nested segments of patterns. Each segment rotates rigidly around the galaxy centre 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.

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-- Edited by Blobrana at 23:19, 2007-07-25