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Barred galaxies
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Title: Rings and spirals in barred galaxies. I Building blocks
Authors: E. Athanassoula, M. Romero-Gomez, J. J. Masdemont
(Version v2)

In this paper we present building blocks which can explain the formation and properties both of spirals and of inner and outer rings in barred galaxies. We first briefly summarise the main results of the full theoretical description we have given elsewhere, presenting them in a more physical way, aimed to an understanding without the requirement of extended knowledge of dynamical systems or of orbital structure. We introduce in this manner the notion of manifolds, which can be thought of as tubes guiding the orbits. The dynamics of these manifolds can govern the properties of spirals and of inner and outer rings in barred galaxies. We find that the bar strength affects how unstable the L1 and L2 Lagrangian points are, the motion within the 5A5A5Amanifold tubes and the time necessary for particles in a manifold to make a complete turn around the galactic centre. We also show that the strength of the bar, or, to be more precise, of the non-axisymmetric forcing at and somewhat beyond the corotation region, determines the resulting morphology. Thus, less strong bars give rise to R1 rings or pseudorings, while stronger bars drive R2, R1R2 and spiral morphologies. We examine the morphology as a function of the main parameters of the bar and present descriptive two dimensional plots to that avail. We also derive how the manifold morphologies and properties are modified if the L1 and L2 Lagrangian points become stable. Finally, we discuss how dissipation affects the manifold properties and compare the manifolds in gas-like and in stellar cases. Comparison with observations, as well as clear predictions to be tested by observations will be given in an accompanying paper.

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RE: Spiral galaxies
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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: Spitzer's View of Edge-on Spirals
Authors: B. W. Holwerda, R. S. de Jong, A. Seth, J. J. Dalcanton, M. Regan, E. Bell, S. Bianchi

Edge-on spiral galaxies offer a unique perspective on disks. One can accurately determine the height distribution of stars and ISM and the line-of-sight integration allows for the study of faint structures. The Spitzer IRAC camera is an ideal instrument to study both the ISM and stellar structure in nearby galaxies; two of its channels trace the old stellar disk with little extinction and the 8 micron channel is dominated by the smallest dust grains (Polycyclic Aromatic Hydrocarbons, PAHs).
Dalcanton et al. (2004) probed the link between the appearance of dust lanes and the disk stability. In a sample of bulge-less disks they show how in massive disks the ISM collapses into the characteristic thin dust lane. Less massive disks are gravitationally stable and their dust morphology is fractured. The transition occurs at 120 km/s for bulgeless disks.
Here we report on our results of our Spitzer/IRAC survey of nearby edge-on spirals and its first results on the NIR Tully-Fischer relation, and ISM disk stability.

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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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Title: The formation of spiral arms and rings in barred galaxies
Authors: M. Romero-Gomez, E. Athanassoula, J.J. Masdemont, C. Garcia-Gomez

We propose a new theory to explain the formation of spiral arms and of all types of outer rings in barred galaxies. We have extended and applied the technique used in celestial mechanics to compute transfer orbits. Thus, our theory is based on the chaotic orbital motion driven by the invariant manifolds associated to the periodic orbits around the hyperbolic equilibrium points. In particular, spiral arms and outer rings are related to the presence of heteroclinic or homoclinic orbits. Thus, R1 rings are associated to the presence of heteroclinic orbits, while R1R2 rings are associated to the presence of homoclinic orbits. Spiral arms and R2 rings, however, appear when there exist neither heteroclinic nor homoclinic orbits. We examine the parameter space of three realistic, yet simple, barred galaxy models and discuss the formation of the different morphologies according to the properties of the galaxy model. The different morphologies arise from differences in the dynamical parameters of the galaxy.

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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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Posts: 131433
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Edge-on Spirals
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Title: Spitzer's View of Edge-on Spirals
Authors: B. W. Holwerda, R. S. de Jong, A. Seth, J. J. Dalcanton, M. Regan, E. Bell, S. Bianchi

Edge-on spiral galaxies offer a unique perspective on disks. One can accurately determine the height distribution of stars and ISM and the line-of-sight integration allows for the study of faint structures. The Spitzer IRAC camera is an ideal instrument to study both the ISM and stellar structure in nearby galaxies; two of its channels trace the old stellar disk with little extinction and the 8 micron channel is dominated by the smallest dust grains (Polycyclic Aromatic Hydrocarbons, PAHs). Dalcanton et al. (2004) probed the link between the appearance of dust lanes and the disk stability. In a sample of bulge-less disks they show how in massive disks the ISM collapses into the characteristic thin dust lane. Less massive disks are gravitationally stable and their dust morphology is fractured. The transition occurs at 120 km/s for bulgeless disks. Here we report on our results of our Spitzer/IRAC survey of nearby edge-on spirals and its first results on the NIR Tully-Fischer relation, and ISM disk stability.

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Galaxy features
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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

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RE: Spiral galaxies
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Title: The formation of spiral arms and rings in barred galaxies
Authors: M. Romero-Gomez, E. Athanassoula, J.J. Masdemont, C. Garcia-Gomez

In this and in a previous paper (Romero-Gomez et al. 2006) 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. The results presented here will be discussed and compared with observations in a forthcoming paper.

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This montage of galaxies M61, NGC 4449, NGC 4725, NGC 5068, NGC 5247, and NGC 5775/5774 (from top left to bottom right) are spiral galaxies like our own Milky Way.


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The colour images reveal distinct pink patches marking the glowing hydrogen gas clouds in star forming regions along the graceful spiral arms.
While Virgo cluster galaxy M61 is perhaps the most striking of these spirals, the interesting galaxy pair NGC 5775/5774 neatly contrasts the characteristic spiral edge-on and face-on appearance.

The one exception to this parade of photogenic spiral galaxies is the small and relatively close irregular galaxy NGC 4449 (top middle). Similar to the Large Magellanic Cloud, companion galaxy to the Milky Way, NGC 4449 also sports young blue star clusters and pink star forming regions.
All the galaxies were imaged with a small (16 inch diameter) reflecting telescope and digital camera by public participants in the Kitt Peak National Observatory Visitor Centre’s Advanced Observing Program.


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