scholarly journals Numerical Experiments on the Response Mechanism of Barred Spirals

1983 ◽  
Vol 100 ◽  
pp. 207-208
Author(s):  
K. O. Thielheim ◽  
H. Wolff
Keyword(s):  
Numerical Experiments ◽  
Spiral Structure ◽  
Stellar Disk ◽  
Disk Galaxies ◽  
Density Waves ◽  
First Order ◽  
Spiral Density Waves ◽  
Stellar Component ◽  
Spiral Structures ◽  
Self Gravity

As a generating mechanism of spiral structure, we have recently studied the driving of density waves in the stellar component of disk galaxies by growing barlike perturbations or oval distortions. Numerical experiments (Thielheim and Wolff 1981, 1982) as well as analytical calculations using the first-order epicyclic approximation (Thielheim 1981; Thielheim and Wolff 1982) have been performed, demonstrating that this mechanism is capable of producing two-armed trailing spiral density waves in disks of noninteracting stars. These regular, global spiral structures are similar to those found in N-body experiments on self-consistent stellar disks that show bar instabilities which are weak enough to allow spiral patterns to persist (e.g., Hohl 1978; Berman and Mark 1979; Sellwood 1981). On account of this similarity, we take the view that the spiral structure observed in N-body experiments is primarily not an effect of the self-gravity of the stellar disk but a response phenomenon, caused by the formation of a weak central bar and its subsequent growth due to angular momentum extraction by interaction with the spiral as described by Lynden-Bell and Kalnajs (1972).

scholarly journals Spiral patterns beyond the optical radius: numerical simulations and synthetic HI observations

2016 ◽  
Vol 11 (S321) ◽  
pp. 81-83
Author(s):  
Sergey Khoperskov ◽  
Giuseppe Bertin
Keyword(s):  
Large Scale ◽  
Spiral Structure ◽  
Stellar Disk ◽  
Small Scale ◽  
Density Waves ◽  
Spiral Density Waves ◽  
Hydrodynamical Simulations ◽  
Lindblad Resonance ◽  
Regular Patterns ◽  
Optical Radius

AbstractThe outer parts of many galaxy disks exhibit extended spiral arms far beyond the optical radius. To understand the nature and the origin of such outer spiral structure, we investigate the propagation in the outer gaseous regions of large-scale spiral density waves excited in the bright optical disk. By means of 3D hydrodynamical simulations, we show that spiral density waves, penetrating in the gas through the outer Lindblad resonance, can indeed give rise to relatively regular patterns outside the bright optical stellar disk. The amplitude of spiral structure increases rapidly with radius. Beyond the optical radius, spirals become nonlinear and develop small-scale features related to shear-induced instabilities. We also construct the synthetic 21-cm data cubes extracted from simulated gaseous disks. Our synthetic HI observations point to the existence of specific kinematical features related to the presence of spiral pattern perturbations that should be found in deep HI observations.

Density waves in disk galaxies

1967 ◽  
Vol 31 ◽  
pp. 313-317 ◽  
Author(s):  
C. C. Lin ◽  
F. H. Shu
Keyword(s):  
Mathematical Analysis ◽  
Spiral Structure ◽  
Stellar System ◽  
Collective Modes ◽  
Disk Galaxies ◽  
Spiral Pattern ◽  
Density Waves ◽  
The Galaxy ◽  
Detailed Mathematical Analysis

Density waves in the nature of those proposed by B. Lindblad are described by detailed mathematical analysis of collective modes in a disk-like stellar system. The treatment is centered around a hypothesis of quasi-stationary spiral structure. We examine (a) the mechanism for the maintenance of this spiral pattern, and (b) its consequences on the observable features of the galaxy.

scholarly journals Selfpropagating Stochastic Star Formation and Spiral Structure

1983 ◽  
Vol 100 ◽  
pp. 141-142 ◽  
Author(s):  
J. V. Feitzinger ◽  
P. E. Seiden
Keyword(s):  
Star Formation ◽  
Spiral Structure ◽  
Relative Importance ◽  
Formation Model ◽  
Density Waves ◽  
Spiral Density Waves ◽  
Two Component ◽  
Grand Design ◽  
Dynamic Phenomena

Spiral structure in galaxies can arise from both dynamic and non dynamic phenomena: spiral density waves and stochastic selfpropagating star formation. The relative importance of these effects is still not known. Deficiences of the original selfpropagating star formation model (where only stars are taken into account) are overcome by explicitly considering the stars embedded in and interacting with a two-component gas (Seiden and Gerola, 1979; Seiden, Schulman and Feitzinger, 1982; Seiden and Gerola, 1982). The two-component gas is essential because it is the means by which we get feedback in the interaction between stars and gas. The coupling between stars and gas regulates and stabilizes star formation in a galaxy. Under proper conditions this model can give good grand design spirals (Fig. 1).

scholarly journals Outer spiral structure in disk galaxies

2016 ◽  
Vol 11 (S321) ◽  
pp. 123-123
Author(s):  
P.A. Patsis
Keyword(s):  
Periodic Orbits ◽  
Spiral Galaxies ◽  
Spiral Structure ◽  
Disk Galaxies ◽  
Spiral Arms ◽  
Chaotic Orbits ◽  
Grand Design ◽  
Pattern Speed ◽  
Spiral Structures ◽  
Barred Spiral Galaxies

AbstractIn several grand design barred-spiral galaxies it is observed a second, fainter, outer set of spiral arms. Typical examples of objects of this morphology can be considered NGC 1566 and NGC 5248. I suggest that such an overall structure can be the result of two dynamical mechanisms acting in the disc. The bar and both spiral systems rotate with the same pattern speed. The inner spiral is reinforced by regular orbits trapped around the stable, elliptical, periodic orbits of the central family, while the outer system of spiral arms is supported by chaotic orbits. Chaotic orbits are also responsible for a rhomboidal area surrounding the inner barred-spiral region. In general there is a discontinuity between the two spiral structures at the corotation region.

scholarly journals The propagation and absorption of spiral density waves

1970 ◽  
Vol 38 ◽  
pp. 323-325 ◽  
Author(s):  
F. H. Shu
Keyword(s):  
Angular Momentum ◽  
Group Velocity ◽  
Wave Energy ◽  
Radial Direction ◽  
Wave Action ◽  
Spiral Waves ◽  
Stellar Disk ◽  
Density Waves ◽  
Spiral Density Waves

An ‘anti-spiral theorem’ holds with limited validity for the neutral modes of oscillation in a stellar disk - namely, whenever the effects of stellar resonances can be ignored. In the regions between Lindblad resonances, a group of spiral waves will propagate in the radial direction with the group velocity found by Toomre. This propagation occurs with the conservation of ‘wave action’, wave energy, and wave angular momentum.

scholarly journals C. Spiral Structure

1985 ◽  
Vol 19 (1) ◽  
pp. 425-427
Keyword(s):  
Spiral Structure ◽  
Density Wave ◽  
Wave Theory ◽  
Density Waves ◽  
External Perturbations ◽  
Gravitational Theories ◽  
Recent Developments ◽  
Star Formation In Galaxies ◽  
Spiral Structures ◽  
Density Wave Theory

The spiral structure of galaxies is probably related to density waves, primarily governed by gravitational forces. Density waves may result from an inherent instability of galaxies against spiral perturbations as conceived in the conventional density-wave theory or may be forced by other internal or external perturbations of the gravitational field, such as neighbouring galaxies or oval distortions and bars in the inner regions of the galaxies. Reviews of recent developments on the various aspects of gravitational theories of spiral structure have been given by Ambastha and Varma (30.151.029), Athanassoula (33.151.051, 1984), Contopoulos (32.151.021, 34.151.103), Donner (30.151.085), Hunter (34.151.053), James and Wilkinson (29.151.023), Kalnajs (33.151.024), Kormendy (32.151.049), Lin (32.151.040, 33.151.025, 33.151.071), Lin and Bertin (30.151.068, 1984), Lin and Roberts (30.151.045), Martinet (30.151.043), McElroy (34.157.160), Norman (33. 157.088), Pasha and Tsitsin (34.151.042), Sorensen (29.151.024), Thonnard (31.158. 357), and Toomre (30.151.021). Seiden and Gerola (31.151.084) reviewed the theory of formation of spiral structures by stochastic self-propagating star formation in galaxies.

scholarly journals Dynamical Mechanisms for Discrete Unstable Spiral Modes in Galaxies

1983 ◽  
Vol 100 ◽  
pp. 119-120 ◽  
Author(s):  
Giuseppe Bertin
Keyword(s):  
Differential Rotation ◽  
Growth Rates ◽  
Spiral Structure ◽  
Long Waves ◽  
Density Waves ◽  
Physical Mechanisms ◽  
Spiral Density Waves ◽  
Propagation Properties ◽  
Normal Spiral

Progress in understanding physical mechanisms for the excitation and maintenance of spiral structure has considerably benefited from investigations of tightly wound spiral density waves (e.g., see Bertin 1980). These studies have identified the existence of four basic kinds of density waves (trailing and leading waves, and in each case short and long waves) with different propagation properties. In addition, they have led to the conclusion that some realistic galaxy models can support self-excited global normal spiral modes. These owe their maintenance to the presence of trailing waves with opposite propagation properties and are excited mostly as a result of a WASER (superreflection) mechanism at corotation. In discussing the dynamics of spiral structure and in comparing theory with observations a number of important issues should be kept in mind (Lin and Bertin 1981). Here we just recall that the calculation of spiral modes is being pursued by many researchers, using different methods. In general the structure and the growth rates of the dominant modes are determined by the radial distributions of the active disk density, the differential rotation, and the dispersion speed through the dimensionless functions εo, j, and Q (Haass, Bertin and Lin 1982).

scholarly journals Stability of a Collisionless Stellar Disk

1996 ◽  
Vol 169 ◽  
pp. 519-520
Author(s):  
E. Griv
Keyword(s):  
Asymptotic Analysis ◽  
Spiral Structure ◽  
Stellar Disk ◽  
Spiral Pattern ◽  
Density Waves ◽  
Collective Effect ◽  
Modal Theory

The study of stability of the stellar disks of flat galaxies is the first step towards an understanding of the phenomena of spiral structure. To explain the spiral pattern, Lin and Shu (1964) further developed the Lindblad's idea of density waves by considering the spiral structure as the collective effect in a self-gravitating system. As an extension to the original Lin-Shu theory, unstable spiral modes have been obtained from an asymptotic analysis (e.g., Bertin [1980]). In such “modal theory”, the Jeans-type instability (even weak) is important to the maintenance of spiral structure.

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