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.

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 Synthetic HI observations of spiral structure in the outer disk in galaxies

2015 ◽  
Vol 81 (6) ◽  
Author(s):  
Sergey A. Khoperskov ◽  
Giuseppe Bertin
Keyword(s):  
Spiral Structure ◽  
Density Wave ◽  
Stellar Disk ◽  
Spiral Pattern ◽  
Outer Disk ◽  
Wave Trains ◽  
Hydrodynamical Simulations ◽  
Low Amplitude ◽  
Lindblad Resonance ◽  
Armed Spiral

By means of 3D hydrodynamical simulations, in a separate paper we have discussed the properties of non-axisymmetric density wave trains in the outermost regions of galaxy disks, based on the picture that self-excited global spiral modes in the bright optical stellar disk are accompanied by low-amplitude short trailing wave signals outside corotation; in the gas, such wave trains can penetrate through the outer Lindblad resonance and propagate outwards, forming prominent spiral patterns. In this paper we present the synthetic 21 cm velocity maps expected from simulated models of the outer gaseous disk, focusing on the case when the disk is dominated by a two-armed spiral pattern, but considering also other more complex situations. We discuss some aspects of the spiral pattern in the gaseous periphery of galaxy disks noted in our simulations that might be interesting to compare with specific observed cases.

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.

scholarly journals On the maintenance of spiral structure

1979 ◽  
Vol 84 ◽  
pp. 151-153
Author(s):  
James W-K. Mark ◽  
Linda Sugiyama ◽  
Robert H. Berman ◽  
Giuseppe Bertin
Keyword(s):  
Spiral Structure ◽  
Travelling Waves ◽  
Integral Condition ◽  
Stimulated Emission ◽  
Stellar Disk ◽  
Wave System ◽  
Wave Amplification ◽  
Corotation Radius ◽  
The Galaxy ◽  
Central Regions

A concentrated nuclear bulge with about 30% of the galaxy mass is sufficient (Lin, 1975; Berman and Mark, 1978) to eliminate strong bar-forming instabilities which dominate the dynamics of the stellar disk. Weak bar-like or oval distortions might remain depending on the model. In such systems self-excited discrete modes give rise to global spiral patterns which are maintained in the presence of differential rotation and dissipation (cf. especially the spiral patterns in Bertin et al., 1977, 1978). These spiral modes are standing waves that are physically analyzable (Mark, 1977) into a superposition of two travelling waves propagating in opposite directions back and forth between galactic central regions and corotation (a resonator). Only a few discrete pattern frequencies are allowed. An interpretation is that the central regions and corotation radius must be sufficiently far apart so that a Bohr-Sommerfeld type of phase-integral condition is satisfied for the wave system of each mode. The temporal growth of these modes is mostly due to an effect of Wave Amplification by Stimulated Emission (of Rotating Spirals, abbrev. WASERS, cf. Mark 1976) which occurs in the vicinity of corotation. In some galaxies one mode might be predominent while other galaxies could exhibit more complicated spiral structure because several modes are present. Weak barlike or oval distortions hardly interfere with the structure of these modes. But they might nevertheless contribute partially towards strengthening the growth of one mode relative to another, as well as affecting the kinematics of the gaseous component.

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 Spiral Structure: Density Waves or Material Arms?

1983 ◽  
Vol 100 ◽  
pp. 161-162 ◽  
Author(s):  
S. V. M. Clube
Keyword(s):  
Stationary Distribution ◽  
Globular Cluster ◽  
Spiral Structure ◽  
The Sun ◽  
Density Waves ◽  
Structure Density

Recent studies (Frenk and White 1980, 1982) of the (x, y, z) and (Π, θ) distributions of the Galactic globular cluster population have given Ro = 6.8±0.8 kpc and Π = 51±26 kms−1 for the inner halo. The observations leading to these solutions are well illustrated by the ρ vs. x plot in figure 1 (Clube and Watson (CW), 1979) for the globulars with the best determined data within |l|, |b| < 20°. The independently determined Ro value clearly divides the distribution in such a way that most of the objects on the nearside of the G.C. approach the Sun while those on the farside recede, the probability that this arrangement arises by chance from a “stationary” distribution being ~ 0.002.

scholarly journals Interpretation of Observations of Spiral Structure in Terms of the Density Wave Theory

1974 ◽  
Vol 58 ◽  
pp. 399-412 ◽  
Author(s):  
Per Olof Lindblad
Keyword(s):  
Spiral Structure ◽  
Density Wave ◽  
Wave Theory ◽  
Density Waves ◽  
Density Wave Theory

A review is given of recent theoretical and observational work on the density wave theory of spiral structure. Emphasis is put on the kinematic picture, and the question whether modern observations reveal the existence of density waves is discussed.

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