Elementary Process of Galactic Spiral Arm Formation: Phase Synchronization of Epicyclic Motion by Gravitational Scattering

Aside from the well-known spiral arm tracers such as the OB associations, young galactic clusters, WR stars and possibly the long-period classical cepheids, the more common stars in the neighborhood of the sun within 2 kpc show little or no relationship to the local spiral structure of the galaxy.

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Spiral-arm instability: giant clump formation via fragmentation of a galactic spiral arm

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stx2978 ◽

2017 ◽

Vol 474 (3) ◽

pp. 3466-3487 ◽

Cited By ~ 9

Author(s):

Shigeki Inoue ◽

Naoki Yoshida

Keyword(s):

Galactic Spiral ◽

Spiral Arm

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Galactic spiral structure

Proceedings of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rspa.2009.0036 ◽

2009 ◽

Vol 465 (2111) ◽

pp. 3425-3446 ◽

Cited By ~ 9

Author(s):

Charles Francis ◽

Erik Anderson

Keyword(s):

Pitch Angle ◽

Milky Way ◽

Spiral Galaxies ◽

Stellar Orbits ◽

Grand Design ◽

Galactic Spiral Structure ◽

Pattern Speed ◽

Galactic Spiral ◽

Almost All ◽

Spiral Arm

We describe the structure and composition of six major stellar streams in a population of 20 574 local stars in the New Hipparcos Reduction with known radial velocities. We find that, once fast moving stars are excluded, almost all stars belong to one of these streams. The results of our investigation have led us to re-examine the hydrogen maps of the Milky Way, from which we identify the possibility of a symmetric two-armed spiral with half the conventionally accepted pitch angle. We describe a model of spiral arm motions that matches the observed velocities and compositions of the six major streams, as well as the observed velocities of the Hyades and Praesepe clusters at the extreme of the Hyades stream. We model stellar orbits as perturbed ellipses aligned at a focus in coordinates rotating at the rate of precession of apocentre. Stars join a spiral arm just before apocentre, follow the arm for more than half an orbit, and leave the arm soon after pericentre. Spiral pattern speed equals the mean rate of precession of apocentre. Spiral arms are shown to be stable configurations of stellar orbits, up to the formation of a bar and/or ring. Pitch angle is directly related to the distribution of orbital eccentricities in a given spiral galaxy. We show how spiral galaxies can evolve to form bars and rings. We show that orbits of gas clouds are stable only in bisymmetric spirals. We conclude that spiral galaxies evolve toward grand design two-armed spirals. We infer from the velocity distributions that the Milky Way evolved into this form about 9 billion years ago (Ga).

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On Passing of Interstellar Clouds Through a Galactic Spiral Arm

Mathematical Physics and Computer Simulation ◽

10.15688/mpcm.jvolsu.2020.2.4 ◽

2020 ◽

pp. 41-56

Author(s):

Alexander Zemlyakov ◽

Mikhail Eremin ◽

Ilya Kovalenko ◽

Elena Zhukova

Keyword(s):

Galactic Disk ◽

Vertical Direction ◽

Interstellar Gas ◽

Hydrodynamic Simulation ◽

Interstellar Clouds ◽

Coriolis Forces ◽

Adiabatic Flow ◽

Low Mass ◽

Galactic Spiral ◽

Spiral Arm

It is believed that the taxonomy of interstellar clouds in their vicinity can serve as an indicator of the features of the geometry and intensity of galactic shock waves. In this paper, the authors present the results of a detailed two-dimensional hydrodynamic simulation of the passage of a cloud through the spiral arm of a galaxy and provide a brief analysis of the effects arising from this motion. The model of interstellar gas used assumes adiabatic flow in the spiral arm. The external gravitational field of the galactic disk and spiral arm is taken into account. The transverse dimensions of the arm in the calculations are taken as follows: the half-width of the arm is 1 kpc along the plane of the disk and 0.6 kpc in the vertical direction. A fragment of the flow is considered near and inside the spiral arm, the effects of the curvature of the arm and the influence of the Coriolis forces are neglected. It is shown that clouds passing through the arm are strongly deformed and lose a significant part of the mass or are completely destroyed in the case of low-mass clouds. The boundary value of the cloud mass at which complete destruction occurs lies in the interval between 3 000 and 6 000 M.

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Substructure within galactic spiral arms as derived from studies at 21 cm

Symposium - International Astronomical Union ◽

10.1017/s0074180900051147 ◽

1959 ◽

Vol 9 ◽

pp. 355-359

Author(s):

R. D. Davies

Keyword(s):

Fine Structure ◽

High Resolution ◽

Velocity Dispersion ◽

Detailed Structure ◽

Hydrogen Line ◽

Spiral Arms ◽

Frequency Structure ◽

Galactic Spiral ◽

Spiral Arm

Detailed structure within the spiral arms of our Galaxy is suggested by hydrogen-line spectra taken with high resolution in frequency [1]. The spectra show much detail in each maximum (spiral arm). It is not clear, however, if this frequency structure refers to fine structure in depth or in velocity dispersion or in both. Fine structure in position and depth has been inferred from 21-cm drift curves taken across the nearby spiral arms. The results of three investigations will be discussed. Two have been published in some detail [2, 3] and will only be summarized here.

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Cold H I in turbulent eddies and galactic spiral shocks

Proceedings of the International Astronomical Union ◽

10.1017/s1743921307001731 ◽

2006 ◽

Vol 2 (S237) ◽

pp. 363-367 ◽

Cited By ~ 2

Author(s):

Steven J. Gibson ◽

A. Russell Taylor ◽

Jeroen M. Stil ◽

Christopher M. Brunt ◽

Dain W. Kavars ◽

...

Keyword(s):

Large Scale ◽

Galactic Disk ◽

Line Emission ◽

Molecular State ◽

Small Scale ◽

Turbulent Eddies ◽

Background Line ◽

Galactic Spiral ◽

Co Emission ◽

Spiral Arm

AbstractH I 21cm-line self-absorption (HISA) reveals the shape and distribution of cold atomic clouds in the Galactic disk. Many of these clouds lack corresponding CO emission, despite being colder than purely atomic gas in equilibrium models. HISA requires background line emission at the same velocity, hence mechanisms that can produce such backgrounds. Weak, small-scale, and widespread absorption is likely to arise from turbulent eddies, while strong, large-scale absorption appears organized in cloud complexes along spiral arm shocks. In the latter, the gas may be evolving from an atomic to a molecular state prior to star formation, which would account for the incomplete HISA-CO agreement.

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