In Publications of the Astronomical Society of Australia, Volume 31, 2014.
http://arxiv.org/abs/1407.5062

For a PDF version of the article, click here.

DAWES REVIEW 4: SPIRAL STRUCTURES IN DISC GALAXIES

Clare Dobbs 1 and Junichi Baba 2


1 School of Physics and Astronomy, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK
2 Earth-Life Science Institute, Tokyo Institute of Technology 2-12-1-I2-44 Ookayama, Meguro, Tokyo 152-8551, Japan


Abstract: The majority of astrophysics involves the study of spiral galaxies, and stars and planets within them, but how spiral arms in galaxies form and evolve is still a fundamental problem. Major progress in this field was made primarily in the 1960s, and early 1970s, but since then there has been no comprehensive update on the state of the field. In this review, we discuss the progress in theory, and in particular numerical calculations, which unlike in the 1960s and 1970s, are now commonplace, as well as recent observational developments. We set out the current status for different scenarios for spiral arm formation, the nature of the spiral arms they induce, and the consequences for gas dynamics and star formation in different types of spiral galaxies. We argue that, with the possible exception of barred galaxies, spiral arms are transient, recurrent and initiated by swing amplified instabilities in the disc. We suppose that unbarred m = 2 spiral patterns are induced by tidal interactions, and slowly wind up over time. However the mechanism for generating spiral structure does not appear to have significant consequences for star formation in galaxies.


Keywords: galaxies: spiral, galaxies: kinematics and dynamics, galaxies: structure, galaxies: star formation

The Dawes Reviews are substantial reviews of topical areas in astronomy, published by authors of international standing at the invitation of the PASA Editorial Board. The reviews recognise William Dawes (1762-1836), second lieutenant in the Royal Marines and the astronomer on the First Fleet. Dawes was not only an accomplished astronomer, but spoke five languages, had a keen interest in botany, mineralogy, engineering, cartography and music, compiled the first Aboriginal-English dictionary, and was an outspoken opponent of slavery.


Table of Contents

INTRODUCTION
Historical overview

GENERATION OF SPIRAL STRUCTURE
Quasi-stationary density wave theory
Dispersion relations of tight-winding density waves
Propagation of tight-winding density waves
Swing amplification
Global mode theory
Simulations of long-lived spiral patterns
Dynamic spirals
Swing amplified spirals
Corotation scattering and radial migration of stars
Recurrent mechanisms for dynamic stellar spiral arms
Bar driven spirals
Spirals corotating with bars
Decoupling between spirals and bars
Non-linear coupling between spirals and bars
Non-stationary spiral arms in barred galaxies
Tidal interactions
Historical overview
Tidally induced arms: stationary, kinematic or material arms?
Prograde and retrograde encounters, and the orbit of the perturber
Longevity of tidally-induced spirals
Stochastic star formation
Exotic mechanisms

BEHAVIOUR OF GAS IN SPIRAL GALAXIES
Stability of a star and gas disc
Damping of spiral arms
Physical processes in the ISM
Substructure along spiral arms
Quasi-stationary density waves
Resonances
Stability and structure of the shock
Tidally induced spiral arms
Gas flow in dynamic spiral arms

OBSERVATIONAL EVIDENCE FOR DIFFERENT MECHANISMS OF GENERATING SPIRAL STRUCTURE
Pattern speeds
Pitch angles
Stellar cluster ages
Resonances and interference patterns
Observations of grand design and flocculent structure
The Milky Way
Spiral arm triggering of star formation

SUMMARY AND DISCUSSION
Quasi-stationary density wave theory
Dynamic spirals
Tidal interactions
Bar driven spirals
Other mechanisms
Observations

REFERENCES

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