1. Introduction

Spiral galaxies represent about 60% of bright galaxies in the local universe. Their disk shape with wound arms is one of the most recognizable structures in the cosmos. But these arms are not fixed ribbons of matter: they are density waves propagating through the galactic disk.

2. Structure of a Spiral Galaxy

A typical spiral galaxy comprises a spheroidal central bulge of old stars, a thin disk of stars, gas and dust, spiral arms where star formation concentrates, a spherical halo of old stars and globular clusters, and a dark matter halo extending well beyond the visible disk.

3. Density Wave Theory

Lin and Shu (1964) proposed that spiral arms are quasi-stationary density waves. Stars and gas pass through the arms like cars passing through a slowdown on a highway: they slow in the dense region, temporarily accumulate there, then move on. The arm itself moves at an angular velocity different from that of the stars.

This theory explains why spiral arms are regions of intense star formation: gas compressed while crossing the density wave reaches the Jeans threshold and collapses into stars.

4. Dark Matter and Rotation Curves

If a galaxy contained only visible matter, distant disk stars should orbit more slowly than stars near the center (like planets in the solar system). Yet observations show flat rotation curves: orbital velocity remains constant to the edges of the visible disk. This implies the existence of a halo of non-luminous matter — dark matter — whose mass dominates that of the disk.

5. The Milky Way

The Milky Way is a barred spiral galaxy (SBbc) about 100,000 light-years in diameter. It has 4 main arms: Perseus, Scutum-Centaurus, Sagittarius and Norma. The Sun is located in a minor arm, the Orion Arm, about 26,000 light-years from the galactic center. It takes about 225 million years to complete one revolution.

6. Conclusion

The spiral arms of galaxies are not fixed material structures but dynamic density waves. Their existence, shape and persistence depend on gravity, differential rotation of the disk and the dominant presence of dark matter. The galactic spiral is a dynamic equilibrium solution at the scale of hundreds of thousands of light-years.

References

  1. [1]
    Einstein, Albert (1915). Die Feldgleichungen der Gravitation. Sitzungsberichte der Preussischen Akademie der Wissenschaften, p. 844–847
  2. [2]
    Lin, C. C., Shu, F. H. (1964). On the Spiral Structure of Disk Galaxies. The Astrophysical Journal, 140, p. 646–655. DOI: 10.1086/147955
  3. [3]
    Hubble, Edwin P. (1926). Extragalactic Nebulae. The Astrophysical Journal, 64, p. 321–369. DOI: 10.1086/143018
  4. [4]
    de Vaucouleurs, Gérard (1959). Classification and Morphology of External Galaxies. Handbuch der Physik, 53, p. 275–310
  5. [5]
    Binney, James, Tremaine, Scott (2008). Galactic Dynamics. Princeton University Press. ISBN: 978-0-691-13026-2
  6. [6]
    Bertin, Giuseppe, Lin, C. C. (1996). Spiral Structure in Galaxies: A Density Wave Theory. MIT Press. ISBN: 978-0-262-02396-2
  7. [7]
    Shu, Frank H. (1982). The Physical Universe: An Introduction to Astronomy. University Science Books. ISBN: 978-0-935702-05-7
  8. [8]
    Grand, R. J. J., Kawata, D., Cropper, M. (2012). Spiral Arm Formation in Live Dark Matter Halo Simulations. Monthly Notices of the Royal Astronomical Society, 421(2), p. 1529–1538. DOI: 10.1111/j.1365-2966.2012.20411.x
  9. [9]
    Dobbs, C. L., Baba, J. (2014). Dawes Review 4: Spiral Structures in Disc Galaxies. Publications of the Astronomical Society of Australia, 31, p. e035. DOI: 10.1017/pasa.2014.31
  10. [10]
    Kepler, Johannes (1619). Harmonices Mundi. Gottfried Tampach
  11. [11]
    Newton, Isaac (1687). Philosophiæ Naturalis Principia Mathematica. Royal Society
  12. [12]
    Contopoulos, George (2002). Order and Chaos in Dynamical Astronomy. Springer-Verlag. DOI: 10.1007/978-3-662-04917-4. ISBN: 978-3-540-43360-0
  13. [13]
    Toomre, Alar (1964). On the Gravitational Stability of a Disk of Stars. The Astrophysical Journal, 139, p. 1217–1238. DOI: 10.1086/147861
  14. [14]
    Zwicky, Fritz (1933). Die Rotverschiebung von extragalaktischen Nebeln. Helvetica Physica Acta, 6, p. 110–127
  15. [15]
    Sagan, Carl (1980). Cosmos. Random House. ISBN: 978-0-394-50294-6
  16. [16]
    Penrose, Roger (2004). The Road to Reality: A Complete Guide to the Laws of the Universe. Jonathan Cape. ISBN: 978-0-224-04447-9
  17. [17]
    Poincaré, Henri (1892). Les Méthodes nouvelles de la mécanique céleste. Gauthier-Villars
  18. [18]
    Penrose, Roger, Rindler, Wolfgang (1984). Spinors and Space-Time, Vol. 1. Cambridge University Press. DOI: 10.1017/CBO9780511564048. ISBN: 978-0-521-33707-6
  19. [19]
    Mihos, J. Christopher, Hernquist, Lars (1996). Gasdynamics and Starbursts in Major Mergers. The Astrophysical Journal, 464, p. 641–663. DOI: 10.1086/177353
  20. [20]
    Springel, Volker, White, Simon D. M., Jenkins, Adrian (2005). Simulations of the Formation, Evolution and Clustering of Galaxies and Quasars. Nature, 435, p. 629–636. DOI: 10.1038/nature03597
  21. [21]
    Rubin, Vera C., Ford, W. Kent Jr. (1970). Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions. The Astrophysical Journal, 159, p. 379–403. DOI: 10.1086/150317
  22. [22]
    Oort, Jan H. (1977). The Galactic Center. Annual Review of Astronomy and Astrophysics, 15, p. 295–362. DOI: 10.1146/annurev.aa.15.090177.001455
  23. [23]
    Elmegreen, Bruce G., Elmegreen, Debra Meloy (1987). Arm Classifications for Spiral Galaxies. The Astrophysical Journal, 314, p. 3–9. DOI: 10.1086/165034