Disturbed Universe

http://home.cc.umanitoba.ca/~umcoll14/minorproject.html

http://casa.colorado.edu/~danforth/science/spiral/

Arm Formation in Spiral and Barred Spiral Galaxies

By Robert Collister

Spiral and barred spiral galaxies can be loosely classified into two types based on how distinct their arms are. Galaxies with thin, well-defined spiral arms are referred to as Grand Design sprials while those with fuzzy, loosely-defined spirals are described as flocculent. Ever since Edwin Hubble first began classifying galaxies based on the appearance of their spiral arms, there has been the problem of explaining the existence of these structures.

M51 - A Grand Design Spiral NGC 7339 - A flocculent galaxy

M51 - A Grand Design Spiral NGC 7793 - A Flocculent Spiral

Early Ideas

The very first idea about how spirals arms came to be was that they simply formed like that along with the galaxy. However, this model soon was proven false due to what is called the "winding problem". Since the disk of the galaxy rotates differentially, that is all different radii have the same speed but different length paths, a radial arm would quicky curve as the galaxy rotates. As time contiues to pass this "spoke" becomes more and more tightly wound, eventually obliterating the spirals. This model for spiral arms would last only a couple rotations of the galaxy, and thus better models were needed.

winding problem diagram

The "winding problem"

winding2

Tighter winding over time

The other model that proved untenable suggested that magnetic fields were responsible for the formation of spiral arms. This idea was never very well developed and soon was shown to be false. The strength of the fields that were needed to accomplish this feat were about five times stronger than what was being observed. Nevertheless, some good came out of the analysis of this model. It was discovered that while magnetic fields were not responsible for the arms, they do follow along the arms themselves.

Current Models

Spiral galaxies come in many, many different forms. The vast differences in the shape of one spiral galaxy to the next lead to the idea that there is more than one process involved in the formation of their spiral arms. There currently are three widely accepeted mechanisms for this. They are the Density Waves model first put forth by American astronomers Chia Chiao Lin and Frank Shu in the 1960s, self-propagating star formation put forth by M. W. Mueller and W. David Arnett in 1976. The third way is that the arms are a product of the collision between two galaxies.

First I will examine the collision model for spiral arms. When two galaxies approach one another they exert incredible tidal forces upon each other. The matter in each galaxy is pull together by these forces producing a bulge along the disk and even the formation of a star bridge between the two galaxies. As the collision progresses, the motion of the galaxies is changed and after the merging or brush-by, new spiral arms remain. It is very difficult to explain but I have found a supercomputer simulation of such an interaction between two galaxies. In it, you can see the leading bridges and trailing tails of the galaxies before collision.

Galaxy Collision Simulation

During the interaction those structures aquire the rotation that develops the spiral arms. Afterwards you can clearly see the two arms rotating about the center nucleus of the merged galaxies. This process works fine for explaining how the arms are formed, but does little to explain why they continue to exist.

This brings us to the self-propagating star formation model. When the first large, hot stars formed in the galaxy, their radiation and stellar winds push against and compress nearby gas and dust, causing more star formation. When those same stars go supernova, the resulting shockwaves produce the same process. Together, these two events contribute to a continuing cycle of star formation.

Propagation

Self-Propagating Star Formation

Stars do not form behind the front as in the diagram because the gas and dust is too hot to be packed tight enough for fusion to start. Now, since the all parts of the galaxy have the same angular speed around the nucleus but the inner areas have a shorter circumference to travel, the inner edge of the star forming region advances faster than the outer edge. The result is a spiral arm formed by the bright OB stars and their nearby emission nebulae. However, due to the already described motion around the galaxy, the spiral arm quickly becomes spread out and indistinct. Furthermore, these arms come and go seemingly at random. Also of note is that there is no explanation of how there could be two or more of these self-propagating waves moving around a galaxy. Hence it is believed this model is accurate for the more irregular spirals, such as the LMC, and the more filamentary aspects of other galaxies. This model does not explain Grand Design sprials very well.

LMC

Large Magellanic Cloud

Thirdly, the Density Wave model will be discussed. The initial idea is credited to Swedish astronomer Bertil Lindblad who in the 1940s proposed that spiral arms were a pattern that moved through a galaxy like waves on a pond. Lin and Shu expanded upon this idea to produce the Density Wave model. In this, the arms are a pattern of density waves moving around the galaxy in a steady-state. While this is probably not the exact case, I will discuss the modification later.

One important aspect of the Density Wave theory is that the wave itself moves more slowly than the galaxy itself rotates; the stars and interstellar gas and dust are moving through the wave, not the wave moving through the stars, gas and dust. As the gas and dust are compressed, they form nebulae which in turn become stars. All of the OBAFGKM classes of stars are formed. However, the O and B stars have very short lifetimes and after about 3-15 million years move off the main sequence and become red giants. This means that they only travel a short distance off of the spiral arm before they pass on and explains why the regions between spiral arms are much darker than the arms themselves. The luminous OB stars and any ionized hydrogen emission nebulae near them have become extinct leaving only the much fainter stars behind that will orbit around the galactic disk.

Density Wave

Density Wave Model

This model also explains why the stars in the disk of a galaxy are largely population I stars, that is they are composed of a small amount of metals, while the galatic nucleus is formed exclusively of population II stars, that is composed solely of hydrogen and helium. The model explains this because it shows that the OB stars that have died and gone nova have scattered metals in their general area. Once this part of the galaxy that contains these metals passes through the next density wave, the metals get combined with hydrogen and helium to form new population I stars. The nuclues where the density wave is absent, does not have the level of star formation that the disk has, hence new population I stars are lacking leaving only older population II stars. Furthemore, this explains why galactic nuclei appear more red and yellow than the disk, since the stars in the nucleus are cooler stars with longer lifetimes.

Current Research

The density wave model is still undergoing development. The single largest problem with the model is that it is unlikely the density waves are a steady-state on their own. Currently research is focused on finding a driving mechanism for the density waves that keeps them moving around the galaxy. Since it requires a great deal of energy to compress such a large amount of matter, it would be expected that the density wave would eventually die out, just like the wave on a pond. So far three of these mechanisms have been suggested. One proposed mechanism involves interactions from nearby companion galaxies (such as dwarfs) that again provide the required tidal forces. Another idea is that assymetrical distribution of globular clusters and other star formations in the halo of the galaxy provide the same differing tidal forces. A third idea suggests that a barred nucleus provides enough differing tidal forces to keep the wave going. Recent high resolution observations have detected barred structures in the majority of Grand Design spirals, lending much credence to this idea.

References

Toomre, A. (1981), in The Structure and Evolution of Normal Galaxies, eds. Fall, S.M. and Lynden-Bell, D. (Cambridge University Press, London). 111-136.

Danforth, Charles. (1998) The Origins of Spiral Arms. http://www.pha.jhu.edu/~danforth/index.html

Freedman, Roger A. and Kaufmann, William J. III. (2002) Universe, 6th ed. (Library of Congress Cataloging-in-Publication Data)