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What is the origin of spiral structure in galaxies?

What is origin of spiral gravitational waves in a spiral galaxy? I read that close to the very core of a spiral galaxy, star density could be so high that stars orbital motions were somehow linked by mutual gravity forces, leading to slow down inner stars and to accelerate outer stars in violation of Kepler's laws. Is this spiral gravitationnal wave some kind of consequence of this violation ? Thanks for trying to give me some light !

This is a very interesting question and it's something that extragalactic astronomers have thought about quite a lot. Spiral arms are seen in about two thirds of the brightest 1500 galaxies (as seen from Earth) and we ourselves live in a spiral galaxy, so it must be something very generic that causes it.

It turns out that if you assume that the arms are material (i.e. made up of stars that always remain in the arm and are just denser than the stars outside the arm) then very quickly the arms would wind up as the galaxy rotates (well quickly compared to the age of the universe!). So the process which make spiral arms must maintain them too (or they would disappear).

There are also different types of spiral structure. Some galaxies (like M51 below) are what we call "Grand Design" spirals, meaning that they have a clearly outlined and well organised spiral structure. Other galaxies, like NGC 4414) are called "flocculent" spirals and have much harder to trace arms. The arms can also be more or less tightly wound and some galaxies with spiral arms have bars (see NGC 1300) while others (lie M51) don't. Any theory of how spiral structures has to take this all into account.

M51: The Whirlpool Galaxy

There are actually two popular theories, one of which is more commonly used to explain grand design spirals, the other for flocculent spirals.

  • Density Wave theory.
    This is the preferred model for grand design spirals. The spiral arms in this model are over dense regions of the disk which move round at a different speed to the stars themselves. Stars thus move in and out of the spiral arm (which fits in nicely with ideas of there being more star formation in the arms since many galaxies are observed to have more new stars in the place where the arm should just have moved through). How these density waves are set up is unclear, but it may have to do with interactions (many grand design spirals have smaller companions - just like M51). Once they are set up they can last for a long enough time to be consistent with the number of spiral galaxies we see.
  • Stochastic Self-Propagative Star Formation
    This model probably cannot explain grand design sprials, but it might be something like what causes flocculent spiral structure. Basically what it says is that if you have star formation which triggers star formation in areas adjacent to it (which is not an unrealistic idea) then as the galaxy rotates, this "self-propagative star formation" will lead to the appearance of a spiral pattern. The "stochastic" part is because there is also a small probability of random star formation in all areas in the disk which keeps things going. In a computer simulation this does infact create reasonable flocculent spirals.

September 2002, Karen Masters (more by Karen Masters) (Like this Answer)

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