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What is a spiral arm?

Submitted by Kevin Jardine on 7 June, 2013 - 08:08

Many galactic astronomers use the term "arm" in an inconsistent and confusing way. As a good example, this image of the Milky Way on the Chandra website shows eight arms. In a more recent example, this press release promotes the (in my view) ill-advised term "Local Arm" for the structure this website calls the "Orion Spur".

This confusion is becoming a greater problem as we get closer to the great dream of mapping the Milky Way. For example, it is increasingly the case that the question "How many spiral arms does the Milky Way have?" has no clear answer not because we lack data, but because astronomers disagree over what an arm is, or use the term imprecisely.

So today I'm proposing a definition:

A spiral arm is a large scale structure of atomic hydrogen that:

  • lies outside the galactic centre region,
  • is warmer and denser than its surroundings, and
  • wraps around the galactic centre for more than 180°.

In an astronomical context we can use the word "arm" as a short form of "spiral arm" when no confusion will result. We should not use the word "arm" to refer to any other galactic structure than a "spiral arm".

The emphasis on atomic hydrogen makes sense to me because atomic hydrogen is the basic building block of all galaxies and including the reference to atomic hydrogen avoids arguments that a spiral arm is not an arm because it lacks other tracers like molecular gas, red giants or HII regions.

The definition precludes smaller structures that may exist in the galactic bar and the near and far 3kpc "arms" ("ring" or "ring structures" may be better terms for what has been called the 3kpc "arms").

By this definition, the evidence suggests that the Milky Way has four spiral arms.

Also by this definition the Orion Spur does not appear to be an arm. Although it is a large structure, it is found (so far as we currently know) entirely or almost entirely on the near side of the galaxy. The concept of spiral in a spiral arm implies that it wraps around the galactic centre and the Orion Spur does not appear to do this. I also think that it is confusing in an astronomical context to use the term "arm" when we do not mean "spiral arm".

(Whether "Orion Spur" is an appropriate name for this structure is a different question which I'll address in a future blog post.)