Optical tracers of spiral structure

When we look at spiral galaxies we see that the spiral arms are not continuous bands of light. Rather, they appear to contain knots of bright stars and glowing gas. For example, it appears that HII regions and OB associations trace out spiral arms in other galaxies. For this reason, we have tried to see if the HII regions we can see optically in our galaxy form any distinct pattern. By using optical observations we can rely on distances determined by spectroscopic parallax for the stars exciting the HII regions. Similarly, we can also look at the distribution of OB associations. A drawback is that, with optical observations, we cannot see very far along the plane of the galaxy. When we study the distribution of HII regions and OB associations, it is clear that the placement is not entirely random. We seem to see at least pieces of connected chains of HII regions and OB associations. These pieces have been identified as a series of named "arms", identified by the constellation in which they are most prominent. This is all a tantalizing hint of spiral structure, but is not a definitive picture.

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