could the "arms" of a galaxy be refered to as lines of gravitation?

Answer 1

From what I undestand they are density waves. It's like if traffic is going 65mph on the freeway but a few cars decide to go 40mph, everyone has to slow down in order to pass them. As people pass the slowpokes, they speed up again to 65 mph. The slow cars create a clump of other cars also forced to go slow, and this clump is a density wave. Cars pass in and out of this slow clump but the clump remains. Spiral arms in a galaxy behave in a similar way but with gas instead of cars. Faster moving nebula crash into slower moving nebula, causing new stars to form. It is this new star formation that makes up the spiral arms we see in galaxies. I hope this answers your question.

Answer 2

Matter and gravity are a manifestation of space.
I suppose you could refer to them as lines of gravitation.

Answer 3

The real gravity well is the black hole in the center which might generate a gravity well 10 to 100 light years across.