The recently released map of galactic neutral hydrogen density and velocity is really beautiful. The work has been relased by the HI4PI collaboration (HI = neutral hydrogen, $4\pi$ = complete spherical coverage).
My question is fairly simple. Looking at the image near the galactic plane, why do I see near the center a green bump on the left and a blue bump on the right, then near the edges of the image a much stronger blue bump on the left and a green bump on the right. In other words, there is a major and a minor peak in the positive radial velocity gas, and a major and a minor peak in the negative radial velocity gas.
Does this have some simple geometrical explanation?
above x2: neutral hydrogen density/velocity map from here with inset indicating color scale expanded.
above: for those who can not distinguish the green and blue in the image, this plots the average green (solid) and blue (dashed) intensity within the central equatorial band.
Since the stackexchange YouTube option is not turned on in astronomy stackexchange, here is a GIF of screenshots at -30, -20, -10, 0, 10, 20, and 30 km/sec: