How is it possible to have red shift along one axis and blue shift on another in a spiral galaxy arm?

user5389726598465 05/09/2018. 0 answers, 47 views
doppler-effect galaxy-rotation-curve

I don't understand this image:enter image description hereIf the stars are moving in roughly circular orbits why would red shift occur in the direction of the orbit and how would it be possible to have blue shift in two opposing directions?

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