[Lewis Thomson (OP)]

Steve submitted this question to the Naked Scientists for answers.

"In pictures of galaxies the red shift and blue shift visually appear to hover over the galaxy arms by an estimated 10-20 light years, indicating other than direct emission from the arms.
and the question is: as the red/blue shifts are on opposite sides above, could they also be below the galaxy arms, as in blue over blue, red over red, or blue over red and red over blue?, keeping things in (electromagnetic) balance?"

Discuss in the comments below...

[Origin]

"In pictures of galaxies the red shift and blue shift visually appear to hover over the galaxy arms by an estimated 10-20 light years, indicating other than direct emission from the arms.

I don't think that is accurate.  I certainly have not seen that and I can't think of any reason that would be true.

[Origin]

indicating other than direct emission from the arms.
and the question is: as the red/blue shifts are on opposite sides above, could they also be below the galaxy arms, as in blue over blue, red over red, or blue over red and red over blue?, keeping things in (electromagnetic) balance?"

Red and blue shift in a galaxy would have to do with the emission of photons from the stars in the galaxy.  Due to the rotation of the galaxy, half of the galaxy is moving towards us (blue shifted) and the other half is moving away from us (red shifted).  The effect is very small.  I assume any picture of this that you saw was an enhanced picture to show the effect.  The red and blue shift have nothing to do with an electromagnetic balance, it is driven only by the motion between the observer and the source.

[evan_au]

I agree with Origin - the question appears to be about galaxy rotation curves.

In a spiral galaxy (like our Milky Way), most of the visible matter is in a flat disk, rotating in a certain direction, and one side will be red-shifted, and the other side blue-shifted
- It was anomalies in this galaxy rotation curve that hinted at the presence of Dark Matter
- But to see this red/blue shift due to galaxy rotation, you have to subtract the overall red or blue shift due to the whole galaxy moving towards or away from us (the relatively nearby Andromeda galaxy is moving towards us; distant galaxies are moving away from us)
- In spiral galaxies, there are globular clusters orbiting above and below the visible disk. These are thought to be remnants of smaller galaxies, and will also show an individual red or blue shift, but it's not the more organized movement of the flat disk.

There are also elliptical galaxies, which don't show the overall ordered movement of a flat spiral galaxy. These are thought to arise from the collision of two large galaxies, which disrupts the orderly motion of a spiral galaxy.

https://en.wikipedia.org/wiki/Galaxy_rotation_curve