[anomaloustho]

I went to give a similar answer and realized that - in the instance that the two galaxies are sideways facing each other and with the text saying “it gets brighter” not “one side gets brighter” - I’m not sure if this is the actual answer.

I can’t figure out why the Doppler Shift would make the entire galaxy brighter. I assumed it would make the rotation side spinning towards us brighter. But also redshift the side spinning away.