What do we mean when we say that the rotation curve for a spiral galaxy is "flat"? what do we mean when we say that the rotation curve for a spiral galaxy is "flat"? the disk of a spiral galaxy is quite flat rather than spherical like the halo. gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward. the amount of light emitted by stars at different distances is about the same throughout the galaxy. all the galaxy's mass is concentrated in its flat, gaseous disk?