A Rotation Curve Is a Plot of the Rotational Velocity

Multiple Choice

A rotation curve is a plot of the rotational velocity of a galaxy as a function of the distance away from the galactic center. Predictions for rotation curves for spiral galaxies suggest that the curve should decline beyond the galactic bulge. Actual measurements, however, show that the rotation curve is flat beyond the bulge. What does this deviation from prediction state about spiral galaxies, including the Milky Way?

A) Spiral galaxies are not rotating as rapidly as previously believed.
B) The spiral arms move together as a rigid object.
C) There is a great deal of unseen mass surrounding the visible galaxy.
D) The centers of spiral galaxies contain supermassive black holes.