galaxy rotation curve

Imagine standing at the center of a spinning merry-go-round and asking how fast each horse is moving. A galaxy's rotation curve answers the same question for a spiral galaxy: how fast do stars and gas circle the center, measured at different distances out from the middle? Plot orbital speed against distance and you get a curve that tells you, by gravity alone, how the galaxy's mass is arranged.

Here is the shock. If a galaxy's mass were where its light is — concentrated in the bright central regions — then, just as planets far from the Sun orbit more slowly than close ones, stars far out in a galaxy should orbit slowly. Instead, observations show the curve stays flat: out past the visible edge, stars and gas keep circling at roughly the same high speed, around 200 kilometers per second, with no falloff. By Newton's gravity, that high speed at large radius requires far more mass than the stars and gas can account for — and that unseen mass must keep increasing outward, forming a huge invisible halo.

Flat rotation curves, measured carefully by Vera Rubin and others in the 1970s, are one of the strongest pieces of evidence for dark matter. Each spiral seems to sit inside a dark-matter halo several times more massive than everything we can see, extending far beyond the starlight. Rotation curves also let astronomers weigh galaxies and underpin the Tully-Fisher relation, which links how fast a spiral spins to how luminous it is.