Galaxy Rotation Curves: Physical Mechanisms

Gas emission lines and stellar spectra provide line-of-sight velocities across galaxy disks. A mass model includes stars, gas, central components, and an inferred extended dark-matter halo or an alternative gravitational prescription.

For circular motion, orbital speed depends on enclosed gravitating mass. If visible mass converges while speed stays high at larger radius, either additional unseen mass continues accumulating or the effective law of gravity differs from its standard Newtonian behavior in that regime.

Rotation curves across many galaxy types repeatedly show mass discrepancies, supplemented by lensing, satellite dynamics, cluster evidence, CMB results, and structure formation.

Circular speed reflects enclosed gravitational mass. If visible mass stops increasing rapidly but speed remains high, standard gravity requires additional mass at large radius.