Evidence for dark matter

By measuring the dimensionless product (σz/vc)2R0/h one can estimate the ratio of disk mass to total mass interior to the Sun’s orbit, where σz is velocity dispersion perpendicular to the disk appropriate to some tracer population, h is the scale height of that tracer, and vc is the circular velocity at the Sun’s distance from the center of the Milky Way, R0. While there are model dependent factors of order unity which render this ratio uncertain, it is difficult to avoid the conclusion that there is significant dark matter interior to the Sun’s orbit. Beyond the solar circle radial velocity data for disk stars are consistent with a flat rotation curve out to 2R0. Beyond that the interpretation of radial velocity data for blue horizontal branch stars, globular clusters, and satellite galaxies requires assumptions about their transverse velocities. Proper motion measurements less uncertain than 0.5 mas/yr might resolve this ambiguity.By measuring the dimensionless product (σz/vc)2R0/h one can estimate the ratio of disk mass to total mass interior to the Sun’s orbit, where σz is velocity dispersion perpendicular to the disk appropriate to some tracer population, h is the scale height of that tracer, and vc is the circular velocity at the Sun’s distance from the center of the Milky Way, R0. While there are model dependent factors of order unity which render this ratio uncertain, it is difficult to avoid the conclusion that there is significant dark matter interior to the Sun’s orbit. Beyond the solar circle radial velocity data for disk stars are consistent with a flat rotation curve out to 2R0. Beyond that the interpretation of radial velocity data for blue horizontal branch stars, globular clusters, and satellite galaxies requires assumptions about their transverse velocities. Proper motion measurements less uncertain than 0.5 mas/yr might resolve this ambiguity.