The global properties of the Galaxy. II. The galactic rotation parameters from 21-cm H I observations.

Sensitive 21-cm H I line data for the Galaxy at galactocentric distances greater and less than that of the sun are reexamined. The following Galactic rotation parameters are derived: a solar galactocentric distance of 8.5 + or - 0.5 kpc, a local circular velocity of 220 + or - 10 km/s, an Oort constant A of 13 + or - 2 km/s per kpc, and an Oort constant B of -13 + or - 2 km/s per kpc. It is found that the rotation curve is flat at about 220 km/s between 4 and 25 kpc and probably well beyond. An H I mass of about 2 billion solar masses is estimated for the Galaxy, along with a luminosity of 17 billion suns and a Hubble type of Sbc II. It is concluded that the total mass of the Galaxy is around 1 trillion solar masses and mostly in unseen form and that the total extent of the Galaxy is about 90 kpc, which puts the Magellanic Clouds within its halo.