Mass of the Galaxy Inferred from Outer Rotation Curve

Using an outer rotation curve of the Galaxy, we explore the galactic constants and the mass of the Galaxy. We show that \Theta_0 of 200 km/s is more favorable than the IAU standard value of 220 km/s, and also show that if \Theta_0 is smaller than 207 km/s the rotation curve beyond 2R_0 is declining in Keplerian fashion. In the case of \Theta_0= 200 km/s and R_0= 7.6 kpc, the total mass and the extent of the Galaxy inferred from the rotation curve are 2.0+/-0.3x 10^{11} M_\odot and 15 kpc, respectively. These results may significantly change the previous view of the Galaxy, that its outer region is dominated by a massive dark halo extending out to several tens of kpc.