by combining CO line velocities and optical distances (Blitz et al. 1986; Fich et al. 1989), and by the HI disk-thickness method (Honma and Sofue 1997). Optical measurements of carbon stars have added many data points for outer rotation (Demers and Battinelli 2007). An accurate rotation velocity was obtained at a galactocentric radius of 13 kpc using VERA from parallax-proper motion measurements (Honma et al. 2007). These data were recently integrated to derive an accurate rotation curve of the Galaxy (Sofue et al. 2008). Here, we assume the galactocentric distance of the Sun to be 8.0 kpc and its circular rotation velocity of 200 km s$^{-1}$.

Rotation curves are usually decomposed into several components, mostly into a bulge, disk and dark halo (Sofue and Rubin 2001). We have decomposed the newest rotation curve into a de Vaucouleurs bulge, exponential disk, and isothermal dark halo (Sofue et al. 2008). We showed that the outer rotation curve is reasonably fitted by an isothermal model. However, it was also pointed out that the halo model might not be unique: It is difficult to clearly discriminate one model from the other, e.g. isothermal model (Begeman et al. 1991) from NWF (Navarro et al. 1996) 