Rotation Curve of the Milky Way out to $\sim$ 200 kpc

The rotation curve (RC) of the Galaxy, the Milky Way, is constructed starting from its very inner regions (few hundred pc) out to a large galactocentric distance of $\sim 200\kpc$ using kinematical data on a variety of tracer objects moving in the gravitational potential of the Galaxy. We study the effect on the RC due to the uncertainties in the values of the Galactic Constants (GCs) $\rsun$ and $\vsun$ (these being the sun’s distance from and circular rotation speed around the Galactic center, respectively) and the velocity anisotropy parameter $\beta$ of the halo tracer objects used for deriving the RC at large galactocentric distances. The resulting RC in the disk region is found to depend significantly on the choice of the GCs, while the dominant uncertainty in the RC at large distances beyond the stellar disk comes from the uncertainty in the value of $\beta$. In general we find that the mean RC steadily declines at distances beyond $\sim50\kpc$. Also, at a given radius, the circular speed is lower for larger values of $\beta$ (i.e., for more radially biased velocity anisotropy). Considering recent results from large numerical simulations, which find an increasingly radially biased velocity ellipsoid of the Galaxy’s stellar population at large distances, with stellar orbits tending to be almost purely radial ($\beta\to 1$) beyond $\sim 100\kpc$, our results, for the case of $\beta=1$, give a model independent estimate of the total mass of the Galaxy within $\sim 200\kpc$, $M(200\kpc)\gsim (6.8\pm4.1)\times10^{11}\Msun$. The complete RC of the Galaxy given here may be useful for deriving the phase space properties of the Galaxy’s dark matter halo.

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Date added: Fri, 11 Oct 13