The inner rotation curve of the Milky Way

Abstract We derived the inner rotation curve (RC) of the Milky Way by applying the terminal velocity method (TVM) to the longitude–velocity diagrams (LVD) made from the large-scale survey data of the Galactic plane in the H i (HI4PI full-sky survey) and $^{12}$CO lines (CfA-Chile 1.2 m Galactic plane survey, Nobeyama 45 m Galactic plane and Galactic Center surveys, and Mopra 22 m southern Galactic plane survey). The derived RC agrees well with the RCs derived from the astrometric measurements of the maser sources by very long baseline interferometer observations and the GAIA result. We combined them to construct a unified RC from $R=0$ to ${\sim}25$ kpc and decomposed the curve into bulge, disc, and dark halo components with high accuracy. We present the RC as ascii tables for the solar constants of $(R_0,\Theta _0)=(8.178\:\rm{kpc}, 235.1\:\rm{km}\:\rm{s}^{-1})$. We also obtained a detailed comparison of the eastern ($l\ge 0^{\circ }$) and western (${<}0^{\circ }$) RCs in the H i and CO lines, which allowed the creation of an east–west (E/W) asymmetry curve of the velocity difference. The E/W asymmetry is fitted by a sinusoidal function of the radius with the amplitude increasing toward the Galactic Center. We consider the possibility of the origin due to a weak bar inside ${\sim}4\:$kpc.