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 HI (HI4PI whole sky survey) and 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 (VLBI) 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 precision. The dark matter density near the Sun is estimated to be $0.107 \pm 0.003$ GeV cm$^{-3}$. We present the RC as ascii tables for the solar constants of $(R_0,V_0)=(8.178 {\rm kpc}, 235.1 {\rm km/s})$, We also obtained a detailed comparison of the eastern ($l\ge 0^\circ$) and western ($< 0^\circ$) RCs in the HI and CO lines, which allowed the creation of an 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.