The Circular Velocity Curve of the Milky Way from 5–25 kpc Using Luminous Red Giant Branch Stars

Abstract

We present a sample of 254,882 luminous red giant branch (LRGB) stars selected from the APOGEE and LAMOST surveys. By combining photometric and astrometric information from the Two Micron All Sky Survey and Gaia survey, the precise distances of the sample stars are determined by a supervised machine-learning algorithm: the gradient-boosted decision trees. To test the accuracy of the derived distances, member stars of globular clusters (GCs) and open clusters are used. The tests by cluster member stars show a precision of about 10% with negligible zero-point offsets, for the derived distances of our sample stars. The final sample covers a large volume of the Galactic disk(s) and halo of 0 < R < 30 kpc and ∣ Z ∣ ≤ 15 kpc. The rotation curve (RC) of the Milky Way across the radius of 5 ≲ R ≲ 25 kpc has been accurately measured with ∼54,000 stars of the thin disk population selected from the LRGB sample. The derived RC shows a weak decline along R with a gradient of −1.83 ± 0.02 (stat.) ± 0.07 (sys.) km s ^−1 kpc ^−1 , in excellent agreement with the results measured by previous studies. The circular velocity at the solar position, yielded by our RC is 234.04 ± 0.08 (stat.) ± 1.36 (sys.) km s ^−1 , again in great consistency with other independent determinations. From the newly constructed RC, as well as constraints from other data, we have constructed a mass model for our Galaxy, yielding a mass of the dark matter halo of M _200 = (8.05 ± 1.15) × 10 ^11 M _⊙ with a corresponding radius of R _200 = 192.37 ± 9.24 kpc and a local dark matter density of 0.39 ± 0.03 GeV cm ^−3 .

Keywords

• Galaxy rotation curves
• Distance measure
• Milky Way dynamics
• Galaxy kinematics