The milky way's circular-velocity curve between 4 and 14 kpc from apogee data

Abstract

We measure the Milky Way's rotation curve over the Galactocentric range 4 kpc ≲ R ≲ 14 kpc from the first year of data from the Apache Point Observatory Galactic Evolution Experiment. We model the line-of-sight velocities of 3365 stars in 14 fields with b = 0° between 30° ≤ l ≤ 210° out to distances of 10 kpc using an axisymmetric kinematical model that includes a correction for the asymmetric drift of the warm tracer population (σ R 35 km s-1). We determine the local value of the circular velocity to be Vc (R 0) = 218 ± 6 km s-1 and find that the rotation curve is approximately flat with a local derivative between -3.0 km s-1 kpc-1 and 0.4 km s-1 kpc-1. We also measure the Sun's position and velocity in the Galactocentric rest frame, finding the distance to the Galactic center to be 8 kpc < R 0 < 9 kpc, radial velocity V R, = -10 ± 1 km s-1, and rotational velocity V φ, = 242+103 km s-1, in good agreement with local measurements of the Sun's radial velocity and with the observed proper motion of SgrA*. We investigate various systematic uncertainties and find that these are limited to offsets at the percent level, 2 km s-1 in V c . Marginalizing over all the systematics that we consider, we find that Vc (R0) < 235 km s-1 at >99 % confidence. We find an offset between the Sun's rotational velocity and the local circular velocity of 26 ± 3 km s-1, which is larger than the locally measured solar motion of 12 km s-1. This larger offset reconciles our value for Vc with recent claims that Vc ≳ 240 km s -1. Combining our results with other data, we find that the Milky Way's dark-halo mass within the virial radius is 8 × 1011 M © 2012. The American Astronomical Society. All rights reserved.