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\n<~ R <~ 14 kpc from the first year of data from the Apache Point Observatory\nGalactic Evolution Experiment (APOGEE). We model the line-of-sight velocities\nof 3,365 stars in fourteen fields with b = 0 deg between 30 deg < l < 210 deg\nout to distances of 10 kpc using an axisymmetric kinematical model that\nincludes a correction for the asymmetric drift of the warm tracer population\n(\\sigma_R ~ 35 km/s). We determine the local value of the circular velocity to\nbe V_c(R_0) = 218 +/- 6 km/s and find that the rotation curve is approximately\nflat with a local derivative between -3.0 km/s/kpc and 0.4 km/s/kpc. We also\nmeasure the Sun's position and velocity in the Galactocentric rest frame,\nfinding the distance to the Galactic center to be 8 kpc < R_0 < 9 kpc, radial\nvelocity V_{R,sun} = -10 +/- 1 km/s, and rotational velocity V_{\\phi,sun} =\n242^{+10}_{-3} km/s, in good agreement with local measurements of the Sun's\nradial velocity and with the observed proper motion of Sgr A*. We investigate\nvarious systematic uncertainties and find that these are limited to offsets at\nthe percent level, ~2 km/s in V_c. Marginalizing over all the systematics that\nwe consider, we find that V_c(R_0) < 235 km/s at >99% confidence. We find an\noffset between the Sun's rotational velocity and the local circular velocity of\n26 +/- 3 km/s, which is larger than the locally-measured solar motion of 12\nkm/s. This larger offset reconciles our value for V_c with recent claims that\nV_c >~ 240 km/s. Combining our results with other data, we find that the Milky\nWay's dark-halo mass within the virial radius is ~8x10^{11} M_sun.\n