The Distance to a Star-Forming Region in the Outer Arm of the Galaxy

We performed astrometric observations with the Very Long Baseline Army of WB89−437, an H 2 O maser source in the Outer spiral arm of the Galaxy. We measure an annual parallax of 0.167 ± 0.006 mas, corresponding to a heliocentric distance of 6.0 ± 0.2 kpc or a Galactocentric distance of 13.4 ± 0.2 kpc. This value for the heliocentric distance is considerably smaller than the kinematic distance of 8.6 kpc. This confirms the presence of a faint Outer arm toward l = 135°. We also measured the full space motion of the object and find a large peculiar motion of ∼20 km s −1 toward the Galactic center. This peculiar motion explains the large error in the kinematic distance estimate. We also find that WB89−437 has the same rotation speed as the LSR, providing more evidence for a flat rotation curve and thus the presence of dark matter in the outer Galaxy.