Trigonometric Parallaxes of Star-forming Regions beyond the Tangent Point of the Sagittarius Spiral Arm

As part of the BeSSeL Survey, we report trigonometric parallaxes and proper motions of molecular maser sources associated with 13 distant high-mass star-forming regions in the Sagittarius spiral arm of the Milky Way. In particular, we obtain improved parallax distance estimates for three well-studied regions: 1.9(-0.1)(+0.1) kpc for M17, 5.3(-0.9)(+1.3) kpc for W51, and 7.9(-0.7)(+0.9) kpc for GAL 045.5+00.0. Peculiar motions for all but one source are less than 20 km s(-1). We fit a log-periodic spiral to the locations and estimate an average pitch angle of 7 degrees.2 +/- 1 degrees.9. We find that the section of the arm beyond the tangent point in the first quadrant of the Milky Way appears 15 pc below the IAU-defined Galactic plane.