TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. VI. GALACTIC STRUCTURE, FUNDAMENTAL PARAMETERS, AND NONCIRCULAR MOTIONS

2009/02/28 by M. J. Reid, K. M. Menten, Xin Zheng +19 · 68 citations
Mathematics · Physics and Astronomy · #Astrometry #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Astrophysics and Star Formation Studies #Circular motion #Classical mechanics #Declination #Galactic Center #Galaxy #Galaxy formation and evolution #Galaxy rotation curve #Geometry #Kinematics #Mathematics #Milky Way #Physics #Rotation (mathematics) #Spiral (railway) #Spiral galaxy #Stars #Stellar, planetary, and galactic studies #Very-long-baseline interferometry #astro-ph.CO #astro-ph.GA

paper · pdf · doi:10.1088/0004-637x/700/1/137

published as Astrophys.J.700:137-148,2009 · 35 pages, 7 figures, 7 tables

arxiv created 2009/05/01 · openalex publication_date 2009/06/30 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05

Abstract

We are using the Very Long Baseline Array and the Japanese VLBI Exploration of Radio Astronomy project to measure trigonometric parallaxes and proper motions of masers found in high-mass star-forming regions across the Milky Way. Early results from 18 sources locate several spiral arms. The Perseus spiral arm has a pitch angle of 16° ± 3°, which favors four rather than two spiral arms for the Galaxy. Combining positions, distances, proper motions, and radial velocities yields complete three-dimensional kinematic information. We find that star-forming regions on average are orbiting the Galaxy ≈15 km s −1 slower than expected for circular orbits. By fitting the measurements to a model of the Galaxy, we estimate the distance to the Galactic center R 0 = 8.4 ± 0.6 kpc and a circular rotation speed Θ 0 = 254 ± 16 km s −1 . The ratio Θ 0 / R 0 can be determined to higher accuracy than either parameter individually, and we find it to be 30.3 ± 0.9 km s −1 kpc −1 , in good agreement with the angular rotation rate determined from the proper motion of Sgr A*. The data favor a rotation curve for the Galaxy that is nearly flat or slightly rising with Galactocentric distance. Kinematic distances are generally too large, sometimes by factors greater than 2; they can be brought into better agreement with the trigonometric parallaxes by increasing Θ 0 / R 0 from the IAU recommended value of 25.9 km s −1 kpc −1 to a value near 30 km s −1 kpc −1 . We offer a "revised" prescription for calculating kinematic distances and their uncertainties, as well as a new approach for defining Galactic coordinates. Finally, our estimates of Θ 0 and Θ 0 / R 0 , when coupled with direct estimates of R 0 , provide evidence that the rotation curve of the Milky Way is similar to that of the Andromeda galaxy, suggesting that the dark matter halos of these two dominant Local Group galaxy are comparably massive.

Citations

• The Proper Motion of Sagittarius A*. II. The Mass of Sagittarius A*
• The New I.A.U. System of Galactic Coordinates (1958 Revision)
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• The Distance to the Perseus Spiral Arm in the Milky Way
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• Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits
• Constraints on the Proper Motion of the Andromeda Galaxy Based on the Survival of Its Satellite M33
• Implications of recent measurements of the Milky Way rotation for the orbit of the Large Magellanic Cloud

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