Mark J. Reid
Galactic Structure from Trigonometric Parallaxes of Star Forming Regions
The Very Long Baseline Array and the Japanese VERA array are conducting large programs to measure trigonometric parallaxes and proper motions of high-mass star-forming regions across the Milky Way. These measurements give the full 3-dimension locations and space velocities of the star-forming regions. Some spiral arms have now been well located and their pitch angles measured. Star-forming regions have significant departures from circular orbits in the Galaxy and, on average, orbit the Galaxy slower than for circular rotation. We now can accurately estimate the distance to the Galactic Center (R0) and the circular rotation speed (T0). The rotation speed is greater than the IAU recommended value of 220 km s−1 and closer to that of the Andromeda galaxy, suggesting that the dark matter halos of the two galaxies may be comparably massive.