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 { { } ^ { \circ } } \pm 3 { { } ^ { \circ } } , which favors four rather than two spiral arms for the Galaxy .
Combining positions , distances , proper motions , and radial velocities yields complete 3-dimensional kinematic information .
We find that star forming regions on average are orbiting the Galaxy \approx 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 \pm 0.6 kpc and a circular rotation speed { \Theta _ { 0 } } = 254 \pm 16 km s ^ { -1 } .
The ratio { \Theta _ { 0 } } / { R _ { 0 } } can be determined to higher accuracy than either parameter individually , and we find it to be 30.3 \pm 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 two ; they can be brought into better agreement with the trigonometric parallaxes by increasing { \Theta _ { 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 \Theta _ { 0 } and { \Theta _ { 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 .