THE DISTANCE TO A STAR-FORMING REGION IN THE OUTER ARM OF THE GALAXY

K. Hachisuka, A. Brunthaler, K. M. Menten, M. J. Reid, Y. Hagiwara, N. Mochizuki
2009 Astrophysical Journal  
We performed astrometric observations with the VLBA of WB89-437, an H2O 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 degrees. We also measured the full space
more » ... d the full space motion of the object and find a large peculiar motion of ~20 km/s 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.
doi:10.1088/0004-637x/696/2/1981 fatcat:gpc3uujgnzaqznhj5cg2ntpyhq