Astrometry of Galactic Star Forming Region Sharpless 269 with VERA : Parallax Measurements and Constraint on Outer Rotation Curve

TL;DR

This study uses high-precision astrometry with VERA to measure the parallax of S269, providing new insights into the Galaxy's outer rotation curve and dark matter distribution.

Contribution

First detection of a parallax beyond 5 kpc, constraining the Galaxy's outer rotation curve and supporting the presence of dark matter.

Findings

Measured the smallest parallax ever at 189 micro-arcsec

Confirmed the flatness of the Galaxy's outer rotation curve

Provided strong evidence for dark matter in the outer disk

Abstract

We have performed high-precision astrometry of H2O maser sources in Galactic star forming region Sharpless 269 (S269) with VERA. We have successfully detected a trigonometric parallax of 189+/-8 micro-arcsec, corresponding to the source distance of 5.28 +0.24/-0.22 kpc. This is the smallest parallax ever measured, and the first one detected beyond 5 kpc. The source distance as well as proper motions are used to constrain the outer rotation curve of the Galaxy, demonstrating that the difference of rotation velocities at the Sun and at S269 (which is 13.1 kpc away from the Galaxy's center) is less than 3%. This gives the strongest constraint on the flatness of the outer rotation curve and provides a direct confirmation on the existence of large amount of dark matter in the Galaxy's outer disk.